zur Erlangung des Grades eines Doktor-Ingenieurs

der Fakultät für Elektrotechnik und Informationstechnikan der Ruhr-Universität Bochum

Michael Westermeier

Bochum 2011

Dissertation

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IKInvestigation of the emitter-effect

in HID-lamps

by absorption- and emission-

spectroscopy

Dissertation eingereicht am: 21.12.2010Tag der mundlichen Prufung: 28.01.2011

Berichter: Prof. Dr.-Ing. Peter AwakowiczProf. Dr. em. Jurgen Mentel

i

Acknowledgement / Danksagung

Die vorliegende Doktorarbeit prasentiert Forschungsergebnisse, die ich im Laufe meinermehrjahrigen wissenschaftlichen Tatigkeit am Lehrstuhl fur ”Allgemeine Elektrotechnik undPlamsmatechnik”(AEPT) der Ruhr-Universitat Bochum erarbeitet habe. Da das wissenschaft-liche Arbeiten aber erst durch den regen Austausch und die Mithilfe vieler Menschen lebt,sind die vorgestellten Ergebnisse gepragt von der Unterstutzung zahlreicher Wegbegleiter.Ganz alleine hatte ich es sicher nicht so weit gebracht. Bei den Wichtigsten mochte ich michim Folgenden kurz bedanken:

Zuallererst mochte ich mich bei Herrn Prof. Dr.-Ing. Peter Awakowicz dafur bedanken, dasser mich als wissenschaftlicher Mitarbeiter an seinem Lehrstuhl AEPT beschaftigt hat und mirsomit ermoglichte, eine interessante und spannende Promotion auf dem Gebiet der Gasent-ladungslampen durchzufuhren. Herr Awakowicz hat mich, wie auch alle anderen Mitarbeiter,jederzeit bestens in der wissenschaftlichen Arbeit unterstutzt, gab mir die Moglichkeit durchviele Konferenzreisen einen regen, fruchtbaren Austausch mit anderen Wissenschaftlern zufuhren und sorgte als Lehrstuhlchef immer fur eine hervorragende Arbeitsatmosphare. Ihnen,Herr Awakowicz, vielen Dank fur alles und weiterhin besten Erfolg fur Sie und fur die vielenspannenden Aufgaben die am Lehrstuhl AEPT in der nachsten Zeit anstehen.Genauso herzlich mochte ich mich bei Herrn Prof. Dr. em. Jurgen Mentel dafur bedanken,dass er mich fachlich sehr intensiv und zu jeder Zeit auch sehr personlich bei meiner wis-senschaftlichen Arbeit und spater beim Schreiben meiner Doktorarbeit betreut hat. ObwohlHerr Mentel schon langere Zeit emeritiert ist, hat er es sich nie nehmen lassen, jeden Tagzur Uni zu kommen um mich und viele andere junge Wissenschaftler von der Plasmatechnikzu begeistern und mit seinem großen Wissen bei der ein oder anderen scheinbar ausweglosenProblematik weiterzuhelfen. Ich mochte Ihnen, Herr Mentel, fur all dies ganz herzlich dankenund wunsche Ihnen alles Gute damit Sie noch lange Zeit junge Wissenschaftler wie mich beiihrer Promotion begleiten und unterstutzen konnen.

Großer Dank gilt auch der Firma Philips Lighting, Eindhoven, durch die im Rahmen eineswissenschaftlichen Kooperationsprojektes erst interessante Fragestellungen sowie die fach-liche und technische Zusammenarbeit fur meine Doktorarbeit entstanden sind. Stellvertre-tend fur alle Philips Mitarbeiter mochte ich mich hier besonders bei Dr. Gerard Luijks be-danken fur die perfekte fachliche wie auch personliche Zusammenarbeit und den regelmaßi-gen, fruchtbaren wissenschaftlichen Austausch. Speziell unsere gemeinsame Woche beimPhilips Quality-Improvement-Competition Europe-Final in Lissabon behalte ich in sehr guterund freudiger Erinnerung.Naturlich bin ich auch meinen Kollegen aus der Lampengruppe am Lehrstuhl AEPT sehrdankbar fur die gute fachliche und freundschaftliche Unterstutzung wahrend meiner Pro-motion. Herrn Dr. Oliver Langenscheidt dafur, dass er mir als seine studentische Hilfskraftvertraut und mir so den Start in die Gasentladungslampen-Technik bis zum Beginn meiner

ii Danksagung

eigenen Promotion bereitet hat. Herrn Dr. Jens Reinelt mochte ich danken fur die er-folgreiche, freundschaftliche Zusammenarbeit wahrend der wir lange Zeit gemeinsam pro-movierten. Auch den Kollegen/ -innen Cornelia Ruhrmann und Andre Bergner mochte ichherzlich danken, dass sie unter meiner Betreuung ihre Diplomarbeiten in unserer Lampen-gruppe schrieben, mir dadurch viele wichtige Ergebnisse fur meine Doktorarbeit liefertenund schließlich selbst als wissenschaftliche Mitarbeiter unser Lampenteam verstarkt haben.Genauso mochte ich Thomas Hobing dafur danken, dass er unter meiner Betreuung seineBachelorarbeit geschrieben und mir davon einen wesentlichen Teil der Ergebnisse fur meineArbeit zur Verfugung gestellt hat.Generell mochte ich allen Kollegen/ -innen und Mitarbeitern am Lehrstuhl AEPT fur dietolle fachliche und personliche Zusammenarbeit danken, es hat mit immer viel Spaß gemacht,ein Teil dieses guten Teams zu sein. Naturlich wurde ich gerne jeden einzelnen von euch hiermit ein paar personlichen Worten erwahnen, aber seht es mir bitte nach, es passt einfachnicht mehr rein... Vielen Dank, ihr seid ein tolles Team!Fur die gute fachliche und finanzielle Unterstutzung mochte ich auch dem Graduiertenkol-leg 1051 der DFG ”Nichtgleichgewichtsphanomene in Niedertemperaturplasmen” sowie ganzbesonders der Research-School der Ruhr-Universitat Bochum danken. Stellvertretend furalle weiteren Mitarbeiter mochte ich hier Frau Dr. Ursula Justus der RUB Research-Schoolfur ihren begeisternden Einsatz fur junge Wissenschaftler danken, die es mit ihrem Teamgeschafft hat, dass wir auch mal etwas uber den Tellerrand der eigenen Forschung herausge-blickt und viele Soft-Skills erlernt haben.

Mindestens genauso wichtig wie die gute Zusammenarbeit mit Betreuern, Projektpartnernund Kollegen ist sicherlich ein tragfahiger Freundeskreis und eine gute familiare Unter-stutzung. Daher mochte ich an dieser Stelle, neben vielen anderen, besonders meinen Kom-militonen und Freunden Sebastian Theiß, Andreas Herzog und Kai Daniel danken. Dafur,dass wir uns gemeinsam durch die oft anstrengenden aber auch schonen Abschnitte desStudiums getragen haben und uns auch in der Zeit danach nicht aus den Augen verlieren.

Ganz besonders herzlich mochte ich mich bei meinen Eltern, Ulrich und Elisabeth West-ermeier, bedanken. Sie haben mich erst zu einem wissbegierigen, technisch interessiertenjungen Mann gemacht und mir dann mein Studium der Elektro- und Informationstechnik inBochum ermoglicht und finanziert. Ohne euch und eure perfekte Unterstutzung schon vonKind an wurde ich sicher heute nicht diese Zeilen unter meine Doktorarbeit schreiben kon-nen... Naturlich gilt dieser Dank auch meinen Geschwistern Claudia, Christian und Julia, dieunsere Familie erst komplett machen. In jeglicher Phase bis zum Anschluss meiner Promo-tion konnte ich mich voll auf euch alle verlassen und habe immer samtliche Unterstutzungbekommen, die ich brauchte. Danke!Zu guter Letzt bleibt mir naturlich noch besonders Danke zu sagen an meine Freundin Kata-rina Meciarova. Sie hat in der Zeit meiner Promotion und vor allem wahrend des Schreibensmeiner Doktorarbeit doch die ein oder andere schlechte Laune und schwierige Phase vonmir direkt abbekommen und immer toll ausgeglichen. Sie hat mir den Rucken soweit es gingfreigehalten und war immer fur mich da. Kati, ich danke dir sehr dafur und liebe dich.

Ja wunderbarchen, am Ende wird alles gut... Danke euch allen, auch denen, die leider nichteinzeln Erwahnung finden konnten!

Contents iii

Contents

Important symbols and abbreviations v

List of figures xi

Abstract xvii

1 Introduction 11.1 Technology of artificial light sources - a short overview of a long-time devel-

opment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 High-intensity discharge lamps (HID) . . . . . . . . . . . . . . . . . . . . . . 31.3 State of the research into HID-lamps . . . . . . . . . . . . . . . . . . . . . . 5

1.3.1 HID research at the Bochum University - The model lamp . . . . . . 51.3.2 The work of Dabringhausen, Lichtenberg, Redwitz et al . . . . . . . . 61.3.3 The work of Langenscheidt . . . . . . . . . . . . . . . . . . . . . . . . 81.3.4 The work of Scharf . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3.5 The work of Reinelt . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Thesis content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.4.1 Aim of this PhD work - pyrometry and absorption spectroscopy . . . 101.4.2 Thesis structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 Fundamentals 152.1 Arc discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2 Fundamentals of plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.1 Basic plasma processes . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.2 Basic plasma properties . . . . . . . . . . . . . . . . . . . . . . . . . 182.2.3 Thermal equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.4 Radiative processes in plasmas . . . . . . . . . . . . . . . . . . . . . . 21

2.3 Electrodes of arc discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.3.1 The heat balance of HID electrodes . . . . . . . . . . . . . . . . . . . 242.3.2 Emission processes of HID electrodes . . . . . . . . . . . . . . . . . . 282.3.3 The emitter-effect on tungsten surfaces . . . . . . . . . . . . . . . . . 322.3.4 The cathodic sheath model for HID electrodes . . . . . . . . . . . . . 35

3 Experimental setup 413.1 Investigated HID-lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.1 The Bochum model lamp . . . . . . . . . . . . . . . . . . . . . . . . . 413.1.2 The high-pressure sodium lamp (HPS) . . . . . . . . . . . . . . . . . 433.1.3 Ceramic HID-lamps (YAG) . . . . . . . . . . . . . . . . . . . . . . . 46

3.2 Electrical setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

iv Contents

3.2.1 Electric power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.2.2 Electric diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 Optical diagonstic setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.3.1 Spectroscopy and imaging optics . . . . . . . . . . . . . . . . . . . . 513.3.2 2D-Imaging of electrode and arc attachment . . . . . . . . . . . . . . 553.3.3 Absolute calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.4 Triggering for phase resolved measurements . . . . . . . . . . . . . . . . . . 57

4 Measuring methods and data interpretation 594.1 Measurement of the electrode temperature and power loss . . . . . . . . . . 59

4.1.1 Spectroscopic electrode temperature measurement . . . . . . . . . . . 604.1.2 2D electrode temperature measurement . . . . . . . . . . . . . . . . . 634.1.3 Reproducibility and errors of the pyrometric temperature measurements 654.1.4 Calculation of the electrode power loss . . . . . . . . . . . . . . . . . 67

4.2 Spectroscopic particle density measurements . . . . . . . . . . . . . . . . . . 694.2.1 Inverse Abel transformation . . . . . . . . . . . . . . . . . . . . . . . 704.2.2 Density measurement in YAG-lamps by emission spectroscopy . . . . 734.2.3 Density measurement by absorption spectroscopy . . . . . . . . . . . 794.2.4 Reproducibility and errors of the spectroscopic Dy density measurements 85

5 Measurements and results 895.1 Results at the Bochum model lamp . . . . . . . . . . . . . . . . . . . . . . . 89

5.1.1 Determination of the work function by simulation and measurements 905.1.2 Thoriated electrodes with low and high operating frequencies . . . . . 99

5.2 Results at the high-pressure sodium lamps . . . . . . . . . . . . . . . . . . . 1095.2.1 The Ba diffusion process in the HPS-lamp . . . . . . . . . . . . . . . 1095.2.2 The influence of hydrogen in HPS-lamps . . . . . . . . . . . . . . . . 1215.2.3 Electrode temperature measurements in the HPS-lamps . . . . . . . . 127

5.3 Results at the YAG-lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.3.1 Pure Dy-lamps - variation of the Dy-amount of the salt mixture . . . 1345.3.2 Results in the NTD-lamps . . . . . . . . . . . . . . . . . . . . . . . . 1445.3.3 The poisoning effect of thallium on the Dy emitter-effect . . . . . . . 1515.3.4 Comparison of YAG- and PCA-lamps . . . . . . . . . . . . . . . . . . 1585.3.5 Plasma temperature measurements by emission and absorption . . . . 161

6 Conclusions and outlook 1676.1 Results at the model lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1676.2 Results at the high-pressure sodium lamp . . . . . . . . . . . . . . . . . . . . 1686.3 Results at the YAG-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

Bibliography 172

A Appendix A - 1A.1 Relative efficiency curves of the used CCD cameras . . . . . . . . . . . . . . A - 1A.2 Relative filter characteristics of the used interference filters . . . . . . . . . . A - 3A.3 Spectral efficiency of the high power LED used as backlight source . . . . . . A - 5

B Curriculum Vitae B - 1

Symbols and Abbreviations v

Important symbols and abbreviations

Symbols

A Atom

A+ Ionized atom

A∗, A+∗ Excited atom, ion

Ai Atom in excited state i

Aul Transition probability from a state u to a state l

c0 Speed of light

cp Heat capacity

dE Electrode diameter

ε0 Vacuum permittivity

εr Relative permittivity

ελ Spectral emission coefficient

ελ,ul Spectral line emission coefficient

ελ,cont Spectral continuum emission coefficient

εul Total line emission coefficient

ε(λ, T ) Emissivity

εtot Total surface emissivity

e elementary charge

efast Accelerated electron

Ekin Kinetic energy

Er Energy of species in a state r

Ei Ionization energy

EF Fermi energy

E∞ Vacuum energy level

Eu Energy of the upper state

El Energy of the lower state

vi Symbols and Abbreviations

Φ Arc potential

ϕ Work function

f Frequency

flu Oscillator strength

gr Statistical weight of a state r

g0 Statistical weight of the groundstate

h, ~ Planck constant

Iλ Spectral radiance

iarc Arc current

j Current density

jtot Total current density

je Electron current density

ji Ion current density

jem Current density caused by thermionic emission

jep Current density caused by back-diffusion of electrons

jSE Current density caused by secondary electrons

κ(T ) Heat condictivity

kB Boltzmann constant

α Absorption coefficient

λd Debye length

λul Wavelength in the region ul

λHg,1 Mercury line at 576.96nm

λHg,2 Mercury line at 579.07nm

λDyI Dysprosium line at 695.81nm

λDyII Dysprosium ion line at 394.47nm

Lλ Surface radiance

larc Arc length

lE Electrode length

µ0 Vacuum permeability

mj Mass of species j

me Electron mass

mAr Atomic mass of Argon

ν Frequency

νul Discrete photon energy

n Density of states

ne Electron density

ni Ion density

nr Density of excited atoms in state r

n0 Groundstate density

Symbols and Abbreviations vii

ω Angular frequency

p Pressure

Pul Line profile

Pin Lamp input power

Ploss Electrode power loss

Prad Electrode power loss by radiation

Ploss Electrode power loss by conduction

qp Power flux density

qrad Radiated power

qcond Conducted power

qrec Power flux by recombination

qkin Power flux by kinetic energy of ions

qneutral Power flux from back diffusion of neutrals

qem Power flux by emitted electrons

qep Power flux from back diffusion of electrons

qtc Power flux caused by the heavy particles to the electrode

ρm Mass density

rE, RE Electrode radius

σSB Stefan-Boltzmann constant

σ(T ) Electrical conductivity

τ0 Optical thickness, optical depth

Tc Cathode surface temperature

Te Electron temperature

Tj Temperature of species j

Th Temperature of heavy particles j

Tamb Ambient temperature

Ttip Electrode tip temperature

texp Exposure time

tdelay Delay time

uarc, Uarc Arc voltage

ua,el, Ua,el Electrical anode sheath voltage

ua, Ua Anodic sheath voltage

uc, Uc Cathodic sheath voltage

vj Particle velocity of species j

Z0 Atom partition function

Ze Electron partition function

Zi Ion partition function

viii Symbols and Abbreviations

Symbols and Abbreviations ix

Abbreviations

AEPT Institute for Elektrical Engineering and Plasma Technology (german)

AC Alternating current

BBAS Broadband absorption spectroscopy

BMBF Federal Ministry of Education and Research (german)

CCD Charge Coupled Device

CFL Compact Fluorescent Lamp

CRI Colour rendering index

CZC Current Zero Crossing

DC Direct current

DT Dysprosium / Thallium

DyI Dysprosium Iodide

ESV Electrode Sheath Voltage

HF High Frequency

Hg Mercury

HID High Intensity Discharge

HPS High pressure sodium

ICCD Intensified Charge Coupled Device

IR Infrared

LAS Laser absorption spectroscopy

LED Light Emitting Diode

LTE Local Thermal Equilibriuum

pLTE Partial Local Thermal Equilibriuum

ND Sodium / Dysprosium

NT Sodium / Thallium

NTD Sodium / Thallium / Dysprosium

NTCC Sodium / Thallium / Calcium / Cerium

OES Optical Emission Spectroscopy

SNR Signal to Noise Ratio

TD Thallium / Dysprosium

TET Institute for Theoretical Electrical Engineering (german)

UHP ultra-high pressure

UV Ultraviolett

VHF Very High Frequency band

YAG Yttrium-aluminium-garnet

List of Figures xi

List of Figures

1.1 Worldwide electricity consumption by application and lighting sectors . . . . 11.2 Example of an automotive HID D-lamp and sketch . . . . . . . . . . . . . . 41.3 Images of the diffuse and spot arc attachment in the Bochum model lamp . . 7

2.1 U/I-diagramm of different gas discharge categories . . . . . . . . . . . . . . . 162.2 Typical potential distribution along an HID arc discharge . . . . . . . . . . . 172.3 Coordinate definitions for electrode heat balance . . . . . . . . . . . . . . . . 262.4 Sketch of the energy band model for a metal and the gas-phase with emitter-

effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.5 Sketch of three different emitter-storage-concepts . . . . . . . . . . . . . . . 332.6 Black-Box model of the cathodic boundary sheath . . . . . . . . . . . . . . . 362.7 Example of calculated transfer functions in dependence on cathode surface

temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.1 Schematic sketch of the Bochum model lamp . . . . . . . . . . . . . . . . . . 423.2 Images of the electrodes in an HPS-lamp with ceramic burner and with sap-

phire burner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3 Image and schematic sketch of the HPS research lamp . . . . . . . . . . . . . 453.4 Schematic sketch of the H2 dispenser/ getter HPS-lamp . . . . . . . . . . . . 463.5 Image and schematic sketch of a YAG-lamp . . . . . . . . . . . . . . . . . . 473.6 Block diagram of the power amplifier setup with and without commutator . 493.7 Switched-dc current distributions for high frequencies at a YAG-lamp with

and without commutator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.8 Schematic sketch of the spectroscopic measuring setup . . . . . . . . . . . . 523.9 Overview spectrum of the UHP-lightsource . . . . . . . . . . . . . . . . . . . 533.10 Schematic sketch of the 2D photographic camera system . . . . . . . . . . . 553.11 Block diagram of the trigger setup . . . . . . . . . . . . . . . . . . . . . . . . 58

4.1 Possible imaging orientations on the spectrograph entrance slit . . . . . . . . 604.2 Example of an electrode radiation spectrum measured at λ = 718 nm for the

determination of the electrode temperature . . . . . . . . . . . . . . . . . . . 614.3 Relative tungsten emissivity ε at λ = 718 nm for different temperatures . . . 624.4 Example of a measured 2D temperature image of a YAG electrode (NTD1)

in absolute units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644.5 Comparison measurements of the electrode tip temperature Ttip in Bochum

and Philips Lighting Eindhoven . . . . . . . . . . . . . . . . . . . . . . . . 664.6 Example of a measured 1D electrode temperature distribution along a YAG-

lamp electrode (NTD1) and corresponding numerical simulation . . . . . . . 684.7 Schematic sketch of the geometrical relation for the Abel transformation . . 71

xii List of Figures

4.8 Example of the spectrum emitted by the two main Hg-lines λHg,1 and λHg,2

and integrated radial intensity profile . . . . . . . . . . . . . . . . . . . . . . 744.9 Example of a resulting radial distribution of the plasma temperature Tpl mea-

sured by Hg emission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.10 Example of a recorded emission spectrum around middle wavelength λspec,Dy =

697 nm including two Dy spectral lines . . . . . . . . . . . . . . . . . . . . . 764.11 Zoomed excerpt of the investigated Dy emission spectral line λDy,696 with

integration limits and continuum background . . . . . . . . . . . . . . . . . . 774.12 Example of a resulting radial distribution of the Dy atom density nDy mea-

sured by emission spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 794.13 Example of a measured Dy absorption line profile Iabs,UHPν and correlated

optical depth τ in an NTD1 YAG-lamp . . . . . . . . . . . . . . . . . . . . . 814.14 Example of a measured and corrected optical Dy line depth τν in an NTD1

YAG-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.15 Example of a resulting radial distribution of the Dy atom ground-state density

n0,Dy measured by absorption spectroscopy . . . . . . . . . . . . . . . . . . . 844.16 Relation nDy/n0,Dy for typical plasma temperatures Tpl in HID-lamps . . . . 854.17 Comparison of Dy atom density results determined by emission- and absorp-

tion spectroscopy in an NTD1 and an NTD2 YAG-lamp . . . . . . . . . . . 86

5.1 Measured and simulated Ttip of BSD and G18 cathodes for varying arc currents 905.2 Effective work function ϕ for Th on W depending on relative coverage . . . . 925.3 Measured and simulated Ttip of BSD and G18 cathodes for a varying work

function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.4 Measured power loss Ploss of BSD and G18 cathodes in dependence on arc

current Iarc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.5 Measured and simulated Ttip of BSD and G7 cathodes for varying arc currents 955.6 Measured Ttip of BSD and G18 anodes (dE = 600µm−2 mm) for varying arc

currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.7 Electron emission current density in dependence on surface temperature for

pure tungsten, pure thorium and thoriated tungsten . . . . . . . . . . . . . . 975.8 Ttip for rising frequencies in Dy-YAG-lamps and in the model lamp . . . . . 995.9 Phase resolved Ttip results for lE = 20 mm BSD electrodes in dependence on

frequency with f = 10 Hz - 10 kHz, iarc = 6 A switched-dc . . . . . . . . . . . 1015.10 Phase resolved Ttip results for lE = 20 mm BSD electrodes in dependence on

frequency with f = 10 Hz - 10 kHz, iarc = 4 A switched-dc . . . . . . . . . . . 1025.11 Phase resolved Ttip results for lE = 20 mm thoriated G18 electrodes in depen-

dence on frequency with f = 10 Hz - 10 kHz, iarc = 6 A switched-dc . . . . . 1035.12 Phase resolved Ttip results for lE = 20 mm thoriated G18 electrodes in depen-

dence on frequency with f = 10 Hz - 10 kHz, iarc = 3 A switched-dc . . . . . 1045.13 Phase resolved Ttip results for dE = 360µm electrodes of YAG-lamps with

Th filling in dependence on frequency with f = 10 Hz - 2 kHz, iarc = 800 mAswitched-dc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

5.14 Phase resolved Ttip results for dE = 360µm electrodes of YAG-lamps withTh filling in dependence on frequency with f = 10 Hz - 2 kHz, iarc = 500 mAswitched-dc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

5.15 Radially resolved measurement of relative Ba line-emission at λ = 553 nm inan HPS-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

List of Figures xiii

5.16 Sketch: Orientation of the spectrograph entrance slit to the measured HPS-electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.17 Sketch: Special geometry of a perpendicular absorption measurement in aHPS-lamp between the tungsten coil and the electrode tip . . . . . . . . . . 112

5.18 Radially resolved measurement of the relative Ba line-absorption at the Baresonance line (λ = 553 nm) in a HPS-lamp . . . . . . . . . . . . . . . . . . . 112

5.19 Sketch: The final absorption measuring and evaluation method of Ba alongthe electrode in a HPS-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.20 Ba atom density along the electrode in HPS-lamp 3b3 (dcoil−tip = 1 mm)measured by LED-backlight . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.21 Ba atom density along the electrode in HPS-lamp 2b3 (dcoil−tip = 0.6 mm)measured by LED-backlight . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.22 Comparison: Spectral backlight power output at λ = 553 nm for LED andUHP-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

5.23 Ba atom density along the electrode in HPS-lamp 3b1 (dcoil−tip = 1 mm)measured phase averaged by UHP-backlight . . . . . . . . . . . . . . . . . . 117

5.24 Ba atom density along the electrode in HPS-lamp 3b3 (dcoil−tip = 1 mm)measured phase averaged by UHP-backlight . . . . . . . . . . . . . . . . . . 118

5.25 Ba-density along the electrode for HPS-lamp 3b1 (dcoil−tip = 1.0 mm) shortlybefore and after switch-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

5.26 Ba atom density results along the electrode rod in a dispenser/ getter HPS-lamp for several disp./ getter heating cycles . . . . . . . . . . . . . . . . . . 122

5.27 Time development of averaged Ba atom density results and lamp voltage inthe dispenser/ getter HPS-lamp for several disp./ getter heating cycles . . . 123

5.28 Repetition with longer timescales: Time development of averaged Ba atomdensity results and lamp voltage in the dispenser/ getter HPS-lamp for severaldisp./ getter heating cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5.29 Time development of averaged Ba atom density results, lamp voltage anddispenser-/ getter-temperature in the dispenser/ getter HPS-lamp for severaldisp./ getter heating cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

5.30 Repetition with longer timescales: Time development of averaged Ba atomdensity results, lamp voltage and dispenser-/ getter-temperature in the dis-penser/ getter HPS-lamp for several disp./ getter heating cycles . . . . . . . 126

5.31 Electrode temperature distribution in the HPS-lamp 6, 2 for various low fre-quencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

5.32 Electrode temperature distribution in the HPS-lamp 6, 2 for various high fre-quencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

5.33 Electrode temperature distribution in the HPS-lamp 3b1 (dcoil−tip = 1.0 mm)for various high frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.34 Electrode temperature distribution in the HPS-lamp 4b1 (dcoil−tip = 1.4 mm)for various high frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.35 Ttip for YAG-lamps with different lamp fillings . . . . . . . . . . . . . . . . . 133

5.36 Dy atom groundstate density at the electrode for YAG-lamps with 1 mg, 2 mgand 4 mg pure Dy salt fillings, iRMS = 500− 800 mA, low frequency operation 135

5.37 Dy atom groundstate density at the electrode for YAG-lamps with 1 mg and2 mg pure Dy salt fillings, iRMS = 500− 800 mA, high frequency operation . . 136

xiv List of Figures

5.38 Dy atom groundstate density in the discharge middle for YAG-lamps with1 mg and 2 mg pure Dy salt fillings, iRMS = 500 − 800 mA, low and highfrequency operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

5.39 Electrode tip temperature Ttip for YAG-lamps with 1 mg, 2 mg and 4 mg pureDy salt fillings, iRMS = 500− 800 mA, low frequency operation . . . . . . . . 138

5.40 Electrode tip temperature Ttip for YAG-lamps with 1 mg, 2 mg and 4 mg pureDy salt fillings, iRMS = 500− 800 mA, high frequency operation . . . . . . . 139

5.41 Dy ion density at the electrode for YAG-lamps with 1 mg and 2 mg pure Dysalt fillings, iRMS = 500− 800 mA . . . . . . . . . . . . . . . . . . . . . . . . 140

5.42 Electrical lamp input power PRMS for YAG-lamps with 1 mg, 2 mg and 4 mgpure Dy salt fillings, frequency depending from f = 1 Hz to f = 1 kHz . . . . 141

5.43 ∆λ-method for Dy density at the electrodes in YAG-lamps with 1 mg, 2 mgand 4 mg pure Dy salt fillings for low and high frequency . . . . . . . . . . . 142

5.44 Electrode tip temperature Ttip for YAG-lamps with NTD1 and NTD2 saltfillings, iRMS = 500− 800 mA, low frequency operation . . . . . . . . . . . . 144

5.45 Electrode tip temperature Ttip for YAG-lamps with NTD1 and NTD2 saltfillings, iRMS = 500− 800 mA, high frequency operation . . . . . . . . . . . . 145

5.46 Electrode input power Pin for YAG-lamps with Dy, NTD1 and NTD2 saltfillings, iRMS = 800 mA, low and high frequency operation . . . . . . . . . . . 146

5.47 Dy atom groundstate density at the electrode for YAG-lamps with NTD1 andNTD2 salt fillings, iRMS = 500− 800 mA, low frequency operation . . . . . . 147

5.48 Dy atom groundstate density at the electrode for YAG-lamps with NTD1 andNTD2 salt fillings, iRMS = 500− 800 mA, high frequency operation . . . . . . 148

5.49 Dy atom groundstate density in the discharge middle for YAG-lamps withNTD1 and NTD2 salt fillings, iRMS = 500− 800 mA, low and high frequencyoperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

5.50 Dy ion density at the electrode for YAG-lamps with NTD1 and NTD2 saltfillings, iRMS = 500− 800 mA . . . . . . . . . . . . . . . . . . . . . . . . . . 150

5.51 Ttip for YAG-lamps with different Tl-containing and non Tl-containing lampfillings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

5.52 Ttip for YAG-lamps with and without Tl in dependence on operating frequency1535.53 Phase resolved nDy for YAG-lamps with and without Tl at electrode and in

discharge middle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1545.54 Phase resolved Dy ion density for YAG-lamps with and without Tl . . . . . 1555.55 Spectral absorption conditions for the Tl-line at λTl = 535.05 nm . . . . . . . 1565.56 Phase resolved nCe for YAG-lamp and PCA-lamp with 2.6% Ce amount in

discharge middle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595.57 Phase resolved nCe for YAG-lamp and PCA-lamp with 9.5% Ce amount in

discharge middle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1605.58 Phase resolved plasma temperature Tpl determined from a combination of

absorption- and emission-spectroscopy in a 1mg Dy-lamp . . . . . . . . . . . 1625.59 Phase resolved plasma temperature Tpl for different frequencies in a pure Hg

YAG-lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1635.60 Phase resolved plasma temperature Tpl determined from a combination of

absorption- and emission-spectroscopy in NTD-lamps . . . . . . . . . . . . . 164

A.1 Relative efficiency curve of the PCO SensiCam for 2D electrode temperatureinvestigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 1

List of Figures xv

A.2 Relative efficiency curve of the PCO SensiCam qe for spectroscopic investigationsA - 2A.3 Relative filter transmission of the IR interference filter . . . . . . . . . . . . A - 3A.4 Relative filter transmissions of the Ba and Dy interference filter . . . . . . . A - 3A.5 Relative filter transmissions of the Tl and Ce interference filter . . . . . . . . A - 4A.6 Relative spectral radiation curve of the high power LED used as backlight

source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 5

xvi List of Figures

Abstract xvii

Abstract

Nowadays, high-intensity discharge (HID) lamps are used in a variety of high power light-ing applications like car headlights, street lighting or video projectors. The lifetime of thesehighly efficient HID-lamps, generating their light output in a high pressure arc plasma, can bedistinctly increased by means of the so-called “emitter-effect”. The emitter-effect is inducedon the tungsten HID-lamp electrode surface by certain rare earth emitter materials (e.g. Th,Ba, Dy or Ce) introduced to a certain amount into the lamp gas-phase during operation.The effective work function of the HID-lamp electrode, determining the energy demand foran electron leaving or entering the solid material, can be reduced by a monolayer of theseemitter materials on the tungsten surface. In the consequence, the HID-lamp electrode withan active emitter-effect stays significantly cooler for a constant lamp power operation, whichcan extend the lamps lifetime more than a factor of ten.Within the scope of this PhD work, two new measuring methods and -setups are developedand optimised to determine on the one hand the temperature of an operating lamp electrodeand on the other hand the correlated particle density of a certain emitter material adjacentto the electrode. For a phase resolved investigation of the electrode temperature distribu-tion, a 2D infrared photography setup consisting of a scientific CCD zoom camera and anoptical interference filter is established. This absolutely calibrated measurement provides di-rectly a two dimensional picture of the electrode temperature distribution while evaluatingthe Planck radiation law corrected with the tungsten emissivity ε(λ, T ). To measure emit-ter particle densities with a high accuracy, a broadband absorption spectroscopy method(BBAS) by means of a powerful UHP (ultra-high pressure) lamp is developed. The mainadvantage of this BBAS method is that the absolute emitter density can be evaluated froma spectroscopically measured, relative luminosity ratio. In the consequence, the developedBBAS measurement does not need an absolute calibration and is independent of optical dis-turbances within the measuring path like an unknown transmission of the HID-lamp burnermaterial.Finally, the two measuring methods are applied to several different HID-lamp systems withinthis PhD work to investigate fundamental correlations and optimal conditions for the emitter-effect. It is found that independently of its storage concept, the emitter material is trans-ported mainly by an ion flux towards the cathodic lamp electrode. It is approved by means ofan additional simulation that the work function of a pure tungsten cathode (ϕ = 4.55 eV) isreduced to ϕ = 3 eV by thorium in a research model lamp. For a higher operating frequency,even an anodic emitter-effect can be verified in closed, commercial like HID-lamps. Withina high-pressure sodium (HPS) lamp, the surface diffusion process of the emitter materialBa along the electrode is investigated by the BBAS measurement. The obtained results areused as input parameters for a theoretical simulation of the lamp behaviour from our projectpartner Philips Lighting, NL. Additionally, the emitter-effect of dysprosium is approved in

xviii Abstract

research YAG-lamps (yttrium-aluminium-garnet). The burner vessel of these lamps is formedby YAG, a transparent material which allows optical observations. Furthermore, it is shownby systematic measurements that the Dy emitter-effect is distinctly reduced by an additionof thallium to the lamp filling whereas an addition of sodium has not significant influence.In the end, the application possibilities and limitations of the developed BBAS density mea-surement concerning non-transparent, commercially used ceramic HID-lamps are discussedand demonstrated. Moreover, a combination of emission- and absorption spectroscopy resultsis presented leading to absolute plasma temperature Tpl results independently of a formerlyused Hg lamp-filling with a high accuracy. The developed, alternative Tpl measurement mightbecome an important instrument in HID-lamp research with regard to the actual technologydevelopment towards Hg-free HID-lamps.

1

1. Introduction

Artificial lighting has become so self-evident within our modern, comfortable life that oftenpeople in developed countries do not recognise its importance anymore. However, our lifewould be totally different if there were no electrical lighting solutions installed in publicareas and in private houses. The possibility to work or to make other activities at any timeindependently of the sunlight hours cannot be taken for granted and was gained long timeago by the invention of artificial lighting at first by candles and later by electricity.Within the actual political framework of environmental problems and worldwide energy sav-ing efforts, the electrical lighting technology plays again a very important role. Just recently,the European Union introduced a new directive which should avoid step by step the use ofinefficient incandescent light bulbs to reduce the electrical energy consumption in Europe[1]. As it is shown in figure 1.1, 19 % of the worldwide electrical energy consumption is used

lighting19%

mechanicalactuators

20%

heating/ cooling16%

information technology14%

others31% 1% traffic lighting

8% street lighting

18% industrial lighting

43% commercial lighting

30% residential lighting

Figure 1.1: Worldwide electricity consumption by application (in 2005 [2]) and specification ofthe lighting fraction by application sectors(in 2007 [3])

for lighting applications. Considering that roughly 40 % of the overall energy is supplied byelectricity, lighting accounts for a fraction of 7.6 % of the total worldwide energy consumption[2, 3]. Thus it is obvious that actual and future research and development within the lightingtechnology is important and has a huge potential for energy- and environmental saving byan improvement of the lighting efficacy.

2 1. Introduction

1.1 Technology of artificial light sources - a short overviewof a long-time development

There is some historical evidence that artificial light sources by oil lamps were already in-vented more than 10.000 years ago. Roughly 8.000 years later, the invention of candles madethe burning light sources more handy but it still had many disadvantages like the low ef-ficiency and the risk of an open fire. It was Thomas Alva Edison, who revolutionised thelighting technology in 1879 by his invention of the first stable electrical light bulb [4]. Today,130 years later, the principle concept of Edison’s invention is still in use within incandescentlamps.

The light generation mechanism in incandescent lamps consists of a tungsten wire whichis heated by an electrical current and glows within a vacuum or noble gas atmosphere.This glowing tungsten wire emits radiation as described by the fundamental Planck’s lawlike any solid material with a certain temperature. Unfortunately, the radiation of tungstenand other comparable materials have their maximum radiative power in the infrared spec-tral range for temperatures below their melting point which is invisible for the human eye.Therefore, the incandescent lamp is very inefficient for lighting applications with an efficiencyof roughly 3− 4 % visible light output power referring to its electrical energy consumption.The introduction of halogen lamps as an alternative to incandescent lamps allows slightlyhigher temperatures of the tungsten wire by a back diffusion process of the material. Butwith its efficiency of 5− 7 % the halogen lamp doesn’t really solve the efficiency problems ofthe classical, so-called “thermal radiator” lamps.

A further step in lighting technology was the invention of gas-discharge lamps. The gas-discharge principle is quite different to thermal radiators as it uses a plasma to excite certaingas atoms. The excited states decay by emitting photons. The wavelength of the radiatedphotons is determined by the energy difference of the excitation levels and thereby a char-acteristic property of a certain chemical element. Accordingly, the gas-discharge lamp canbe filled during its production with specific elements which emit a defined optical spectrum.This optical spectrum is optimised to have its maximum power in the visible spectral rangewhich enhances the lamp efficacy significantly compared to incandescent lamps. On the otherhand, the emission spectrum of the discharge lamps can be matched to the sunlight spectrumby choosing its lamp fillings. This spectral optimisation is a very important parameter tocontrol the so-called “colour-rendering index” (CRI), which describes the degree of natural-ness of a reflecting colourmap illuminated by the investigated lamp.Gas-discharge lamps can be divided into two main categories: low pressure discharge lampswith an operating gas pressure in the order of 100 Pa and high pressure discharge lamps withan operating gas pressure of usually 0.1− 10 MPa. Low pressure discharge lamps, generallyknown as fluorescent lamps or compact fluorescent lamps (CFL), are already installed withina variety of home and professional lighting applications. The fluorescent lamp typically con-sists of two electrodes at both ends of a glass tube and a low pressure noble gas filling of thetube as buffer gas. The light within this low pressure discharge is generated by an additionof mercury because it has a high vapor pressure and can easily be excited. Due to the lowoperating pressure, the gas discharge within a fluorescent lamp can be characterised as atypical low pressure plasma with a low gas temperature and accordingly a low glass tubetemperature which is one of the advantages of the fluorescent and CFL lamp. The major

1.2. High-intensity discharge lamps (HID) 3

disadvantage of this lamp type is that the excited mercury atoms emit their main radiativepower in the UV spectral range. Therefore the inner glass tube of a fluorescent lamp is alwayscovered by a phosphor layer which converts the UV radiation into visible light. This spectrallight conversion leads to a reduced efficacy of fluorescent and CFL lamps.High pressure gas-discharge lamps, also called “high-intensity discharge lamps” (HID), arealso filled with noble gases and a mercury addition but the plasma discharge is much moredense due to the high operating pressure. This high particle and energy density in high pres-sure discharge lamps can be used to produce the light emission in a small discharge volumeand directly within the visible spectral range by additions like Na or Tl. Hence the HID-lamphas a very high efficacy, even higher than fluorescent lamps. However, HID-lamps need com-plex electronic ballasts, have a relatively long run-up time and cannot be reignited instantly.Moreover, the thermal load and with it the temperature of the lamp tube is extremely high(Tglass > 1000 K). It makes their application often uncomfortable, especially for lighting inprivate houses. As this work deals exclusively with the high pressure gas-discharge lamp, itsadvantages and problems will be further discussed in the following chapters.

Just a few years ago, the solid state lighting by means of high power “light emitting diodes”(LEDs) emerged to a remarkable importance in actual lighting applications and aims fora substitution of incandescent and gas-discharge lamps in the near future. Within an LEDthe emitted light is produced by electrons crossing the p-n junction of a so-called “III-V-semiconductor”. The current through the LED is transported by electrons which drop atthe junction from the conduction band of the n-doped semiconductor with a high energylevel to the valence band of the p-semiconductor with a lower energy level and emit thegained energy as photons. Hence, the spectral light output of an LED can be changed andoptimised by the sort of semiconductor materials and their energy distances comparable tothe excitation levels of atoms in gas-discharge lamps. An LED can be optimised to a veryhigh efficacy but the production of white light LEDs with an acceptable similarity to thesunlight spectrum and a good CRI is still a challenging task for solid state research. On theother hand, the LED lighting principle needs a certain cooling of the semiconductor becauseit cannot sustain temperatures above 350−400 K. Moreover the efficiency of LEDs decreaseswith increasing temperature. As a consequence of this thermal problem, single high-powerLEDs are still not available above input powers of 5− 10 W.However, if at all it will last several years or decades until LED technology can really sub-stitute typical high power applications of HID-lamps like stadium lighting, street lightingor video projections. Hence, for optimisation of high-intensity discharge lamps an intensiveresearch into fundamentals and physical aspects of HID-lamps, as presented within this PhDwork, is still of high interest.

1.2 High-intensity discharge lamps (HID)

High pressure gas-discharge lamps are used in a variety of mainly professional lighting appli-cations with input powers of 30 W up to a few kW like for example in video projectors, carheadlights, street lighting, horticulture lighting and stadium lighting. They usually consist oftwo tungsten electrodes which are sealed gas-tight into a round, bulgy or cylindrical shaped

4 1. Introduction

ceramic discharge vessel forming a closed discharge volume. The discharge vessel, also calledburner, is filled with a noble buffer gas like argon or xenon and further additions like mercuryand light producing elements (e.g. Na, Tl). To achieve a sufficiently high vapour pressure,theses elements are mostly added as iodides. Figure 1.2 gives an example of a widely used

Figure 1.2: Example for an automotive HID D2-lamp (left) and corresponding schematic sketch(right)

HID-lamp for automotive headlights and a sketch of the principle design of the burner.By applying a high voltage peak for the lamp ignition, an electric field is formed between thetwo lamp electrodes. This electric field accelerates some statistically existing free electronswhich have a very low mass and gain thereby energy. After moving a certain free path throughthe electric field the electron encounters heavy particles and performs so-called “elastic” and“inelastic collisions”. The length of this free path is inversely proportional to the gas pres-sure. By inelastic collisions part of the electron energy is transferred to the heavy particles.This effect leads to several processes like excitation and ionisation of atoms or dissociationof molecules. Shortly after lamp ignition, the heating of electrons by the electric field andtheir collisions with and energy transfer to heavy particles reaches some equilibrium. Theaccumulation of energetic electrons, excited and ionised atoms and dissociated molecules iscalled a plasma. In a high pressure plasma as it is realised in HID-lamps, the density of parti-cles is so high that the free path of accelerated electrons is very short and collisions betweenelectrons and atoms, ions or molecules are very likely and happen often. As a consequenceof this high pressure, the supplied energy by the electric field is distributed nearly equally toall species within the plasma. All particles in the plasma of an HID-lamp can be assumed tohave the same temperature, associated to their energy, which is called “local thermodynamicequilibrium” (LTE) [5, 6]. The temperature of a high pressure HID plasma is usually in therange of 1400 K to 8000 K.The high particle density combined with a high particle temperature in an HID-lamp leadsto a high degree of excitation of atoms and ions. Due to the assumption of LTE in theHID-lamp plasma, the population density of excited states for a certain atom or ion speciescan be theoretically described by the so-called “Boltzmann distribution”. Electrons withinthe atoms are lifted to a higher energy level Eu during the excitation process which leads toan unstable energy configuration of the atom. A short time after the excitation, determined

1.3. State of the research into HID-lamps 5

by the transition probability, the excited electron within the atom drops down to a lowerenergy level El by emitting the gained energy difference ∆E = Eu − El as a photon withthe correlated wavelength λ = h · c0

∆E. The radiation in a high pressure gas-discharge lamp is

generated by optical transitions from excited to lower excited states as well as from excitedstates to the groundstate. Thus, an HID-lamp has the main advantage that it can emit ahuge number of spectral lines directly in the visible spectral range leading to a high colour-rendering index whereas a low pressure lamp mostly needs phosphors and often has a poorCRI. The use of high pressure gas-discharge lamps leads to even higher lighting efficaciesthan fluorescent lamps have but HID-lamps always need igniters to produce a high voltagepeak and complex electronic ballasts as they have a negative impedance characteristic.

1.3 State of the research into HID-lamps

An HID-lamp, as described in the previous chapter, seems to be a very simple physicalsystem from a first point of view. However, the high pressure arc plasma during operationand its interaction with the tungsten electrodes is a very complex physical phenomenon andhas a high potential for scientific research. The tungsten electrodes are the most importantcomponents of the whole high pressure lamp and are usually within the main focus of actualHID investigations. The electrodes determine key parameters of the lamps like efficiencyand lifetime and have to be designed and operated very carefully. Thus, to analyse andoptimise the behaviour of HID-lamp electrodes, an intensive knowledge of the arc plasmaand its attachment to the electrodes within the so-called“plasma boundary layers”have to beachieved by fundamental and specific research. Luhmann and Dabringhausen already gave awide overview of several important scientific approaches and publications within the field ofhigh pressure arc discharges in [7] and [8]. Thus, this work is limited to the recapitulation ofthe main research results which were found by the HID research group in Bochum and formthe scientific background of this PhD work.

1.3.1 HID research at the Bochum University - The model lamp

Within the HID-lamp research group at the institute “Allgemeine Elektrotechnik und Plas-matechnik” (AEPT) of the Ruhr-University Bochum phenomena and physical aspects of thetungsten electrodes, the high pressure arc plasma and their interaction have been studiedsince a couple of years. Right at the beginning it was realised that commercially producedHID-lamps cannot be changed in any geometrical or physical parameter and do not offerenough possibilities for plasma investigations as they always have a closed volume. To over-come these problems in fundamental HID research, the so-called “Bochum model lamp” wasdeveloped by Luhmann, Nandelstadt and Dabringhausen and became very famous in thescientific community of high pressure arc discharges [7, 9]. The Bochum model lamp is anopen, flexible HID-lamp system which offers the unique possibility to change parameters likeelectrode length and -diameter, electrode material, electrode distance, electrode position,gas type and gas pressure very easily, some even during lamp operation. Thus, the Bochummodel lamp allows the measurement of several fundamental arc plasma parameters whichare not accessible in standard HID-lamps. The concept of the Bochum model lamp will be

6 1. Introduction

described in detail in chapter 3.1.1 whereas the corresponding main research results will besummarised in the following part.

1.3.2 The work of Dabringhausen, Lichtenberg, Redwitz et al

At the beginning of the HID model lamp research in Bochum, all investigations were per-formed on DC arc discharges to characterise the fundamental parameters of the anode andcathode and their plasma boundary layers.Nandelstadt started with the investigation of the cathodic power balance. He validated andspecified some existing research approaches and worked out a mathematical correlation be-tween the cathode fall voltage along the cathodic boundary layer Uc and the electrode powerloss Pl:

Uc =Pl

Iarc

+ ϕ+5

2

kBTe

e(1.1)

with the arc current Iarc, the work function ϕ of the tungsten cathode, the electron tempera-ture Te, the Boltzmann-constant kB and the elementary charge e. Nandelstadt proved thisrelation by model lamp experiments [9]. Afterwards, Nandelstadt investigated systematicallythe influences of geometrical electrode parameters like electrode length and diameter on thecathode temperature, its power balance and cathode fall voltage [10]. He also showed that adoping of the tungsten electrode material with ThO2 results in a distinctly lower electrodetemperature and investigated different types of arc attachment according to the current den-sity and electrode cooling. Nandelstadt found two main types of arc attachment in the modellamp: A diffuse one with a high global cathode temperature and an almost homogenous cur-rent density on the electrode end face and a constricted one, a so-called “spot-mode” with alower global but high spot cathode temperature directly at the arc attachment. An examplefor both types of arc attachment within the Bochum model lamp is given in image 1.3.

Luhmann also worked on the cathodic power balance of DC arc discharges [7]. But insteadof using the model described in equation 1.1, which had some uncertain assumptions, heused the flexibility of the Bochum model lamp to perform Langmuir-probe measurements ofthe arc voltage and Uc directly within the arc discharge [11]. By this technique, Luhmannwas able to approve the correctness of the model used by Nandelstadt (cf. equation 1.1) andhe could also reproduce all important trends found by Nandelstadt like the influence of thecathode geometry on the cathode fall voltage Uc. Besides, Luhmann made first attempts todetermine the HF impedance of the cathodic and anodic boundary layer by adding a HFcurrent onto the main operating DC current.

Schmitz worked out a more specific and detailed theoretical description of the cathodicboundary layer within his PhD work [12]. For his model Schmitz divided the boundary layerof the cathode in three characteristic parts: A collisionless space charge zone directly in frontof the tungsten surface, a so-called “Knudsen-layer” ahead and an ionisation zone. The de-veloped model of Schmitz was physically consistent but it was not able to confirm or evenpredict the results of experimental investigations due to some inadequate assumptions whichwere not well understood at that time.

Later, within the framework of a second BMBF (German federal ministry of education and

1.3. State of the research into HID-lamps 7

(a) Diffuse arc attachment (b) Spot-mode arc attachment

Figure 1.3: Images of the two main kinds of arc attachment on the electrodes within the Bochummodel lamp: Diffuse arc attachment with a nearly homogenous radiation (left) and the spot-mode attachment with a high arc constriction at the electrode edge (right). Parameters: electrodediameter dE = 1.5 mm, electrode length lE = 20 mm, current i = 4 A DC, gas argon at a pressureof p = 0.26 MPa

research) funded fundamental project of the Bochum HID research group, Dabringhausencontinued the model lamp investigations on the cathode by correlating experimental andtheoretical results. He also introduced some first attempts to measure and to model theanodic electrode behaviour [8]. Dabringhausen performed pyrometric electrode temperaturemeasurement along their axial direction and worked out a corresponding theoretical modelof the electrode heat flux balance through the solid material

πr2E

∂

∂z

(κ(T )

∂T

∂z

)= 2πrEεtotσSBT

4, (1.2)

where rE is the electrode radius, z is the axial position along the electrode, κ(T ) is the heatconductivity of the tungsten material, εtot is the total emissivity of tungsten and σSB is theStefan-Boltzmann constant [13]. By fitting the theoretical power balance model to theobtained experimental result, Dabringhausen was able to calculate the exact electrode tiptemperature Ttip and also the cathode power loss Pl = Prad + Pcond divided into the fractionfor the radiative power loss of the electrode rod Prad and the power Pcond, dissipated byheat conduction to the electrode holders. Furthermore, Dabringhausen developed parts of anew modeling concept for the cathodic boundary sheath which allowed him to simulate thetransition probabilities from a diffuse arc attachment to the spot arc attachment and couldbe validated by experimental data [14]. Dabringhausen also made a first attempt to modelthe anodic boundary sheath of DC electrodes. He discovered that the anode voltage fall Ua

is not directly coupled to the power flux density from the plasma arc as it was found for thecathode and that the development of an anodic model which is consistent to experimentsis much more complicated than previously predicted. Finally, Dabringhausen made the firstmeasurements for switched-dc and sinusoidal currents within the model lamp. He measuredthe so-called “electrode sheath voltage” ESV = Ua + Uc which is the sum of anodic andcathodic fall voltage and found re-ignition peaks during the current zero crossing (CZC)depending on the current amplitude and frequency.

8 1. Introduction

Parallel to the work of Dabringhausen, Lichtenberg worked besides some pyrometric andprobe measurements mainly on the theoretical description and improved accuracy of thecathodic HID boundary model [15]. Lichtenberg introduced the concept of transfer functionsfor the power flux (qp)

qp(Tc, Uc) = qrec + qkin + qneutral + qem + qep + qtc, (1.3)

with the recombination energy of ions at the electrode surface qrec, the kinetic energy ofthe ions qkin, the energy of neutrals produced by recombination of ions qneutral, the energyof thermionically emitted electrons reduced by the Schottky effect qem, the electron backdiffusion energy qep and finally the heat energy flux transported by heavy particles to thecathode qtc as well as the corresponding transfer function for the current density (j)

j(Tc, Uc) = jem + ji − jep, (1.4)

from the plasma arc to the cathode into the sheath model [16]. By means of these trans-fer functions, the behaviour of the cathode can be described with Tc and Uc, completelydecoupled from the plasma arc parameters in a so-called “black-box model”. In addition,Lichtenberg studied the transition probability and influences of the arc attachment from thediffuse attachment to the spot attachment and investigated the behaviour of the so-called“super-spot mode” experimentally [17, 18]. Lichtenberg also worked out a first simple modelfor the anodic boundary sheath and the corresponding anode fall voltage Ua and he madefirst considerations and proposals for the modeling of the anodic and cathodic electrodesheaths in the case of AC and switched-dc operation.

Besides Dabringhausen and Lichtenberg, Redwitz was the third team member working onthe characterisation of HID electrodes in the Bochum model lamp within the second BMBFproject. Redwitz performed the first spectroscopic investigations on the model lamp witha high spectral resolution [19]. He measured the electron temperature Te and the electrondensity ne spectroscopically in front of the anode and in front of the cathode and discov-ered some distinct deviations from the LTE assumption within the sheath zones. Redwitzcompared his experimental results with modeling data from Dabringhausen and Lichten-berg which showed mostly a good agreement [20]. He also worked out experimental data forsome detail improvements of the cathodic boundary model and described theoretically a firstanodic boundary model appropriate for his experimental findings [21].

1.3.3 The work of Langenscheidt

Langenscheidt was the direct successor of Dabringhausen, Lichtenberg and Redwitz, workingon some important open questions concerning the fundamental HID model lamp investiga-tions [22]. Together with Dabringhausen, Langenscheidt used the cathodic boundary layermodel of Lichtenberg to establish a 3-dimensional simulation of the electrode heat balance.With this approach, Langenscheidt was able to find simulation results for the constrictedspot mode attachment on HID electrodes by choosing inhomogenous initial conditions forthe heat distribution within the 3D-electrode. His theoretical modeling results provided exactparameters for the transition conditions from a diffuse to a spot arc attachment which couldbe confirmed in experimental model lamp investigations. Langenscheidt was also the first

1.3. State of the research into HID-lamps 9

one performing systematic measurements of the type of arc attachment and ESV behaviourin the model lamp driven with AC and switched-dc currents. He developed a high speedtriggering system [23] and analysed the model lamp electrodes with focus on their transitionfrom the anodic to the cathodic mode and vice versa during the current zero crossing (CZC)[24].Parallel to his research into the AC operation of the Bochum model lamp, Langenscheidtstarted the investigation of the so-called “gas-phase emitter-effect” in yttrium-aluminium-garnet (YAG) HID-lamps. The emitter-effect is used within HID-lamps to reduce their elec-trode temperature while keeping the other operating parameters constant. The emitter-effectwill be described in more detail in chapter 2.3.3 as it is also the main focus of this PhD-work. Based on preparatory work of Redwitz, Langenscheidt developed a new pyrometricmeasurement of the electrode temperature distribution by means of a 1D-spectrograph. Hismeasuring approach has a better time- and space-resolution than the measurement performedby Dabringhausen. For his YAG-lamp investigations, Langenscheidt combined the electrodetemperature results with absolute particle densities of the dysprosium emitter material de-termined by means of absolutely calibrated emission spectroscopy. Langenscheidt analysedsystematically the influence of the emitter material on the electrode temperature for severallamp operating conditions and found optimal operating points but also disturbances on theemitter-effect by other YAG-lamp ingredients like sodium and thallium [25, 26, 27].

1.3.4 The work of Scharf

Parallel to the mostly experimental work of Langenscheidt, Scharf developed a new theo-retical model of the boundary layers of HID electrodes at the “Institut fuer TheoretischeElektrotechnik” (TET) at the Ruhr-University Bochum [28]. For his HID sheath modeling,Scharf analysed and improved several existing fluid models for the cathodic sheath in detail[29, 30] and finally used a new kinetic approach for his model. Scharf solved the achievedmodel equations by use of the so-called “Laguerre polynomials” and “Hermite polynomi-als”. In the second part of his PhD work, Scharf enhanced the cathodic boundary model ofLichtenberg by re-calculating the transfer functions under different conditions and therebyestablished and proved the theoretical prediction of mode changes of HID arc attachmentstogether with Langenscheidt.

1.3.5 The work of Reinelt

In his PhD work, Reinelt investigated advantages and disadvantages of the high frequency(HF) operation of HID-lamps in the Bochum model lamp and in YAG-lamps [31]. Reineltwas able to operate the Bochum model lamp at high frequencies up to f = 2 MHz as themodel lamp has a very large gas volume which does not lead to problems with acousticresonances like in standard HID-lamps. Reinelt measured the electrode temperature distri-bution spectroscopically and investigated the type of arc attachment by photography in themodel lamp [32]. He extended the cathodic boundary model for DC currents of Nandelstadtand Lichtenberg to a time dependent model by adding a term for the heat capacity of the

10 1. Introduction

electrode (cf. equation 1.2):

πr2E

∂

∂z

(κ(T )

∂T

∂z

)= 2πrEεtotσSBT

4 + πr2Eρmcp(T )ω

∂T

∂ϕ. (1.5)

Here, ρm is the mass density of the tungsten material and cp is the tungsten heat capacity.Reinelt developed a different approach to determine the electrode input power comparedto Dabringhausen by integrating the heat conductivity equation and fitting numerically theaxial position z to the measuring results. By this procedure, Reinelt was able to calculate theelectrode input power Pl and the electrode tip temperature Ttip with respect to the dynamicbehaviour of the electrode during AC operation. Reinelt found that for low operating current,the electrode is cooled in the anodic phase and heated in the cathodic phase represented bychanges of Ttip and for high operating currents it is vice versa. But for higher frequenciesabove roughly f = 2 kHz, the electrode tip temperature Ttip reaches a constant value andis not modulated anymore in the anodic and cathodic phase due to the heat capacity ofthe electrode. Reinelt showed that for high frequency operation the electrode behaviour isdominated by the cathode and anodic effects are only of minor importance [33]. With hisapproach of the time dependent power balance, Reinelt also determined the time dependentcathodic fall voltage uc and by additional ESV measurements the anodic fall voltage ua, too.He found evidence for a heating of the electrode from the plasma during CZC, when theelectrical input power is zero, by thermionically emitted electrons re-entering the electrode.Reinelts investigations showed that all phase modulation effects of the electrode temperatureand input power vanish above an operating frequency of f = 1−2 kHz. Accordingly, a phaseresolved investigation is not very significant for higher frequencies.Within the YAG-lamps, Reinelt investigated the effects of a high frequency operating cur-rent on the gas-phase emitter-effect. He determined the electrode tip temperature and thedensity of the dysprosium emitter close to the electrode by spectroscopy as it was alreadydone by Langenscheidt. Besides the known electrode temperature reduction by the emitter-effect, Reinelt found a decreasing electrode temperature with increasing operating frequencywhich was not observed in the model lamp without emitter-effect. These results led Reineltto propagate the existence of an anodic emitter-effect for higher frequencies. A decreasingelectrode temperature for an increasing frequency can only be explained by a monolayerof emitter material on the electrode surface which survives the anodic phase because it isnot desorbed anymore during short periodic times. This surviving dysprosium monolayerproduces obviously an emitter-effect also within the anodic phase. As this aspect is a veryimportant starting point for parts of this PhD work, it will be investigated in more detail inchapter 5.1.2.

1.4 Thesis content

1.4.1 Aim of this PhD work - pyrometry and absorption spectroscopy

This PhD work deals with the investigation of the gas-phase emitter-effect in different HID-lamps. As already mentioned, the emitter-effect is used in HID-lamps to reduce their elec-trode temperature while keeping the lamp current and lamp power input quasi constant. Theemitter-effect is dependent on different lamp parameters which can be optimised, its physical

1.4. Thesis content 11

background will be discussed in detail in chapter 2.3.3. The main use of the emitter-effect isto distinctly extend the HID-lamp lifetime.During operation, the hot HID electrodes loose part of their tungsten material by evapora-tion. This evaporated tungsten condenses at the coldest lamp parts, the inner glass wall, andreduces the transmission of the glass burner. As a consequence, the produced light inside thelamp cannot be radiated completely by the lamp as it is partly absorbed at the glass wall.This effect leads to a lower light emission of the lamp and to a further increase of the wholeHID-lamp temperature which increases again the tungsten evaporation of the electrodes. Thedescribed lamp ageing process is self-amplifying and ends in an overheating and destructionof the HID-lamp. Accordingly, it is of major interest for the lamp producer to investigateand understand the emitter-effect which reduces the HID electrode temperature and therebyslows down the lamp ageing.

To investigate the gas-phase emitter-effect in detail, mainly two measurements are needed:The distribution of the electrode temperature, especially its tip temperature Ttip, has to bemeasured by pyrometry and the density of emitter atoms in front of the electrode by spec-troscopy.Langenscheidt and Reinelt already measured the axial electrode temperature distribution by1D emission spectroscopy in the infrared (IR) wavelength region in [22, 31]. By means ofabsolutely calibrated measurements combined with the Planck-radiation function for solidstate radiators they were able to determine the absolute temperature of electrodes in HID-lamps. However, this one-dimensional spectroscopic measurement is quite complex leadingto a relatively high error and it does not give any information about the type of arc attach-ment which is often needed for interpretation purposes. To overcome these disadvantages,a new temperature measurement is developed within this PhD work by means of a nar-row IR-filter on an absolutely calibrated 2D photography camera. The results give a directtwo-dimensional temperature image of the whole HID-electrode which provides informationabout the type of arc attachment and can be used to extract an averaged one-dimensionaltemperature distribution in an arbitrary direction.The density of dysprosium emitter atoms and ions was also measured by Langenscheidt andReinelt in ceramic YAG-lamps. They used an absolutely calibrated emission spectroscopymeasurement to determine the radially resolved dysprosium densities in front of the elec-trode. Besides some numerical problems due to the dependence of the results on two inverseAbel transformations (cf. chapter 4.2.1), the determination of particle densities by emissionspectroscopy has three major disadvantages: On the one hand, an emission measurement inHID-lamps is naturally only applicable at positions where light radiation is produced. On theother hand, the determination of groundstate densities in plasmas by emission spectroscopyalways requires a known plasma temperature. A third disadvantage is the transmission ofthe YAG-lamp material. The YAG transmission can be considered during calibration but itis changed by condensed tungsten material and lamp salt particles during operation. Accord-ingly, the changes of the YAG transmission lead to a systematic measuring error and reducethe accuracy of the results. To solve these emission spectroscopy problems, a new absorp-tion spectroscopy measurement for particle densities is developed and applied to HID-lampswithin this PhD work.

Optical absorption spectroscopy is already widely used to measure molecule- and atom-densities within plasmas and vapours. Optical absorption measurements of high accuracywere developed since monochromatic laser systems are available [34, 35, 36, 37]. Actually,

12 1. Introduction

laser absorption systems are applied within a variety of diagnostic setups for flexible andnon-intrusive particle density and temperature measurements [38, 39, 40]. Nevertheless, laserabsorption spectroscopy (LAS) setups are usually very complex and therefore rather expen-sive. A tuneable laser system is needed to scan the spectral profile of an absorption line.Thus, a laser absorption system is always limited to a narrow wavelength interval and can-not be used for measurements of different absorption lines.In contrast to laser absorption, the so-called “broadband absorption spectroscopy” (BBAS)uses non-monochromatic light sources like discharge lamps or light emitting diodes (LED).Since the efficacy and flexibility especially of high-pressure discharge lamps (HID) has beensignificantly improved recently, BBAS is a reasonable alternative to LAS in many opticalmeasuring systems [41, 42, 43, 44, 45, 46]. Broadband absorption setups are very often lesscomplex and allow a direct and simple measurement of the whole absorption profile by aspectroscopic setup. Additionally, BBAS density measurements can be applied to differentatoms or molecules if a broadband light source combined with appropriate filters is chosen.To realise a high signal-to-noise ration (SNR), broadband light sources like xenon-arc-lamps[41, 42, 44, 46] or deuterium lamps [45] are used which have an optimised intensity within aspecific wavelength interval, e.g. in the UV-region. Moreover, cascaded arcs have been usedas broadband light sources for optical absorption spectroscopy [43]. However, all these high-intensity broadband light sources have problems to provide a stable emission over a longertimescale. But a stable radiance of the light source is required during the recording time forhigh accuracy absorption measurements presented in [41, 42, 44, 45, 46]. It is a prerequisite,if a lock-in technique is applied.A recent development in BBAS is the use of high-power light emitting diodes (LED) as broad-band light sources [47, 48, 49]. LEDs can be optimised for a defined wavelength interval, theyare available with a power of some watts and they have a much higher temporal stability ofemission than Xe- or deuterium-lamps. Thus, the application of high-power LEDs in broad-band absorption setups can distinctly improve the measuring accuracy. However, up to nowhigh-power LEDs are not available for all required wavelength intervals, especially around550 nm. Additionally, the LED radiant power is limited until now to a few watts, which leadsto a spectroscopic measuring time of at least a few ten milliseconds [47]. Consequently itdoes not allow a good phase-resolution if lamps are operated with higher frequencies.

As it will be shown in more detail later, the absorption measurement within this PhD work isrealised by mean of a broadband ultra-high-pressure (UHP) background light. This solutionis less complex, more flexible and less expensive than comparable measurements with tune-able laser absorption setups. It will be shown in this work that the BBAS measurement canbe applied at an arbitrary position within the HID-lamp, that it is independent of the YAGtransmission and that it does not even need an absolute calibration. The groundstate atomdensities can be measured directly without any information of the plasma temperature andhave a good accuracy. The BBAS measurement will be discussed, implemented and testedin detail within this PhD work and finally it will be used to investigate the emitter-effect ofbarium and dysprosium in different HID-lamps.

1.4. Thesis content 13

1.4.2 Thesis structure

After this introduction, chapter 2 deals with the basic aspects of electrical gas discharges andwith fundamental plasma parameters and properties. Chapter 2 focuses mainly on the exci-tation of atoms and ions by collisions and their de-excitation by light emission. The chapteralso discusses the plasma-electrode interactions and their associated theoretical models andit gives an insight into the physical principles of the emitter-effect.The following chapter 3 shows the experimental setup and the various measuring meth-ods used within this work. Three kinds of HID-lamps are introduced and described in thatchapter: The Bochum model lamp, high-pressure sodium (HPS) lamps with barium emittermaterial and ceramic YAG-lamps with Dy emitters.In chapter 4, details about the measuring methods and the evaluation of quantitative resultsin physical units are given. The 4th chapter is focused on the electrode temperature mea-surement by 2D pyrometry and on the determination of particle densities by emission andabsorption spectroscopy.Finally, in chapter 5, the measuring results and interpretations concerning the investigationof the gas-phase emitter-effect are presented. Chapter 5 is divided into three main parts:The first part deals with fundamental investigations of the emitter-effect in the Bochummodel lamp, the second part shows results for the Ba emitter diffusion and its impact onthe electrodes in sapphire HPS-lamps and the last part includes a systematic analysis of thegas-phase emitter-effect of Dy in ceramic YAG-lamps.This PhD thesis ends in a summary of the whole work in chapter 6 and gives a concludingoutlook for possible future absorption measuring methods and investigations of the emitter-effect.

14 1. Introduction

15

2. Fundamentals

The gas discharge within an HID-lamp represents a sort of the general concept of technicalplasmas, it is a high pressure arc plasma. In this chapter, a classification of HID arc plasmasand their fundamental physical aspects will be given. The first part of this chapter focusesmainly on the energy transfer and the production of light radiation in plasmas as well astheir theoretical description by physical models and distribution functions. The second partof this chapter deals with the interaction of the plasma arc with the HID electrodes, thecorresponding theoretical models and the emitter-effect.

2.1 Arc discharges

From a general point of view, gases are isolators and do not conduct an electric current intheir natural state. But if a high voltage is applied to the gas e.g. by electrodes, the resultingelectric field strength accelerates charge carriers which produce more charge carriers (elec-trons and ions) by collisions. This acceleration and production of charge carriers leads to acertain current flow through the gas, a gas discharge, so-called “plasma”, is produced.According to the transported current density and the voltage along the plasma discharge,

several categories of gas discharges can be distinguished. Figure 2.1 gives a condensedoverview of the different kinds of gas discharges arranged in a characteristic U/I-diagram.The absolute current and voltage values in this figure are given for a simple discharge tubefilled with neon at a pressure of p = 133 Pa, a discharge length of 50 cm and an electrodeend face area of 10 cm2. The different discharges will not be discussed in detail as this workfocuses exclusively on arc discharges characterised by a high discharge current combined witha relatively low discharge voltage but negative impedance characteristic (cf. figure 2.1).As the HID-lamps are usually driven with a constant current or power control, the corre-lating potential distribution along the arc axis is a very important physical parameter. Atypical potential along the arc axis between an anodic and a cathodic electrode is plotted infigure 2.2. Especially the potential drops in front of the electrodes reflect important physi-cal details about the anodic and cathodic plasma boundary layers. As shown in figure 2.2,the arc potential distribution can be divided into three general sections: The cathodic fallregion with a high potential drop over a small region of a few µm, the arc column with anapproximately linear potential distribution over a length of several cm and the anodic fallregion with a dimension of some mm and a potential drop which is reversed immediately infront of the anode surface.The cathodic fall Uc is influenced by space charges which occur due to heavy ions accel-

erated by the electric field to the cathode and thermionically emitted electrons from the

16 2. Fundamentals

10-15

10-20

10-12

10-5

10-4

10-3

10-2

10-1

1 10

0

200

400

600

800

arc

transitionregion

abnormalglow

discharge

normal glow discharge

sub-normalglowdischarge

coronaTownsendand Dark,

self sustaineddischargeVB

randombursts

photo electriccurrents

increasing illuminationA

B C

D

E F

G

arc

volt

age

/V

discharge current / A

Figure 2.1: Different categories of gas discharges sorted in a characteristic U/I-diagram for adischarge with the following standard parameters: neon gas filling at p = 133 Pa, dischargelength 50 cm, electrode end face area 10 cm2

tungsten electrode. The accelerated ions transport a certain energy from the plasma to thecathode and heat it up. The hot cathode in contrary emits electrons thermionically whichionise atoms and transport the current along the arc axis. The electron and ion fluxes at thecathode establish an equilibrium between power input and output of te cathodic boundarylayer which is given by the arc current and determines the electrode temperature. Withinthe cathodic sheath region, the local thermodynamic equilibrium (LTE) is highly disturbed,large differences between electron and gas temperature, charge carrier densities and potentialdistribution occur.Within the arc column, the electric field strength Ecolumn is nearly constant, leading to alinear potential distribution (cf. figure 2.2). Accordingly, the voltage along the arc can bedescribed as

Ucolumn = larcEcolumn. (2.1)

larc is the length of the arc column which can be estimated being equal to the electrodedistance in a vertically operated HID-lamp due to the negligible dimensions of the electrodeboundary sheaths.The potential distribution in front of the anode, given in figure 2.2, is based on spectroscopic[19] and theoretical [50] investigations. An inversion of the general potential gradient isrecognised directly in front of the anode due to thermionically emitted electrons which gainenergy in the plasma sheath and diffuse back to the anode. By this effect the plasma in frontof the anode is cooled, resulting in an adjacent dark zone layer without light emission, andan additional heating power flux onto the anode is generated. The measurable integral anodefall Ua,el cannot distinguish this potential inversion and reflects only the deviation from theanode potential to the extrapolated column voltage (cf. equation 2.1).

2.2. Fundamentals of plasmas 17

Ua,el

Ucolumn

Uc

po

ten

tia

l a

lon

g d

isch

arg

e a

rcΦ

(z)

/V

position along discharge arc z

~ mm~ cm~ mμ

Cathodefall region

Anodefall region

Arc column~

~~ ~

Figure 2.2: Scheme of a typical potential distribution along the arc discharge in a standard HID-lamp. The position axis is not true to scale.

2.2 Fundamentals of plasmas

2.2.1 Basic plasma processes

As already mentioned, a plasma is a high energetic gas consisting of atoms, ions, molecules,electrons and excited atoms/ ions. The lightweight, mobile electrons are accelerated by elec-tric fields, gain energy and collide with other species within the plasma discharge. There is avariety of different possible collisions between the species in a plasma which are explained indetail in [51]. Generally, two global sorts of collisions can be distinguished: Elastic collisions,which change only the velocity and direction of the collision partners and inelastic collisionswhich change additionally their inner energy (e.g. dissociation, excitation, ionisation). Theoverall energy and momentum balance before and after a collision within a plasma is alwayskept constant.For the light generation in HID-lamps, inelastic collisions, namely the excitation of atoms Aor ions A+ by fast electrons efast are the most important collisions:

A+ efast ⇒ A∗ + e (2.2)

A+ + efast ⇒ A+∗ + e (2.3)

By this excitation collision the excited atoms A∗ or ions A+∗ gain a higher energy level Eu

compared to their original energy level El. If the upper energy level Eu is not depopulatedimmediately again during a cascaded excitation process or by the inverse process, a collisionalde-excitation, the atom or ion is usually de-excited to its original energy level El by an opticaltransition due to its unstable energetic state:

A∗ ⇔ A+ hν (2.4)

A+∗ ⇔ A+ + hν (2.5)

18 2. Fundamentals

The discrete energy difference ∆E = Eu−El = hνul is thereby radiated as a photon with thecharacteristic frequency νul. The energy of the photon is described by the so-called“Planck-constant” h or ~, respectively. Accordingly, this process produces a discrete line spectrum.The inverse process to the photon emission in equation 2.4 is described by the absorptionof photons within the plasma. When a photon with the energy hνul = ∆E = Eu − El isabsorbed, an atom or ion is excited from a lower energy level El to a higher one Eu.A further light generating process in HID-lamps is the emission of a recombination continuumaccording to

A+ + e⇔ A+ hν. (2.6)

As the emitted photon with the energy hν in equation 2.6 contains also the continuous ki-netic energy of the electron e according to its velocity distribution function, the emitted lightrepresents a continuum spectrum and not a discrete line spectrum.A third important light producing process in HID-lamps is the emission of a free-free contin-uum by deceleration of electrons influenced by the electric field of ions. This process is alsocalled “Bremsstrahlung”.

2.2.2 Basic plasma properties

A plasma is an ionised gas which contains a significant amount of ions, electrons and otherparticles, thus it is often associated to be the fourth aggregate state. From a global view faraway outside of the plasma it appears without any positive or negative charge, it is so-called“quasi-neutral”. This behaviour implies the same amount of ions and electrons inside theplasma bulk. In the general case of a multiple ionised plasma, the quasi-neutrality conditioncan be written as

ne =N∑

k=1

k · ni,k . (2.7)

ne : electron density k : degree of ionisationN : highest ionisation stage ni : ion density

For a multi component plasma composed of a gas mixture, all components have to be summedup on the right hand side of equation 2.7.The quasi neutrality can be disturbed on a very small scale, e.g. adjacent to the electrodesurface, which is described theoretically by the Debye-length:

λd =

√ε0kBTe

e2ne

(2.8)

ε0 : vacuum permittivity kB : Boltzmann constantTe : electron temperature e : elementary charge

The pressure of a plasma p is described by the sum of all partial pressures pj of the differentspecies j. The partial pressures can be described according to Daltons law:

p =m∑

j=1

pj =m∑

j=1

njkBTj (2.9)

2.2. Fundamentals of plasmas 19

j : index of certain species m : total number of speciesnj : density of the species j Tj : temperature of species j

In a plasma, the temperatures of different species Tj can differ e.g. for electrons and atomsor ions. In high pressure arc discharges, as investigated within this work, the densities ofspecies are so high that collisions between them are very likely leading to high collisionrates. This fact implies Maxwellian velocity distributions for the particles which assigntemperatures to the different species. The temperatures Tj are defined according to theMaxwell distribution:

Tj =mj〈v2

j 〉3kB

(2.10)

mj : mass of one particle of species j 〈v2j 〉 : mean square of particle velocity

While inserting the mean kinetic energy 〈Ekin〉 =mj

2〈v2

j 〉 in equation 2.11

〈Ekin〉 =3

2kBTj (2.11)

indicates that the temperature of a particle also represents its mean kinetic energy.As the atoms and ions have approximately identical masses, the energy exchange betweenthem by elastic collisions is very effective. As a consequence, a common temperature Th ofthe heavy particles is adjusted.In typical high pressure arc plasmas containing high electron densities the electrons can alsobe described by the Maxwell distribution due to the strong electron-electron interaction[5]. The electron passes a distance between two collision, its “mean free path”, which is muchsmaller than the dimensions of the plasma volume. Thus, the macroscopic field has only aminor influence on the instantaneous movement of the electrons and can be considered asa superimposed drift motion of the electrons. The electron movement is dominated by theircollisions and therefore undirected. Accordingly, a defined temperature Te can be given forthe electrons.The electrons obtain energy from the electric field and transfer the energy by collisions to theheavy particles. The energy transfer by elastic collisions is proportional to the temperaturedifference Te − Th. As a consequence, Te > Th as long as a plasma is heated electrically.

2.2.3 Thermal equilibrium

In contrast to low pressure plasmas, high pressure plasmas, as used in HID-lamps, can bedescribed by assuming a thermal equilibrium. The thermal equilibrium is a very importantphysical characteristic of high pressure plasmas allowing theoretical modeling and spectro-scopic density measurements. It has to be distinguished between a total thermal equilibrium,a local thermal equilibrium (LTE) and a partial local thermal equilibrium (pLTE). A detaileddescription of the different stages of thermal equilibrium is given in [19] and [52] whereasthis work only concludes the main aspects in the following section.

20 2. Fundamentals

Total thermal equilibrium

The assumption of a total thermal equilibrium implies some strict conditions which are usu-ally not fulfilled in technical plasmas. For a total thermal equilibrium every plasma processmust run as often as its correlated reversal process (so-called “detailed equilibrium”). Addi-tionally, all velocities of the included plasma particles have to be distributed according toMaxwell and every species has to have the same temperature. As a last precondition forthe total thermal equilibrium, all temperature and density distributions have to be indepen-dent of space and time within the plasma volume.If all these conditions for a total thermal equilibrium are met, the distributions of all energeticstates are given by the Boltzmann distribution:

nr

n0

=gr

g0

exp

(− Er

kBT

)(2.12)

nr : density of the excited atoms Er : energy of the excited atomn0 : density of the groundstate atoms T : absolute temperaturegr : stat. weight of the excited state g0 : stat. weight of the groundstate

The thermal equilibrium also includes a balance between ionisation and recombination ofions. Thus, the ratio between the density of the atoms and ions can be described by theSaha-Eggert equation [5]:

neni

na

=2ZeZi

Za

exp

(− Ei

kBT

)(2.13)

ne, ni, na : density of electrons, ions and atomsZe, Zi, Za : partition function of electrons, ions and atomsEi : ionisation energy of the neutral gas

with the partition functions defined as the product of the statistical weight and the Boltz-mann factor:

Za,i =∑

j

gj exp

(− Ej

kBT

)Ze =

(2πmekBT

h2

)3/2

(2.14)

The partition functions sum up all possible energetic states and probabilities of their popu-lation in dependence on the plasma temperature T [53].

Local thermal equilibrium (LTE)

As already mentioned above, a total thermal equilibrium can not be realised in technicalplasmas. Due to the spatial variation of the density and temperature of species within tech-nical plasmas, irreversible processes occur as heat conduction and diffusion. The concept ofthermal equilibrium can only be applied to infinitesimal small local volumes with some con-straints. Hence, the densities and temperatures have to be treated in dependence on space forthe concept of a local thermal equilibrium (LTE). All other assumptions of the total thermalequilibrium are assumed to be almost valid in an LTE plasma, especially the Boltzmanndistribution and the Saha equation keep a validity.

2.2. Fundamentals of plasmas 21

Partial local thermal equilibrium (pLTE)

The assumption of a partial local thermal equilibrium (pLTE) is often useful to describe realplasmas. In contrast to the LTE assumption, a pLTE plasma has defined deviations fromthe thermal equilibrium. Especially the energy distribution of the excited energy levels aredecoupled from the ground state density. Hence, if the pLTE plasma shows strong deviationsfrom the LTE assumption, only atoms with excitation energies in the vicinity of the ionisationlimit can be described by the Boltzmann distribution. Due to this decoupling of energystates, the electron temperature Te within a pLTE plasma differs significantly from theheavy particle temperature Th. Plasmas which have a massive deviation from LTE can onlybe analysed by collision-radiative models.High pressure arc plasmas usually show strong deviations from the LTE assumptions withintheir boundary layers, especially in front of the cathodic electrode. The main deviations areTe 6= Th, lower charged particle densities in comparison to Saha and a significant disturbanceof the quasi-neutrality. For these conditions in HID-lamps, a modified Saha equation can beapplied according to [6].

2.2.4 Radiative processes in plasmas

As an arc plasma inside an HID-lamp is optimised for light emitting purposes, the radiationof photons is naturally the most important process of a lighting plasma. There are severalradiative processes inside a plasma which account more or less for the overall lighting efficacyof an HID-lamp. These processes will be briefly introduced in the following part.

Continuous spectral radiation

There are two main mechanisms radiating light over a continuous wavelength range: Recom-bination radiation of electrons with ions and Bremsstrahlung of electrons.The recombination radiation is produced when an ion recombines with a free electron toan uncharged atom. The constant ionisation energy of the ion and additionally the kineticenergy of the free electron are gained during this process and radiated in one photon withthe total energy Ephoton = Eionisation − Eexcitation + Ekin,electron = hν. As the energy fractionof the electron is distributed continuously according to Maxwell, the radiated photons arespread over a wide wavelength range but with a minimum threshold energy correspondingto the ionisation energy.Bremsstrahlung is generated due to an acceleration and deceleration of free electrons bythe electric field of an adjacent ion. These free-free transition processes also radiate pho-tons continuously over a broader wavelength range but without any threshold. For the sumof all continuous emission of a plasma, an emission coefficient ελ,cont(λ) can be defined independence on the wavelength λ.

22 2. Fundamentals

Spectral line emission

The most important light producing effect in HID-lamps is the line emission of excited atomsor ions. If an atom in the discrete excited energy state Eu decays to a lower, also discreteenergy level El, a corresponding photon is radiated with the discrete energy Eul = hνul.Thus, every atom or ion possesses a discrete, characteristic spectral line distribution withthe corresponding wavelengths

c0 = λul · νul (2.15)

wherein c0 is the velocity of light in vacuum.The line emission of an atom by a certain energy transition can be described theoreticallyby an emission coefficient ε [5]:

ελ,ul(λ) =1

4πAulh

c0

λul

nuPul(λ) (2.16)

Aul : transition probability from the upper energy level u to the lower energy level lnu : density of the upper energy level

Pul(λ) is the profile function of the emission line. The profile function describes the specificbroadening of the spectral line in dependence on wavelength. There are several mechanismswhich lead to a spectral broadening of a measured spectral line which should be infinitesimalnarrow from a theoretical point of view. The most important broadening mechanisms are:

1. Apparatus profile: Every spectrograph performs a specific line broadening due to itsmeasuring principle, e.g. the limited resolution of the diffraction grating

2. Doppler effect: Line broadening caused by the thermal movement of the emittingparticle

3. Pressure broadening: Collisions of the light emitting excited particles with neutralslead to a line broadening due to detuning of the atomic oscillators

4. Stark-broadening: The emitted spectral lines are split by the microscopic field ofcharged particles within the plasma. [54] The Stark-broadening is the dominant mech-anism in high density plasmas like in HID-lamps.

The profile function Pul(λ) in equation 2.16 is normalised to∫ ∞0

Pul(λ) = 1. (2.17)

Thus, for measuring purposes the spectral emission coefficient can be integrated without aconcrete information of the relevant broadening mechanisms. The resulting emission of aspectral line becomes thereby independent of the line profile:

εul =1

4πAulh

c0

λul

nu (2.18)

This line emission coefficient will be used within this work to calculate particle densities inplasmas by means of absolutely calibrated measurements of emitted spectral lines.

2.2. Fundamentals of plasmas 23

Spectral line absorption

Besides the emission of spectral lines, a high pressure plasma in HID-lamps can also absorbspectral lines when photons pass through the plasma volume. In contrast to a collisionalexcitation, a photon with the energy hν = Eul is used to excite an atom or ion from alower energy level El to a higher energy level Eu. The absorbed photon is thereby consumedwithin the plasma volume and cannot contribute to the total light emission of the HID-lampanymore.According to the definition of a line emission coefficient, also a line absorption coefficientcan be given:

αlu = nle2

4πε0

· π

mec0

· flu (2.19)

nl : absolute density of the lower energy level e : elementary chargeε0 : vacuum permittivity me : electron massflu : absorption oscillator strength

This line absorption coefficient αlu can also be treated independent of the profile function bya previous spectral integration (cf. equation 2.17). The absorption oscillator strength flu is acharacteristic constant for the specific optical transition from the energy level l to the upperlever u and can be obtained like the transition probability Aul from spectroscopic databasese.g. the “NIST Atomic Spectral Database” [55].

Radiation transport inside an HID plasma

In high pressure lamp plasmas, mainly the spectral line emission but also the spectral lineabsorption contribute significantly to the emission spectrum of the HID-lamp. The photonswhich are produced in the center of an arc plasma have to pass the outer plasma volumebefore they contribute to the lamp emission. On this way through the plasma discharge,the photons can also be absorbed before leaving the plasma. Accordingly, the light beamleaving the HID-lamp is an integral value along the beam path through the plasma volume,the so-called “line-of-sight”, which can contain several successive emission- and absorptionprocesses. The changes of the spectral radiance Iλ in an infinitesimal small step along theline-of-sight x is described by the radiation transport equation [56]:

dIλ(λ, x)

dx= ελ(λ, x)− αλ(λ, x)Iλ(λ, x) (2.20)

Herein ελ(λ, x) is the previously discussed spectral emission coefficient and αλ(λ, x) thecorresponding absorption coefficient. An integration of the radiation transport equation 2.20along the line-of-sight through a cylindrical arc discharge starting at x = −R with theboundary condition Iλ(λ, x = −R) = 0 yields

Iλ(λ) =

∫ +R

−Rελ(λ, x) exp

(−∫ +R

x

αλ(λ, ξ)dξ

)dx (2.21)

For spectroscopic measurements a knowledge about the absorption strength of a plasmais very important. Spectral emission lines can only be evaluated from measured spectral

24 2. Fundamentals

radiances in absolute values if their reduction by absorption processes is known or negligiblesmall. In high pressure plasmas for example, spectral lines emitted by certain atoms areoften absorbed within the outer, colder part of the discharge by the same atoms speciesthemselves. This effect can lead to reduced emission line maxima or even a depression of theline center and is called “self-absorption”.A characteristic indicator for the absorption strength of a plasma is the definition of theso-called “optical thickness” or “optical depth” [57]. The optical depth τ0(λ) is defined as theintegral of the absorption coefficient along the line-of-sight through a plasma:

τ0(λ) =

∫ +R

−Rα(λ, ξ)dξ (2.22)

If τ0(λ) � 1, absorption processes can be neglected within the plasma because α(λ, r) ≈ 0.In this case the investigated spectral emission line is called “optically thin”.

2.3 Electrodes of arc discharges

The electrodes of HID-lamps are the most important parts, their interaction with the arcdischarge is the key issue to optimise the whole lamp performance. During operation, the HIDelectrodes have to sustain an intense thermal stress and are driven close to their physical loadlimit. Due to the high operating temperatures up to 3650 K, HID electrodes are exclusivelymade of tungsten or doped tungsten compounds. Tungsten has a melting temperature of3695 K which is the highest of all metal materials.The cathode of an arc discharge emits the electrons for the arc current thermionically. Thus,the cathodic electrode needs a certain temperature to sustain the current demand. Therefore,it has to be heated by a heating power flux from the plasma arc. The anode in contrastis heated by the electrons delivered through the current flow from the arc plasma. Theelectrons entering the anodic electrode release an energy from the work function ϕ whichdetermines mainly the anodic heating. To investigate the emitter-effect on electrodes, somefundamental background concerning the heat balance and the electron emission of a cathodewill be summarised in the following sections.

2.3.1 The heat balance of HID electrodes

As already mentioned, the anodic and cathodic tungsten electrodes within HID-lamps havea distinctly high temperature during lamp operation. This high operating temperature is aresult of the heating by accelerated ions at the cathodic electrode and by electrons enteringthe solid tungsten material at the anodic electrode. For a quantitative description of theelectrode heat and power balance, some assumptions and simplifications have to be made. Itis assumed that the volume V of the electrodes and their mass density ρm does not changewith temperature T or pressure p. Accordingly, the processes within the tungsten electrodesare isochore and isobar.

2.3. Electrodes of arc discharges 25

Thermodynamic description of the HID electrode

As a first modeling step, only an infinitesimal small volume dV of the electrode material hasto be considered. According to the first thermodynamic theorem, the time dependent changeof the inner energy U of this small volume in dependence on time can be described as

dU

dt= ρm

∫V

cp(T )∂T

∂tdV. (2.23)

U : inner energy of the volume V ρm : tungsten mass densitycp : tungsten heat capacity T : material temperature

The capacitive part on the right hand side of equation 2.23 can be balanced by heat sourcesS in the volume dV and losses by a heat flux −→q cond perpendicular through the surface −→n ·Aof the volume dV :

ρm

∫V

cp(T )∂T

∂tdV =

∫V

S dV −∫A

−→q cond · −→n dA (2.24)

If the heat flux is directed into the material (−→q cond ·−→n < 0), the last term on the right handside of this equation becomes positive.By use of the Gausss integral theorem the heat balance can be transformed to∫

A

−→q cond · −→n dA =

∫V

∇ · −→q cond dV. (2.25)

Furthermore the Fourier-law for the heat conduction

−→q cond = −κ(T )∇T (2.26)

can be used. Equation 2.26 describes the heat flux −→q cond which always flows into the directionof the negative temperature gradient and depends on the heat conductivity κ(T ).Inserting equation 2.25 and 2.26 into equation 2.24 leads to∫

V

(ρmcp(T )

∂T

∂t−∇ · (κ(T )∇T )

)dV =

∫V

S dV (2.27)

for the integrated heat flux densities in the volume V . As this expression is valid for allinfinitesimal volumes dV of the whole electrode, it can be written as:

ρmcp(T )∂T

∂t−∇ · (κ(T )∇T ) = S. (2.28)

Its integration describes the complete heat balance of the solid tungsten lamp electrode. Thisequation is a nonlinear partial differential equation of 2nd order for T = T (−→r , t). The sourceterm S on the right hand side considers heat sources within the tungsten volume. In thiscase, the so-called “Joule heating” caused by the limited conductivity of tungsten is theonly effect which can have a significant influence on HID-electrodes. But as Bergner showedin [58], the Joule heating is very small for the standard HID-lamp operating ranges and canusually be neglected. Thus, the inner source term S in equation 2.28 will always be treatedas S = 0 within this PhD work.

26 2. Fundamentals

A3

dz

r

A2

A1

z

Figure 2.3: Definition of the z- and r-coordinate and the normal vectors of a disc volume dV fora cylindrical HID-lamp electrode to solve the electrode heat balance

Solution of the heat balance equation for a cylindric HID electrode

To integrate the local electrode heat balance represented by equation 2.28, the cylindricalelectrode rod is described by cylindrical coordinates according to figure 2.3. The axial di-rection along the electrode is named z with z = 0 at the electrode bottom and the radialcoordinate is named r. The heat flux through the electrode rod is assumed to have a rota-tional symmetry, thus the electrode heat balance becomes independent of the polar angle.As the HID-lamps investigated in this work are driven by DC but also by switched-dc andsinusoidal currents, the time dependence of equation 2.28 can be simplified to a periodicalvariation with the angular frequency ω = 2π

τ. The corresponding phase angle is then defined

as ϕ = ωt = 2πτt. As a result equation 2.28 becomes:

ρmcp(T )ω∂T

∂(ϕ)−∇ · (κ(T )∇T ) = 0 (2.29)

This heat balance for periodically driven HID electrodes can be simplified even more for thespecial case of DC driven HID-lamps. After a startup phase, which is not investigated withinthis work, all time dependent effects of the electrode heat balance vanish for DC operation.Thus, the first term of equation 2.29, representing the time dependency, can be neglectedresulting in a stationary local electrode heat balance:

−∇ · (κ(T )∇T ) = 0 (2.30)

Reinelt showed in [31] and [33] that within the largest part of the electrode this stationaryelectrode heat balance is also applicable for switched-dc operation of thin electrodes as inYAG-lamps and for high frequency operation (f ≥ 1 kHz) of thicker electrodes as in theBochum model lamp. If the electrode heat capacity becomes very small due to thin electrodeswith a small volume or if the time period τ is much smaller than the time constant of the

2.3. Electrodes of arc discharges 27

heat flux, the term ρmcp(T )ω ∂T∂(ϕ)

becomes insignificantly small apart from the end face of theelectrode and can be neglected. These conditions are fulfilled for all investigations within thiswork, accordingly the simplified concept of a stationary electrode heat balance representedby equation 2.30 will be discussed exclusively in the following.As sketched in figure 2.3, a small disc of a cylindrical electrode with the radius RE and thethickness dz is considered. With the indicated definition of three normal vectors on the threeelectrode disc surfaces and the Gauss theorem, eqn. 2.30 can be written in the integral formas ∫

V

∇ · (κ(T )∇T ) dV =

∮A

κ(T )∇T d−→A. (2.31)

The resulting surface integral can be evaluated by summing up the three surfaces of theelectrode disc (cf figure 2.3):∮

A

κ(T )∇T d−→A =

∫A1

κ(T )∇T d−→A +

∫A2

κ(T )∇T d−→A +

∫A3

κ(T )∇T d−→A (2.32)

By means of the assumed rotational symmetry ∇T can be reduced to

∇T =∂T

∂r−→er +

∂T

∂z−→ez . (2.33)

Additionally, the power fluxes through the cylinder barrel A3 can be balanced by the powerflux qp from the plasma to the electrode and the power flux qrad which is radiated by the hotcylinder barrel surface according to the Stefan-Boltzmann law:

qrad = σSBεtot(T )(T 4 − T 4amb) (2.34)

σSB : Stefan-Boltzmann constant εtot : total emissivity of tungstenTamb : ambient temperature

The total emissivity εtot(T ) of a material can be derived by an integration of its spectralemissivity ελ(λ, T ) over the whole wavelength range from 0 < λ <∞ according to:

εtot(T ) =

∫ ∞0

ελ(λ, T ) dλ (2.35)

Assuming that the electrode temperature differs only in the z-direction which is approxi-mately true within HID-lamp electrodes, the heat balance of the cylindrical disc with theradius RE amounts to∮

A

κ(T )∇(T ) d−→A = πR2

Eκ(T )

[(dT

dz

)A1

−(dT

dz

)A2

]+ 2πRE(qp − qrad)dz = 0. (2.36)

This equation is valid for all infinitesimal discs along the whole cylindrical electrode rod. Itcan be written as a second order differential equation for T (z):

−πR2E

d

dz

(κ(T )

dT

dz

)= 2πRE(qp − qrad) (2.37)

Its integral represents the total heat balance of the electrode solved for the special one di-mensional case with temperature distributions only along the z-axis.

28 2. Fundamentals

Two boundary conditions are required to execute the integration. A possible choice is theknown temperature at the bottom of the electrode

T (z = 0) = Tbot = const. (2.38)

and the heat flux through the end face of the electrode with the length lE at the electrodetip

dT

dz

∣∣∣∣z=lE

=(qp − qrad)

κ(T )

∣∣∣∣T=Ttip

. (2.39)

Losses of the HID electrodes

As already mentioned previously, the cathodic HID electrode needs a certain temperature toemit a sufficient electron flux for the arc current thermionically. The radiating processes ofthe electrode are discussed during the subsequent chapter 2.3.2. As the HID-lamp is naturallydesigned for visible light emission, the power transfer from the plasma heating the electroderepresents the most important loss mechanism.The corresponding power loss from the plasma to the electrodes Pl can be divided into twomain fractions:

Pl = Pcond + Prad (2.40)

Herein, Pcond represents the power losses by heat conduction into the electrode mounting.It can be calculated by the electrode temperature gradient at the bottom Tbot and the heatconductivity at the electrode end according to

Pcond = πR2Eκ(Tbot)

dT

dz

∣∣∣∣z=0

. (2.41)

The second term of the power loss equation 2.40, Prad, describes the power losses by elec-tromagnetic radiation of the electrode. Prad is given by the Stefan-Boltzmann law (cf.equation 2.34) evaluated at the electrode end face and along the cylindrical barrel:

Prad =

∫A

qrad−→n · d

−→A = πR2

EσSBεtot(Ttip)(T 4tip − T 4

amb) + 2πRE

∫ lE

0

σSBεtot(T )(T 4 − T 4amb) dz

(2.42)The power loss of an HID electrode Pl is a very important parameter for the optimisationof the HID-lamp operation. Besides the electrode tip temperature Ttip, the power loss Pl

will be used later in this work to specify and characterise the efficiency of the investigatedemitter-effect.

2.3.2 Emission processes of HID electrodes

Every material radiates electromagnetic waves, every metallic material also emits electronsthermionically according to the surface temperature. Both processes withdraw energy fromthe material. Due to their hot temperature, the electrodes of HID-lamps cause radiation

2.3. Electrodes of arc discharges 29

processes, too. Accordingly, in addition to heat conduction, the two main losses of metallictungsten electrodes are on the one hand the thermionic emission of electrons enhanced byseveral effects and on the other hand the radiation of electromagnetic waves. Both emissionprocesses will be discussed within this section but with the main focus on the thermionicelectron emission.

Electrode grey-body radiation

As every solid body with a temperature T > 0 electrodes of HID-lamps emit thermal radia-tion depending on temperature. The emitted radiation is always a continuum radiation due tolattice vibrations by thermal excitation. For usual electrode operating temperatures slightlybelow the tungsten melting point, the maximum of the emitted electromagnetic radiationis in the infrared (IR) wavelength region. From an energetic point of view the emission ofthermal radiation represents a significant cooling effect of the electrode which was consideredby qrad in the previous chapter (cf. equation 2.42). The resulting surface radiance Lλ,b of aso called “black-body radiator” is described by the Planck law depending on temperatureT and wavelength λ:

Lλ,b(λ, T ) =c1

πλ5

[exp

(c2

λT

)− 1

]−1

(2.43)

Herein c1 and c2 are the so-called “Planck-constants” defined as

c1 = 2πhc20 = 3.7415 · 10−16 Wm2

c2 = hc0kB

= 1.4388 · 10−2 Km.

Per definition an ideal black-body radiator has to absorb any electromagnetic wave reachingits surface which is never fulfilled in reality. Thus, a correction factor, the surface emissivityε(λ, T ), depending on wavelength and temperature is introduced into the Planck radiationlaw (eqn. 2.43). For a real radiating surface the emissivity is ε(λ, T ) < 1 and it is called“grey-body radiator”. Accordingly, the surface radiance of a grey-body radiator is given by

L(λ, T ) = ε(λ, T ) · Lλ,b(λ, T ). (2.44)

Thermionic electron emission of HID electrodes

It was already mentioned previously that the emission of electrons by the cathodic electrodeis the major effect leading to a current transport through the arc discharge of an HID-lamp.Therefore, the cathodic electrode needs a specific external heating power which is suppliedby the power flux qp from the arc plasma. As a consequence of the heating, the hot electrodereleases an electron flux from its tungsten surface which is nearly similar to the arc currentand balances the heating power flux qp. By this electron emission the electrode looses energyto the plasma boundary sheath, details of the processes in the plasma boundary sheath willbe discussed later in section 2.3.4. This section briefly summarizes the fundamentals andeffects for the so-called “thermionic emission” of electrons.

30 2. Fundamentals

To describe the thermionic electron emission quantitatively, some fundamentals of the elec-tron behaviour and parameters within metals have to be launched in advance. Metallicmaterials are characterised by conduction electrons which are weakly bounded to the solidcrystal structure. Within the crystal structure of the metal, electrons are quasi free. Theelectrons in metals have only kinetic energy and no potential energy depending on theirposition. This property of metals is represented by their current and heat conductivity. Ac-cording to the model of free electrons by Sommerfeld, which is a further development ofthe famous Drude model, the energetic states of a solid state material are consecutivelyfilled with electrons. For this model the density n depending on the energy E is given by

N(E) =1

2π2

(2me

~2

)3/2√E. (2.45)

In the case of a material temperature T = 0 the occupied and unoccupied energy states areseparated by the so-called “Fermi-energy”EF according to

EF =~2

2me

(3π2ne)2/3 (2.46)

with ne the density of electrons in the solid state volume. For temperatures T > 0 theelectrons gain thermic energy and occupy also energy states above the Fermi-energy. Thedistribution of electron energy states in metals for temperatures T > 0 is described by theso-called “Fermi-Dirac-statistic” with the distribution function

f(E) =

[exp

(E − EF

kBT

)+ 1

]−1

. (2.47)

The corresponding density of states n(E) leads

n(E) = f(E) ·N(E) =1

2π2

(2me

~2

)3/2√E

[exp

(E − EF

kBT

)+ 1

]−1

. (2.48)

To be emitted from the solid metal material, an electron has to overcome kinetically theenergy difference between the Fermi-energy EF at T = 0 within the metal and the vacuumenergy E∞ of the free gas-phase (cf. figure 2.4). This energy, heating up the HID electrodeto T > 0 K, is supplied by the arc plasma and is called ”work function” φ with

φ = E∞ − EF. (2.49)

The work function φ is a characteristic property of every material. For pure tungsten, thestandard material of HID-electrodes, the value of the work function amounts to φ = 4.55 eVaccording to [59]. The effective value of the work function can be reduced by several minorand major effects. This work function reduction mechanisms with focus on the emitter-effectwill be presented in a separate chapter 2.3.3 as they form the main focus of this PhD work.

Finally the thermionic electron emission by the heated HID electrode can be describedquantitatively. The so-called “Richardson-Dushman-equation” determines theoreticallythe electron current density jem which is released by a cathode with the temperature Tc andthe work function φ:

jem =4πmee

h3(kBTc)

2 exp

(− φ

kBTc

)(2.50)

2.3. Electrodes of arc discharges 31

As it can be interpreted from equation 2.50, the electron emission of a cathode increaseswith increasing temperature Tc but it also increases with a reduced work function φ. Thiscorrelation forms the basic idea of a reduced electrode temperature due to a reduction of theeffective work function by the emitter-effect while keeping the current density constant.It has to be mentioned that the Richardson-Dushman-equation 2.50 is of course also validfor hot anodes. Thus, hot electrodes of arc plasmas always emit electrons thermionically in-dependently of their polarity. An anodic electron emission flux has to be considered duringan anodic boundary sheath investigation and leads to a cooling of the plasma in front of theanode.

Besides the described, most important thermionic electron emission, some minor electronemission processes are also active on the cathodic surface which will be briefly mentioned forthe sake of completeness:

Secondary electron emission A so-called “secondary electron” can be emitted from a metalif the material is bombarded by high energetic ions. The ion recombines at the metalsurface with a first electron to an atom and provides its surplus energy (ionisationenergy Ei + kinetic energy Ekin) to the metal. If the provided energy is higher thanthe work function φ of the metal, a second electron from the metal can be released.This effect is called secondary electron emission and can be neglected compared to thethermionic emission of electrons for a stable operating HID-lamp [22]. The secondaryemission might have a significant influence during the run-up phase of the lamp, butits investigation is not intended within this work.

Photo emission From a general point of view, an electron can also be released from a metalby means of a photon striking the surface. If the provided energy from the photon ishigher than the work function φ of the metal, an electron is emitted with the energy

hν = φ+1

2mev

2e (2.51)

with the kinetic energy of the emitted electron in the gas-phase on the right hand side.Thus, a threshold energy respectively a threshold frequency νth can be defined for thephotoemission depending on the material:

νth =φ

h(2.52)

For tungsten with a work function of φ = 4.55 eV, the threshold frequency amounts toνth ≥ 1.1 · 1015 Hz or λth ≤ 272 nm. As the proportion of light production in the highenergetic UV range is usually very small within an HID-lamp, photo emission can alsobe neglected for investigations of the electron emission by HID electrodes.

Field emission To emit an electron from a metal by field emission, a high electric fieldstrength is needed adjacent to the metal surface. This high electric field strength re-duces the thickness of the potential barrier for electrons leaving the material. In con-sequence, the metal electrons can tunnel through the potential barrier to the vacuumlevel even if their kinetic energy is lower than the work function. To have a reason-

able influence of the field emission, a field strength of roughly |−→E | = 1 · 109 V/m is

needed. These high field strength values are not reached for a diffuse arc attachment

32 2. Fundamentals

on HID electrodes and not even for a constricted one in the spot-mode. Accordinglythe field emission does also not have a significant influence which should be consideredin HID-lamps.

2.3.3 The emitter-effect on tungsten surfaces

As previously discussed, the work function of a material is the most important physicalproperty concerning its emission of electrons. If the electrode geometry and arc current ofan HID-lamp is given, the electrode work function φ determines the electrode power loss Pl,the electrode temperature T (cf. equation 2.50) and thereby also the ageing speed of theHID-lamp. Thus, the research into mechanisms reducing the work function of tungsten is ofhigh interest for HID-lamp designers. Some problematic effects exist which can even increasethe work function of tungsten, e.g. a coverage of tungsten oxide on a cold HID electrode.These poisoning effects can almost double the effective work function of tungsten comparedto its pure value and lead to problems during the ignition and run-up phase of HID-lamps.To release an electron from a solid metal material, the energy distance between the elec-

x

E

metal gas-phase

EF

E∞

ϕ =E -eff ∞E EF-Δ

ΔE

ϕ=E -∞

EF

+-+-

+-+-+-

emitter material

Figure 2.4: Schematic energy band model of a metal with the Fermi-energy EF and the adjacentgas-phase with a vacuum energy E∞. The emitter-effect is represented by a dipole-layer on thejunction resulting in a reduction ∆E of the energy barrier, the work function φeff .

tron energy in the metal, the Fermi-energy EF, and the vacuum electron energy E∞ for afree electron in the gas-phase has to be overcome. This energy distance equates to the workfunction φ of the metal and is illustrated schematically as an energy band model in figure2.4. One effective mechanism to reduce the work function of lamp electrodes is the so-called“emitter-effect” which is used in almost every actual HID-lamp. The investigation of thisemitter-effect in different lamp types is the key issue of this PhD work.The emitter-effect is formed by an atomic monolayer of a certain emitter material on thetungsten electrode surface. The emitter material must have a lower so-called “electronega-tivity” than tungsten to increase the electron emission of the electrode. The dimensionlesselectronegativity χ describes the affinity of an atom to bond electrons and can be specified

2.3. Electrodes of arc discharges 33

in different scaling systems, e.g. the Pauling system which is used for this work [60]. Ac-cording to [61], the electronegativity of tungsten is χW = 2.36, thus elements like thorium(χTh = 1.3), barium (χBa = 0.89), dysprosium (χDy = 1.22) and cerium (χCe = 1.12) butalso other rare earth can be used to generate the emitter-effect on tungsten electrodes. Ifone of these emitter materials is present as a monolayer on the tungsten surface, it forms adipole layer which pulls electrons out of the tungsten material by an additional electric field.This physical effect results in a bent electron energy band at the transition from the solidmetal to the gas-phase and is also illustrated as a red line in figure 2.4. In conclusion, theemitter-effect leads to a reduced energy barrier for the electrons by ∆E indicated by a lowerso-called “effective work function” φeff = E∞ − EF −∆E.It has to be mentioned that the emitter-effect is not a simple physical phenomenon scalinglinearly with the electronegativity distance ∆χ or any other parameter. Several investiga-tions [25, 22, 31, 58] showed that the emitter-effect on tungsten HID electrodes is a rathercomplex and sensitive mechanism with some optimal conditions depending on the ionisationenergy of the emitter material, the surface coverage, the adsorption- and desorption-rate ofemitter materials, the electrode temperature, influences of other HID-lamp ingredients andsome other minor parameters. However, a systematic investigation of the optimal conditionsfor the emitter-effect on tungsten electrodes as it will be presented within this work is ofhigh scientific interest even for many other fields of plasma technology.

Within commercial HID-lamps, there exist several concepts to store and transport the emit-ter materials to the discharge arc attachment on the electrode surface during operation. Thethree most common concepts, which are all investigated in the results chapter of this work,will be described briefly:

tungstenelectrode

emitter-materialatom

(a) (b) (c)

tungstencoil

emitter-material liquid

emittersalt-pool

Figure 2.5: Sketch of the three most common storage concepts for emitter material in an HID-lamp: (a) electrode doping, (b) coil storage, (c) liquid salt pool

As shown in figure 2.5 (a), the easiest way to supply emitter materials to the electrode sur-face is to dope the pure tungsten electrode rod with a certain amount of emitter atoms. Theemitter atoms which are usually introduced into the tungsten material by sintering tech-niques diffuse to the surface of the operated electrode. They influence the electrode workfunction either directly by diffusion out of the body material or by an ionised backflow pro-

34 2. Fundamentals

cess through the gas-phase. The described concept of emitter-doped tungsten electrodes isapplied within the Bochum model lamp.Another storage solution for emitter-material in an HID-lamp is presented in figure 2.5 (b).To realise this technology, a small tungsten coil is winded around the main lamp electrodeduring its production with a certain electrode tip distance. Afterwards, the tungsten coilis filled with emitter material while diving it into a liquid emitter solution. During lampoperation, the tungsten coil storage is heated by the hot inner electrode. The emitter atomsare released from a compound (e.g. Ba from Ba2CaWO6) and diffuse along the electrode rodto its tip where the material is consumed for the emitter-effect. This concept of a tungstencoil storage for the emitter material is applied e.g. within high-pressure sodium (HPS) lampswhich are introduced and investigated later on.Finally, a last concept of emitter supply, implemented in most of the HID general lightinglamps, is the so-called “gas-phase emitter-effect”. For the gas-phase emitter-effect the emitteratoms are introduced as solid salts like iodide compounds (e.g. DyI3, ThI, CeI3) togetherwith light producing materials into the discharge tube during lamp production. During lampoperation, the solid emitter materials melt, form a liquid salt pool and evaporate to thegas-phase. In conventional HID-lamps the emitter and light producing salts are always onlypartly evaporated and form a certain partial pressure, these lamps are called saturated lamps.Modern, recently developed HID-lamp technologies use unsaturated partial pressures of lampadditions to facilitate e.g. lamp dimming [62]. As measuring results in this work will showlater, the evaporated emitter atoms within the gas-phase are transported to the electrodesurface mainly by ionisation in the plasma and a consecutive ion flux. On the surface of thetungsten electrodes, the emitter material supplied from the gas-phase forms again an atomicmonolayer with dipole character and contributes thereby to the emitter-effect. This conceptof the gas-phase emitter-effect is realised within YAG-lamps which are also investigated laterin this work.

Besides the described emitter-effect other mechanisms can also contribute more or less sig-nificant to a reduction of the effective work function of a material. These effects will besummarised briefly in the following.

Crystal structure The work function of a material may vary according to its crystal struc-ture orientation with respect to the material surface. Accordingly, a given value for thework function of a certain material is always averaged in a complicated manner overall possible crystal structure orientations.

Schottky-effect The so-called “Schottky-reduction” is a special type of the previouslydiscussed field emission. The thermionically emitted electrons adjacent to the electrodesurface create an electric field strength reducing the potential barrier when they leavethe metal. This self-reduction of the potential barrier by emitted electrons can onlyhave a significant influence for field strength above 107V/m and is called Schottky-effect.

Anomalous Schottky-effect The “Anomalous Schottky-effect” is characterised by a re-duction of the work function caused by a combination of the crystal structure andthe general Schottky-effect. By means of strong electric fields the Schottky-effectlowers the work function at surface areas with an optimal crystal structure orientation.Accordingly, surface areas with an awkward crystal orientation are deactivated by this

2.3. Electrodes of arc discharges 35

anomalous Schottky-effect resulting in a reduced effective electron emission area ofthe metal.

2.3.4 The cathodic sheath model for HID electrodes

By means of the previously given fundamentals, a systematic experimental and theoreticalinvestigation of the operating behaviour of HID electrodes and their power- and temperaturefluxes through the tungsten body can be performed. Also the arc plasma in the bulk canbe analysed and described quantitatively by means of the discussed concept of an LTE as-sumption. However, to investigate the whole HID-lamp behaviour, a detailed understandingof the coupling mechanisms between the arc plasma and the electrode is necessary.To understand the interactions between the arc plasma and the tungsten electrodes, theelectrode boundary sheath has to be described theoretically. For this theoretical descriptiona model for the plasma sheath has to be developed which approximates and simplifies themajor physical effects. A careful choice of these model approximations and simplificationsis important to obtain modeling results which remain comparable to reality. According tospecial requirements and plasma properties a few approaches exist for the plasma boundarysheath modeling of low and high pressure plasmas from DC to RF-operation. Besides pos-sible kinetic approaches, a multi-fluid plasma model suites best for the description of theelectrode boundary sheath of a thermal high pressure plasma as in an HID-lamp [29].For the electrode sheath modeling presented in this work the so-called “Black-Box”-modelis considered which was introduced by Lichtenberg [15] and further developed by Dabring-hausen [8] and Langenscheidt [22]. Thus, the following chapter gives a rough overview overthe underlying assumptions and model simplifications and it summarises the main resultsand the important model equations.

Black-Box model

The basic idea of the Black-Box model which was developed within the Bochum HID re-search group is the description of the plasma-electrode interaction by power flux balancesindependently of the bulk plasma. The complex physical effects and interactions within thecathodic boundary sheath are summarised by a Black-Box which only balances the ingoingand outgoing power fluxes. As it will be shown later, this model can be solved self-consistentand is completely sufficient to describe the real interaction of the arc discharge at least withthe cathode.The most important equation of the Black-Box model for a cathodic HID electrode wasachieved by general considerations and empirical research:

UcI = Ploss + φI +5

2

kBTe

eI (2.53)

This equation balances the experimental findings of the cathode voltage drop Uc by electricprobe measurements, the cathode power loss Ploss by the previously discussed electrode heatbalance determined by pyrometry and finally the electron temperature Te measured by spec-troscopy. The power balance given by equation 2.53 and the directions of the power fluxesare visualised additionally in figure 2.6.

36 2. Fundamentals

The term UcI on the left hand side of equation 2.53 describes the power which is supplied

Black-Box P i uel arc c=

Ploss

Prad Pcond

5 k TB e

2 ei arc i arc Á

Figure 2.6: Scheme of the cathodic electrode boundary layer in an HID-lamp including the defi-nitions of the main power fluxes

electrically into the cathodic boundary sheath by the sheath voltage drop Uc and the arccurrent I. This power is balanced on the right hand side by the power losses Ploss heatingthe electrode by an ion flux and the power φI which is needed to release the electrons fromthe tungsten material with the work function φ (cf. chapter 2.3.2). Additionally, an electronenthalpy flux into the plasma bulk is considered on the right hand side of equation 2.53 bythe term 5

2kBTe

eI.

As already mentioned, the concept of the Black-Box model assumes that the cathodic bound-ary sheath can be described by power fluxes completely decoupled from the bulk plasma. Thisassumption is in good accordance with reality as the cathodic boundary sheath has a veryhigh power input on a thin layer scale compared to the bulk plasma due to its high voltagedrop Uc. Accordingly, the cathodic boundary sheath is only determined by the thermion-ically emitted electrons from the cathode and the electrically introduced power UcI. Twoparameters, the so-called“transfer functions”are introduced to describe the Black-Box modelmathematically: The total power density flux qp which is coupled into the electrode and thetotal current density j from the electrode to the arc plasma. These transfer functions arealready included in equation 2.53 by an integral relation according to:

Ploss =

∫A

qp−→n · d

−→A and I =

∫A

j−→n · d−→A (2.54)

The main equation 2.53 of the Black-Box model can be rearranged to obtain the electrodepower loss

Ploss =

(Uc − φ−

5

2

kBTe

e

)I. (2.55)

By means of the defined transfer functions qp and j this equation can also be written inintegral form: ∫

A

qp−→n · d

−→A =

∫A

(Uc − φ−

5

2

kBTe

e

)j−→n · d

−→A (2.56)

2.3. Electrodes of arc discharges 37

This correlation is valid for an arbitrary electrode geometry with the surface area A. Thus,the equation can also be written in its differential form:

qp =

(Uc − φ−

5

2

kBTe

e

)j (2.57)

Finally, a correction factor ξ has to be introduced to consider an increase of the enthalpyflux with growing ionisation degree of the arc plasma. Equation 2.57 leads:

qp(Uc, Te) =

[Uc − φ−

(5

2+ 0.7ξ

)kBTe

e

]j and ξ =

nep

nip + n0p

(2.58)

Herein nep, nip, n0p describe the electron-, ion- and neutral-atom-density at the edge fromthe boundary sheath to the bulk plasma, respectively. The correction term 0.7ξ has to beconsidered because the cathode sheath of HID-lamps usually has a massive power loss tothe plasma bulk by electron enthalpy fluxes due to a heating of the thermionically emittedelectrons and ionisation collisions.The Black-Box model for the cathodic boundary sheath is described consistently by thetransfer functions q(TC) and j(TC) and their correlation expressed in equation 2.58. However,to use the model for power input calculations into the cathode, a more detailed theoreticalinvestigation of the boundary sheath has to be performed afterwards.

Solution of the Black-Box model - the transfer functions

To solve the Black-Box model for a numerical simulation of the cathodic boundary sheath, anextensive investigation of the different fractions of the power- and current fluxes in the sheathis needed. As this investigation was already performed by Lichtenberg [15] and Dabring-hausen [8] and replicated in [22, 31, 58], this work only gives the main ideas.A detailed description of the cathodic boundary sheath which will be used within this workwas given by Schmitz and Riemann in [63, 12]. Schmitz and Riemann divided the cathodicboundary sheath in three sub zones according to the dominant physical effects:

Saha-plasma The first zone adjacent to the bulk plasma is called “Saha-plasma”. It ischaracterised by the Saha equilibrium (cf. equation 2.13) with a balance betweenionisation and recombination. The densities and temperatures within the Saha-plasmazone are assumed to be independent of the bulk plasma.

Presheath The “presheath” adjacent to the Saha plasma has a thickness in the range of√λCX · λi with the mean free path of charge exchange collisions λCX and the mean

free path of ionisation collisions λi. It can be further divided into two sections: Thefirst section is a collision layer wherein ions are produced by electron collisions, thesecond section is a so-called Knudsen-layer which accelerates the ions coming fromthe collision layer to the Bohm velocity (cf. Riemann [63]).

Space charge sheath The last zone between the presheath and the electrode surface iscalled “space charge sheath” with a thickness of roughly one Debye length λD. Withinthe space charge sheath the ions are further accelerated towards the electrode surfaceand gain energy for the cathode heating. This zone is assumed to be collision free.

38 2. Fundamentals

By means of this zone classification the transfer functions for the current density j and forthe power density qp can be traced back to fundamental physical effects within the singlezones.

For the total current density j a sum of four single contributing effects can be found:

j = jem + ji − jep + jSE (2.59)

jem : current density by electrons emitted thermionically from the cathodeji : current density by ions produced in the presheath and entering the cathodejep : current density by an electron back diffusion into the cathodejSE : current density by secondary electron emission due to ion bombardment

The interpretation of the current density j with the Schmitz-Riemann model can be used foranother very important aspect concerning the solution of the Black-Box model: As shownpartly in equation 2.58, the transfer functions j and qp depend on the electron temperatureTe in the Black-Box model. This electron temperature Te is not directly available as a modelparameter and should therefore be substituted by the cathode temperature Tc and the cath-ode voltage fall Uc. The current density investiagtions lead to a direct balance between theelectron- and ion fluxes:

jem(Tc)Uc = ji(Te)Ei

e= ji(Te)Ui (2.60)

The left term of equation 2.60 describes the power which is coupled into the boundarysheath by thermionically emitted electrons from the hot cathode. This power is balanced onthe right hand side of equation 2.60 by the power removed from the boundary sheath dueto ions reaching the cathode surface. By means of the power balance in equation 2.60 theelectron temperature Te can be calculated from the cathode temperature Tc and the cathodevoltage fall Uc. This is an important aspect as with this intermediate step the power fluxdensity qp into the cathode can be expressed by the cathode temperature Tc and a cathodefall Uc as variable parameter. This possibility is one of the major advantages within theconcept of the Black-Box model.

For a numerical simulation of the total power density qp within the cathodic boundarysheath, the electron power balance as well as the heavy particle and the cathodic powerbalances have to be analysed separately.The electron power balance qe for all three zones can be expressed by

qe = qef − qi − qtf + qe,ps = 0. (2.61)

qef : energy gain of the emitted electrons by the electric fieldqi : energy consumption by impact of ionisationqtf : thermic energy flux of the electrons from and into the sheathqe,ps : energy gain of the electrons within the presheath

Similar to the electron power density balance another power density balance qh can be derivedfor the heavy particles:

qh = qi,ps + qi,sc + qtf,n − qi,kin − qhc = 0 (2.62)

2.3. Electrodes of arc discharges 39

qi,ps : energy gain of heavy particles within the presheathqi,sc : energy gain of heavy particles within the space charge sheathqtf,n : thermal energy gain by neutralised atoms diffusing back from the electrodeqi,kin : kinetic energy flux by the ion current jiqhc : energy loss due to heat conduction to the cathode

In the end, the power density flux towards the cathode surface qp has to be described tocalculate transfer functions. It is also a sum of several physical effects which were alreadyincluded as losses within the previous two power density balances:

qp = qrec + qkin − qtf,n − qem + qep + qhc (2.63)

qrec : energy gain due to a recombination of ions at the electrode surfaceqkin : energy gain by kinetic ions accelerated towards the electrodeqtf,n : thermal energy loss by neutralised atoms diffusing back from the electrodeqem : energy loss by emission of electrons from the cathode surfaceqep : energy gain by electrons diffusing from the Saha plasma to the cathodeqhc : energy gain due to heat conduction from the boundary sheath to the cathode

Finally, the discussed three power density balances and the mathematical description of thefundamental processes represented by each term form an equation system. The equationsystem consists of 12 equations and 14 unknown parameters. Thus, the equation systemcan by solved which was done by Lichtenberg and Dabringhausen within a MatLab-programin dependence on two parameters. The results of this equation system lead to the transferfunctions in dependence on the cathode surface temperature Tc and the cathode fall Uc:

qp(Tc, Uc) =

[Uc − φ−

(52

+ 0.7ξ

)kBTe(Tc,Uc)

e

]j(Tc, Uc) (2.64)

j(Tc, Uc) = jem(Tc, Uc) + ji(Tc, Uc)− jep(Tc, Uc) + jSE(Tc, Uc) (2.65)

These two equations conclude the whole concept of the transfer functions as they decouplethe electrode input power density and the correlated current density from the plasma bulk.

Some examples for calculated transfer functions qp(Tc, Uc) and j(Tc, Uc) are presented infigure 2.7. The transfer functions are plotted in dependence on the cathode surface temper-ature Tc for different cathode fall voltages Uc which may occur in HID-lamps. The plots arevalid for arc discharges in an argon atmosphere with p = 0.1 MPa and a work function of4.55 eV within the Bochum model lamp.

40 2. Fundamentals

2500 3000 3500 4000 4500 50000

0.5

1

1.5

2

2.5x 10

9

cathode surface temperature / KTc

pow

er f

lux d

ensi

ty /

Wm

qp

-2

50 V45 V40 V35 V30 V25 V20 V15 V10 V

(a) qp(Tc, Uc)

2500 3000 3500 4000 4500 5000

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

8

cathode surface temperature / KTc

curr

ent

densit

y/A

mj

-2

50 V

45 V

40 V

35 V

30 V

25 V

20 V

15 V

10 V

(b) j(Tc, Uc)

Figure 2.7: Example of calculated transfer functions for the power density qp (left) and the currentdensity j (right) in dependence of the cathode surface temperature Tc for different cathode fallsUc. Parameters: argon with a pressure of p = 0.1 MPa, work function: eϕ = 4.55 eV

41

3. Experimental setup

To investigate the previously introduced emitter-effect of HID electrodes systematically, dif-ferent types of HID-lamps and driving setups as well as electrical and optical measuringsystems are needed. Accordingly, this chapter presents the experimental setup which is usedfor the investigations within this work. The chapter starts with a detailed description ofthe three different HID-lamps in use followed by an introduction of their electrical supply.Afterwards, within the second part of this chapter, the electrical and optical measuring sys-tems are explained including 2D photography with and without infrared filter, emission- andabsorption-spectroscopy. However, the complete data acquisition and evaluation leading toelectrode temperatures and particle densities are discussed later within the separate chapter4.

3.1 Investigated HID-lamps

For the investigation of the emitter-effect and for developing adequate measuring techniqueslike the BBAS, different kinds of HID-lamps have to be considered. Each of them allowsdifferent investigations. Within this work, three different HID-lamp types are used: TheBochum model lamp is used for fundamental investigations of the emitter-effect physics, thehigh-pressure sodium (HPS) lamp is used as a downscaled research model to solve problemsof a real, commercial HID-lamp and finally transparent, ceramic yttrium-aluminium-garnet(YAG) lamps are used to investigate optimal conditions for the emitter-effect by varying saltfillings.

3.1.1 The Bochum model lamp

The Bochum model lamp is a flexible, variable HID research lamp which was invented byNandelstadt and Luhmann [9, 7] and further developed by Lichtenberg, Dabringhausen andRedwitz. Constructed as an open system, the Bochum model lamp offers the unique possi-bility to change general lamp- and electrode parameters (e.g. electrode length and -diameter,electrode distance, gas type and -pressure) fast and easily, some even during operation. Onthe other hand the model lamp provides some feasibilities to perform in situ measurementsof the operating plasma arc for example by Langmuir-probes or ESV (electrode-sheath-voltage) measurements. Therefore, the model lamp is used within this work to investigatesome fundamental aspects of the emitter-effect by comparing pure tungsten electrodes to

42 3. Experimental setup

electrodes doped with emitter materials.Compared to commercial HID-lamps which usually have translucent ceramic discharge tubesand a variety of different salt fillings with overlapping physical effects, the Bochum modellamp can be operated under well defined and less complex physical conditions. For spectro-scopic investigations, for example, the model lamp can be equipped with a discharge tubeincluding an optical window which avoids scattering and distortion of the measured light.Additionally, the gas atmosphere introduced into the model lamp only consists of pure noblegases leading to a low light emission within the arc plasma and thereby a good visibility ofelectrode effects. Including some other advantages due to its flexibility, the Bochum modellamp is an ideal research HID-lamp wherein fundamental effects can be investigated sepa-rately as it will be done for the emitter-effect here.

A principle sketch of the Bochum model lamp is given in figure 3.1. The model lamp

stepping motorstepping motor

spindle drivespindle drive

positioning slidepositioning slide

turnable mountingturnable mounting

electrode feed-throughand glass mountingelectrode feed-throughand glass mounting

gas inlet/ outletgas inlet/ outlet

electrical connectionelectrical connection

electrode holderelectrode holder

electrodeelectrode

discharge glassdischarge glass

connection forcooling liquidconnection forcooling liquid

moveableelectrode holdermoveableelectrode holder

plasmaarc dischargeplasmaarc discharge

tungstenelectrodetungstenelectrode

sealingsealing

cooling liquidflowcooling liquidflow

inner coolingliquid tubeinner coolingliquid tube

Figure 3.1: Schematic sketch of the whole Bochum model lamp (left) and magnified sketch ofthe electrode holder with liquid cooling system (right)

consists mainly of a discharge glass tube which is mounted gastight into the lamp middleby means of metal mounting blocks including a gas in- and outlet. For the investigations

3.1. Investigated HID-lamps 43

within this work, the discharge tube is always equipped with an optical quartz window toavoid distortions and to gain a high optical measuring accuracy. From upside and downside,two electrode holders are inserted gastight and electrically isolated into the discharge vesselthrough the metal mounting blocks. The electrode holders can be positioned with a highmechanical accuracy by means of two stepping motors and linear drives. Accordingly, theposition of the arc discharge and also its length can be varied by this motorised system evenduring operation. As it is magnified on the right hand side of figure 3.1, the tungsten lampelectrodes are soldered into the electrode holders, thus they can easily be changed to obtaindifferent lengths, diameters and materials. To avoid an electrode overheating and to obtaindefined operating conditions, the electrode holder is equipped with a liquid cooling flow atthe electrode sealing point by means of an inner tube (cf. figure 3.1 on the right hand side).To operate the Bochum model lamp, a cooling/heating circulator (Julabo FP50-MV) isused pumping a silicon oil with a high thermal capacity through both electrode holders suc-cessively. By this cooling system, the electrode temperature at the sealing point may be keptat a defined temperature of T = 293 K during lamp operation. The gas volume is pumpedafter each lamp assembling by a two step pumping stage including a turbo molecular pump(Baltzers TMU 65) and a membrane roots pump (Vacuubrand MD-4T) down to a pres-sure of roughly p = 8 · 10−4 Pa. Afterwards, the discharge volume can be filled with noblegases like argon, krypton or xenon at an arbitrary pressure from p = 10 kPa up to p = 1 MPa.The electrical supply for the Bochum model lamp is realised by different electrical power am-plifying systems which will be explained separately later in this chapter.For the investigations within this work, model lamp electrodes with different lengths and di-ameters made of pure tungsten but also tungsten electrodes doped with a certain amount ofthorium emitter are used. The used pure tungsten electrodes are called “BSD-electrodes” or“AKS-electrodes” whereas the so-called “G7-electrodes”, the “G18-electrodes” and the “G30-electrodes”are doped with 0.7%, 1.8% and 3.0% thorium dioxide (ThO2) of the total electrodevolume, respectively. A more detailed description of the Bochum model lamp setup and itsoperating-/ measuring possibilities can be found in [7, 8].

3.1.2 The high-pressure sodium lamp (HPS)

The investigated high-pressure sodium (HPS) lamp is a research HID-lamp model of anoriginal, commercial 1000 W Philips HID system. It is equipped with a sapphire burner tofacilitate optical measurements at the electrodes. For sensitive measurements in a scientificlaboratory, the original 1000 W ceramic lamp design is difficult in its handling and not prac-ticable for optical measurements due to two main disadvantages:On the one hand, the original discharge tube, made of a translucent ceramic, is not useablefor optical observations since a sharp image of the electrode region is impossible to obtain asshown in figure 3.2(a). Instead, the original HPS-lamp was substituted for research purposesby a transparent sapphire tube which is shown in figure 3.2(b). In this photo, the electroderod is clearly visible through the sapphire glass material. Additionally, a tungsten coil aroundthe electrode rod is visible in figure 3.2(b) representing the emitter storage of the HPS-lamp(cf. chapter 2.3.3). Barium oxide in the form of a compound (Ba2CaWO6) is stored withinthe tungsten coil of the HPS-lamp electrode and diffuses along the electrode surface to itstip during operation.On the other hand, for a measurement of particle densities inside the lamp tube by ab-

44 3. Experimental setup

(a) (b)

Figure 3.2: Images of the electrodes in an original HPS-lamp burner made of a translucent ceramicmaterial (a) and in a specially prepared HPS research lamp burner made of transparent sapphirematerial for optical observations (b). The tungsten coil, storing the Ba emitter material, is clearlyvisible around the electrode in (b).

sorption spectroscopy, the 1000 W lamp itself has an undesired high luminous output powerreducing the measuring accuracy. Also the big dimension of the 1000 W lamp and its highpower electronic ballast are difficult to handle in a spectroscopic measuring setup.

However, for an investigation of the barium emitter transport within the HPS-lamp byBBAS, special research HPS-lamps have been designed within our research project withPhilips Lighting, NL. Therefore, the original 1000 W HID-lamp system was downscaled byPhilips to smaller lamps with a nominal power of PHPS = 140 W. All the lamp geometriesand parameters have been reduced by the same factor, so that the resulting HPS researchlamp is an exact model of the original 1000 W system. An image and a schematic magnifi-cation of the complete research HPS-lamp with a transparent sapphire discharge tube and astandard lamp mounting is given in figure 3.3. In contrast to the Bochum model lamp andthe YAG-lamps, the investigated HPS-lamps are always operated horizontally resulting in aslightly bended arc discharge.The 140 W research HPS-lamps shown exemplarily in figure 3.3 have an electrode diameter of900µm, a coil wire diameter of 300µm and are filled with a xenon pressure of p = 47.5 kPa asbuffer gas. Additionally, a certain amount of sodium and mercury is added to the lamp fillingto adjust the electrical operating parameters and the light output. During stable operationroughly 5 min after ignition, the pressure inside the HPS-lamp increases to approximatelyp = 0.4 MPa.The most important variation parameter for a systematic investigation of the Ba emitterdiffusion is the distance dcoil−tip between the end of the tungsten coil and the electrode tip.This parameter dcoil−tip is also indicated in figure 3.3. Hence, for the investigations withinthis work, several HPS-lamps with varying dcoil−tip are used which are equal in all otherparameters. An overview of the investigated HPS-lamps and their coil-tip distances is givenin table 3.1. A measured overview emission spectrum of a HPS-lamp can be found in theappendix of this work. Further details about the HPS-lamp and high pressure sodium lampsin general can be found in [64].

3.1. Investigated HID-lamps 45

gastight sealing tungstenelectrode

tungstenBa-coil

sapphire burner arc discharge

dcoil-tip

Mo-feedthrough

Figure 3.3: Real Photo and magnified schematic sketch of the special research designed HPS-lampwith a sapphire tube and a nominal power of 140 W. The distance dcoil−tip between the tungstencoil and the electrode tip is an important variation parameter and is therefore marked.

The hydrogen dispenser-/ getter HPS-lamp

Besides the presented standard 140 W research HPS-lamps, a special HPS-lamp is investi-gated within this work. The HPS-lamp has been designed to investigate the influence of ahydrogen contamination in the outer lamp envelope on the inner arc discharge. Therefore,the usual HPS-lamp as shown in figure 3.3 is extended by an additional pipe including ahydrogen getter and a hydrogen dispenser. The inner sapphire burner is exactly the same asused within the standard research HPS-lamps. A schematic sketch of this dispenser/ getterHPS-lamp is given in figure 3.4.The extension of the outer bulb, shown in figure 3.4, can be used to modify the H2-pressure

in the outer bulb externally during lamp operation. Therefore, an electrically controlled,

lamp name nominal power dcoil−tip coil wire diameter electrode diameter

6,2 140 W 1.4 mm 300µm 900µm

2b3 140 W 0.6 mm 300µm 900µm

3b3 140 W 1.0 mm 300µm 900µm

3b1 140 W 1.0 mm 300µm 900µm

4b1 140 W 1.4 mm 300µm 900µm

disp./getter 140 W 1.0 mm 300µm 900µm

Table 3.1: Overview of the investigated research HPS-lamps and their characterising geometryparameter dcoil−tip

46 3. Experimental setup

outer bulbsealing

outerlamp bulb

sapphireHPS burner

outer bulbextension

hydrogen getter hydrogen dispenser

Figure 3.4: Schematic sketch of the HPS-lamp extended by an additional pipe on the outer bulb.The pipe contains a hydrogen getter and a hydrogen dispenser to modify the H2-pressure in theouter bulb externally.

cylindrical oven is applied around the extension pipe to heat either the dispenser or the get-ter. If the dispenser is heated above a temperature of Tdisp = 423 K, the hydrogen pressure inthe outer bulb is significantly increased whereas a heating of the getter above Tget = 573 Kreduces the hydrogen pressure, respectively. By this technique, the hydrogen pressure in theouter HPS-lamp bulb can theoretically be varied by more than two orders of magnitude. Thehydrogen variation can be used to investigate its expected impact on the Ba density insidethe sapphire burner. Hence, by means of this H2 dispenser/ getter HPS-lamp it is possible tomeasure the diffusion time constants of hydrogen into the sapphire burner and the influenceof small amounts of hydrogen contamination on the Ba emitter-effect.

3.1.3 Ceramic HID-lamps (YAG)

Finally, as a third type of HID-lamps, yttrium-aluminium-garnet (YAG) lamps, also devel-oped by Philips Lighting, NL, are used to investigate the influence of different salt fillingson the gas-phase emitter-effect. By means of YAG-lamp measurements, the research resultsconcerning the emitter-effect can be stepwise and systematically transferred to commerciallamps by including more and more complex salt mixtures. In the end, a complexity of saltmixtures can be reached in the YAG-lamps which provides suitable research results for usingthe advantages of the emitter-effect in industrial HID-lamp development.Figure 3.5(a) shows a real YAG-lamp with an evacuated outer glass envelope and figure

3.5(b) shows the general design of the inner YAG-lamp burner as a schematic sketch. Theceramic yttrium-aluminium-garnet (Y3Al5O12) of these research burners is transparent andallows optical observations of the arc discharge and the tungsten electrodes which are im-possible in commercial HID-lamps made of translucent ceramics. Langenscheidt and Wester-meier developed a special mounting system of YAG-lamp burners inside the Bochum modellamp which substituted the outer glass envelope and facilitated further research possibilities[22, 25]. However, within this work, all YAG-lamps are operated inside their original outerglass envelopes. The evacuated outer glass envelope features several security functions, e.g.an UV-block and a YAG oxidation protection, and operates as a thermal insulator providingstable thermal operating conditions for the inner YAG burner.The electrodes of the YAG-lamps are made of pure tungsten without any emitter doping byother materials and have diameters of dE = 360µm, dE = 450µm or dE = 500µm. Theyare designed as straight rods without tungsten coils around and have an operating lengthinside the YAG burner of lE = 5 mm (from their ceramic sealing to the electrode tip). The

3.1. Investigated HID-lamps 47

tungstenelectrode

arc discharge

gastightsealing

liquid salt pool

7 mm

0.36-0.5 mm

5 mm

(b)(a)

Figure 3.5: Real image of a whole YAG-lamp (a) and magnified schematic sketch of the innerburner (b)

distance between the two electrode tips, determining the discharge arc length, is larc = 7 mm.However, each YAG-lamp is a unique, handmade research example and might have geometricvariations of a few percent. The inner discharge volume of the YAG-lamp burner amountsto VYAG = 0.314 cm3. It is filled with a mixture of argon and krypton at a pressure ofp = 30 kPa in the cold state. This initial gas filling is called “background-gas” and is neededfor the development of charged streamers during the cold lamp ignition [65]. As an essentialingredient, all investigated YAG-lamps contain 6 mg mercury which evaporates during lampoperation leading to an unsaturated operating lamp pressure of p = 1.98 MPa. The Hg con-tent is needed to achieve a considerably high electrical power input into the discharge arc. Hgatoms inside a plasma have a high “collision cross section” [51] leading to a so-called “ohmicplasma heating” by collisions which is indicated by the significant ohmic resistance of thearc plasma (cf. figure 2.2). However, mercury is a harmful environmental toxin and will bereplaced by other substances in the near future according to actual research perspectives [65].

To gain a significantly high light output mainly in the visible spectral range, YAG-lampscan be filled with different metal halides like commercial HID-lamps. Additionally, the ad-vantages of the gas-phase emitter-effect can be used and optimised inside the YAG-lampsby choosing the right metal halide ingredients and mixing ratios. In contrast to recent HIDdevelopments, the salts inside the YAG-lamp melt during lamp operation and evaporate toa certain, saturated partial pressure inside the discharge volume. Consequently, a liquid saltpool always remains at the bottom of the vertically operated YAG-lamps during their op-eration. Thus, the partial pressures of the evaporated gas fractions are determined mainlyby the temperature of the salt pool. This salt pool temperature, also called “cold spot tem-

48 3. Experimental setup

perature”, is influenced by changes of the lamp operating conditions (e.g. electrical powerinput) leading to problems like changes of the colour temperature and CRI during dimmingof HID-lamps.For the investigations within this work, dysprosium-iodide (DyI3) salts are introduced intothe YAG-lamps as emitter materials to be evaporated partly during lamp operation and toinduce a gas-phase emitter-effect on the electrode surface (cf. chapter 2.3.3). Additionally,other metal halide salt ingredients like sodium (Na) and thallium (Tl) are mixed into theYAG-lamp salts for different light emission purposes. An overview of the investigated YAG-lamps and their nomenclature is given in table 3.2. Some measured overview emission spectraof the most important YAG-lamp types are given exemplarily in the appendix of this work.

lamp Hg dose salt dose Na Tl Dy Ca Ce Th

Hg 6 mg 0 mg - - - - - -

Dy 6 mg 1, 2, 4 mg 0 mol% 0 mol% 100 mol% 0 mol% 0 mol% 0 mol%

Na 6 mg 1 mg 100 mol% 0 mol% 0 mol% 0 mol% 0 mol% 0 mol%

Tl 6 mg 1 mg 0 mol% 100 mol% 0 mol% 0 mol% 0 mol% 0 mol%

Th 4 mg 1 mg 0 mol% 0 mol% 0 mol% 0 mol% 0 mol% 100 mol%

DT 6 mg 2 mg 0 mol% 50 mol% 50 mol% 0 mol% 0 mol% 0 mol%

ND 6 mg 2 mg 50 mol% 0 mol% 50 mol% 0 mol% 0 mol% 0 mol%

NT 6 mg 2 mg 50 mol% 50 mol% 0 mol% 0 mol% 0 mol% 0 mol%

NTD1 6 mg 8 mg 83 mol% 7.2 mol% 9.8 mol% 0 mol% 0 mol% 0 mol%

NTD2 6 mg 8 mg 86 mol% 9 mol% 5 mol% 0 mol% 0 mol% 0 mol%

NTCC1 6 mg 8 mg 13.6 mol% 3.7 mol% 0 mol% 80.1 mol% 2.6 mol% 0 mol%

NTCC2 6 mg 8 mg 12.6 mol% 3.6 mol% 0 mol% 74.3 mol% 9.5 mol% 0 mol%

Table 3.2: Overview of the investigated YAG-lamps and their different salt fillings

3.2 Electrical setup

3.2.1 Electric power supply

As previously discussed, HID-lamps need a high voltage peak (several kV) to be ignited atthe start of their operation. On the other hand, operating HID-lamps show a characteristicnegative differential resistance behaviour resulting in an increasing arc voltage for a decreas-ing arc current. Thus, an HID-lamp cannot be operated at a standard electric power supplylike an incandescent or halogen lamp, it always needs a ballast in the electric supply circuit.There are different possibilities to realise an HID ballast which all contain a high voltageigniter and a current limiter, either by means of a magnetic inductor or by means of anelectronic circuit.The easiest way to operate an HPS- or YAG-lamp, which is used several times within thiswork, is the use of a Philips standard electronic ballast for commercial applications. The

3.2. Electrical setup 49

Philips commercial ballasts have a standard 230 V, 50 Hz sinusoidal input for the publicelectricity network and provide a triangular output voltage at a frequency of 200 kHz forthe HID-lamps by means of electronic conversion. The ballasts operate in a power controlledmode at P = 140 W for the HPS-lamps and P = 75 W for the YAG-lamps. Additionally,they are equipped with an internal igniter providing the high voltage peak for the lamp start.However, the Philips ballasts drive an HID-lamp always at a fixed standard operating pointand do not offer research possibilities like arbitrary voltages, frequencies or waveforms. Evenworse, the Philips ballasts perform an internal live-diagnostic of the HID-lamp performancefor commercial purposes, leading to a security switch-off if the lamp operation is influencedfor research purposes, e.g. by an additional heating or cooling.To overcome the limitations of a standard electronic ballast and to facilitate arbitrary lamp

signalgenerator

poweramplifier

exte

rnal

ign

iter model lamp

HPS-lampYAG-lamp

signalgenerator

poweramplifier

exte

rnal

ign

iter HPS-lamp

YAG-lamp

com

mu

tato

r Rv

signalgenerator

f= 0.1Hz- 2kHz

u= 10mV - 1Vi=

100m

A-

10A

2.5 kV

2.5 kV

U= 3V DC

U=

300V

DC

u=

300V

swit

ched

-dc

i= 100mA-1A switched dc

TTL-signalf= 0.1Hz - 10kHz

(a)

(b)

Figure 3.6: Schematic block diagram of the power amplifier setup to drive HID-lamps with ar-bitrary current waveforms. The power amplifier setup can be operated directly (a) or as a DCvoltage source with a switching commutator (b) to the HID-lamp.

currents, frequencies and waveforms, a power amplifier is used as a second possibility tooperate HID-lamps. Herein, the amplifier DCU/I 2250-28 from Feucht Elektronik op-erates as a current source for the HID-lamp and is controlled by an arbitrary voltage signal.The voltage signal is provided by means of a signal generator (Agilent 33120A) as a si-nusoidal, switched-dc or DC signal and amplified to a corresponding current output of thepower amplifier by a factor of 10. The rise time of the power amplifier output is short enoughto operate an HID-lamp directly without any pre-resistor in spite of its negative differentialresistance. With this electric driving system, an HID lamp can be driven theoretically upto a maximal current of 28 A or a maximal voltage of 400 V (always limited to a maximalamplifier output power of 2250 VA) from DC operation to AC operation with a maximalfrequency of 30 kHz. To ignite a YAG-lamp which is connected to the output of the power

50 3. Experimental setup

amplifier, an external capacitive igniter (Uignition ≈ 4.5 kV) is installed parallel to the lampwhereas HPS-lamps and the Bochum model lamp are ignited by means of a so-called elec-tronic “tesla-coil”. However, all ignition circuits are short-circuited or removed during stablelamp operation to avoid possible influences on electric measurements. A simplified block di-agram of the driving circuit by means of the power amplifier is given in figure 3.6(a).The simple power amplifier setup, presented in figure 3.6(a), can provide switched-dc oper-ating currents of 1− 10 A for the Bochum model lamp with a high accuracy even for higherfrequencies of several kHz. However, the power amplifier has problems to provide low currentslike 100−800 mA for HPS- and YAG-lamps above frequencies of 1−2 kHz in the switched-dcmode as it is designed for higher output powers. The edges of a switched-dc rectangle currentoutput start to be rounded for higher frequencies as shown in figure 3.7(a). Consequently,the introduced average power into an HID-lamp for low operating currents at high operatingfrequencies is significantly reduced if the lamp is operated directly at the amplifier output.Thus, for some special switched-dc measurements in the high frequency range (1− 10 kHz),an additional commutator provided by Philips Lighting is used within the power amplifiersetup. The extended commutator setup is shown in figure 3.6(b). The commutator consistsof a four-path semiconductor switching bridge which changes periodically the polarity of aconstant DC current at the input to a switched-dc current at its output. The constant DCvoltage with a maximum of Umax = 300 V is provided by the previously discussed poweramplifier. The frequency of the commutator is defined by a TTL-signal from a second signalgenerator. As a real switching of a DC current is much more effective than amplifying a givenswitched-dc signal, the commutator output current has a much higher rectangle accuracy forhigh frequency operation as shown in figure 3.7(b). However, the commutator setup can-not drive an HID-lamp with a negative resistance characteristic directly and an additionaltuneable resistor Rv is needed in the circuit to allow a constant current operation (cf. figure3.6(b)).

-1

-0.5

0

0.5

1

phase

HID

-lam

p c

urr

ent

/A

0 π 2π

500 Hz50 Hz

1 kHz2 kHz

-1

-0.5

0

0.5

1

phase

HID

-lam

p c

urr

ent

/A

0 π 2π

50 Hz2 kHz

(a) simple power amplifier setup (b) commutator setup

Figure 3.7: Switched-dc current distributions with iRMS = 800 mA for low and high frequenciesat a YAG-lamp. The YAG-lamp is operated directly at the output of the power amplifier (a) andby means of a commutator (b).

3.3. Optical diagonstic setup 51

3.2.2 Electric diagnostics

To characterise the electrical behaviour of HID-lamps, a measurement of the lamp currentand the lamp voltage has to be performed during operation. In contrast to commercial(Philips) standard drivers with a power control, the HID-lamps operated at the poweramplifier or commutator setup within this work are supplied with a fixed current waveform.The corresponding arc voltage is determined by the lamp operation itself and depends onthe time-variant electrical resistance of the lamp. Accordingly, a measurements of the lampcurrent is only performed to control the amplifier accuracy whereas the measurement of thelamp voltage gives important information about the current lamp status.The power amplifier (Feucht Elektronik DCU/I 2250) in use provides two monitoroutputs with a fixed reference to the actual current and voltage of the high power output.Thus, the lamp current and the lamp voltage of an operating HID-lamp at the power amplifiersetup (cf. figure 3.6(a)) can be measured time resolved with a high accuracy by means of afour channel digital oscilloscope (LeCroy Waverunner LT364). However, if the HID-lampis operated within the commutator setup or at the standard ballast, the current and voltagemeasurements cannot be obtained from the amplifier monitor output anymore. Therefore,an inductive current probe (Tektronix AM503B) and a high voltage differential probe(LeCroy ADP300) are applied directly to the HID-lamp socket. By combining the lampcurrent and voltage measurements, the electrically supplied lamp power is easily accessibleand can be calculated directly and phase averaged within the digital oscilloscope. With thehelp of this in-situ power monitoring, a “power controlled operation” of the HID-lamp can inprinciple be performed “by hand” by adjusting the matching current amplitude.The measured lamp voltage is an important research parameter of the HID-lamp includinginformation about the actual power losses in the electrode boundary sheaths. The lampvoltage can be interpreted for example in combination with a variation of the arc lengthwithin the Bochum model lamp. The resulting linear voltage dependency can be extrapolatedto an arc length of larc = 0 mm and provides thereby the electrode sheath voltage (ESV), thesum of the voltage drops of the cathodic and anodic electrode boundary sheaths [8, 22, 31].

3.3 Optical diagonstic setup

Besides the discussed electrical measurements, optical observations of the HID-lamps areperformed within this work to measure e.g. the electrode temperature and particle densitiesin the gas-phase. Therefore, different optical components, a spectrograph and CCD-camerasare used which are described below. The interpretation of the measured data as well as thecalculation of temperatures and densities are discussed in a separate chapter 4 subsequently.

3.3.1 Spectroscopy and imaging optics

The spectroscopic measuring system is the most important investigation tool concerningoptical HID-lamp research. A schematic overview of the whole spectroscopic setup used inthis work is given in figure 3.8. The investigated YAG- or HPS-lamp, which can also be

52 3. Experimental setup

focus lenses

rotateable Dove-prism

aperture entrance slit

spec

trogra

ph

CCD-camera

projection

HID-lampUHP-lamp

interferencefilter

SensiCam

focus lens

IR-filter

CCD-camera

SensiC

am

x

y

z

Figure 3.8: Schematic sketch of the spectroscopic measuring system including the investigatedHID-lamp, a backlight system, a projection system, a spectrograph with a CCD camera and anIR photography system with a second CCD camera

replaced by the model lamp due to a flexible mechanical mounting, is positioned in thecenter of the spectroscopic measuring system. According to figure 3.8, the spectroscopicmeasuring system consists of three main parts which are described briefly here: An LED/UHP backlight system, some imaging optics and an imaging spectrograph equipped with aCCD camera.

The LED and UHP backlight system

To perform broadband absorption spectroscopy measurements, an additional powerful lightsource is needed in the optical measuring path at a position in front of the investigated HID-lamp. The light beam of the backlight system has to pass the investigated plasma volumeinside the HID-lamp approximately parallel to allow theoretical absorption calculations. Ahigh radiance power output in a specific wavelength region and a temporarily very stableemission are the main requirements for absorption measurements with a high accuracy anda good signal-to-noise ration (SNR) leading to two possible realisations: A high power lightemitting diode (LED) with a suitable wavelength maximum or a filtered ultra-high-pressure(UHP) lamp.The LED solution is realised to obtain first barium absorption results in HPS-lamps bymeans of a 5 W high power LED (Philips Lumiled Luxeon Star). As it is shown in theresults chapter 5, the high power LED which is only available with a wavelength maximum atλLED,max = 530 nm and a FWHM of ∆λ1/2 = 35 nm, is not sufficient to measure Ba absorp-tion at λBa,abs = 553.5 nm with a high accuracy. The relative spectral emission distributionsof all available Lumiled high-power LEDs are given in the appendix of this work. However,in spite of their perfect temporal stability and modulation possibilities, the use of other highpower LEDs is not further intended within this work.

A much better backlight source realisation concerning the measuring accuracy and the SNRis the application of an ultra-high-pressure (UHP) HID-lamp. The UHP lightsource (Philips

3.3. Optical diagonstic setup 53

228L4) used in this work, originally designed for video projection systems, has an operatingpower of P = 120 W and a gas pressure (Hg-vapour) during operation of p = 15 MPa. Aspecially designed reflector is already equipped around the UHP-lamp so that a maximallight output power is emitted as a quasi-parallel light beam by the lamp. The UHP-lampis electrically driven by the original electronic circuit from a video projector which ensuresa very stable luminous output over time according to the high requirements in projectionapplications. It is operated with a 90 Hz switched-dc current, its amplitude amounts to 2.2 A.After a run-up phase of several minutes, the temporal variation of the luminous intensity islower than 2%. However, this temporal variation of the UHP intensity does not influence theaccuracy of the measurements as the UHP reference signal is obtained by a fitting procedureduring data evaluation (cf. chapter 5). It has to be mentioned that a statistical flicker of theUHP light intensity may occur every few seconds. But measurements which are deterioratedby flickering are removed from the results.An overview spectrum of the UHP light output is given in figure 3.9 wherein the mea-

300 400 500 600 700 8000

20

40

60

80

wavelength / λ

rela

tive

inte

nsi

ty /

%

DyBa Ce

Figure 3.9: Relatively calibrated overview spectrum of the UHP backlight source emission. Theinvestigated absorption lines for Ba, Ce and Dy are marked in the spectrum.

sured absorption lines for this work, namely Ba (λBa = 553.5 nm), Dy (λDy = 625.9 nm)and Ce (λCe = 577.4 nm) are marked. In principle, the UHP-lamp is a flexible setup whichcan be used for all absorption line measurements in wavelength regions where the UHPlight output is sufficiently broadened and the spectral UHP light power exceeds the lightemission of the investigated HID-lamp significantly. To avoid influences of the irradiationby the powerful broadband UHP-lamp onto the operation conditions of the HID-lamp, anoptical bandpass filter for every specific measuring wavelength is installed in the backlightsystem (cf. figure 3.8). These interference filters (50 mm Andover Corporation) havea center wavelength of λfilter1 = 553 nm, λfilter2 = 630 nm and λfilter3 = 577 nm to measureBa-, Dy- and Ce-absorption, respectively, and they all have a bandwidth of ∆λfilter = 10 nmFWHM. All filters provide a good transmission of the UHP-light around the specific mea-suring wavelength which can be further adjusted a few nm downwards by a rotation againstthe perpendicular light beam. They are essential to inhibit disturbances by the high UHP-lamp power at other wavelengths. Measured filter curves of all these interference filters can

54 3. Experimental setup

be found in the appendix of this work. The filtered UHP-light is focused by an achromat(f = 80 mm) to an area with a diameter of d = 10 mm around the measuring point. It pro-vides a largely homogenous radiance perpendicular to the axis of the investigated HID-lamp.According to figure 3.8, the beam emanating from the UHP-lamp is not strictly parallelwithin the measured path through the HID-lamp. But the angle of beam spread is so smalland the optical pathlength within the measured volume so short that the beam divergencecan be neglected within this work.

The imaging optics

An image of the investigated HID-lamp and with it of the radiation from the UHP backlightsource passing the HID-lamp is projected onto the entrance slit of a 1/4 m spectrograph.The spectrograph is explained in detail later. The imaging optics between the investigatedlamp and the spectrograph (cf. figure 3.8) consists of two achromats. The object and theimage planes are focal planes of the two lenses. Thus, light beams with punctual startingpoints in the object plane are parallel between them. The achromatic lenses with an equalfocus length of f = 310 mm produce a 1:1 image of the investigated HID-lamp discharge andelectrode onto the spectrograph entrance slit. The lenses have a diameter of d = 80 mm and atransmission factor of 97% within the measured, visible spectral range λ = 400−800 nm. Toavoid vignetting, the aperture angle is limited by an aperture stop. A so-called ”Dove”-prismis mounted between the two lenses. It offers the possibility of rotating the image around theoptical axis by an arbitrary angle. Thereby the axis of the vertically (model lamp and YAG-lamps) or horizontally (HPS-lamps) burning arc can be imaged parallel or perpendicularto the vertical entrance slit of the spectrograph. Accordingly, the one dimensional spatialresolution of the spectrograph can either be used to measure temperatures and densitiesalong the discharge arc axis into the plasma or perpendicular to the arc axis to gain radialresolved distributions after inverse Abel transformation (cf. chapter 4.2.1).

The spectrograph with CCD camera

The spectrograph used for the spectroscopic investigations within this work is a 1/4 m imag-ing type, namely a Chromex/Bruker 250is. Its optics are configured according to theCzerny-Turner principle. The spectrograph is equipped with an adjustable entrance slitwith a slit-width of 10− 2000µm leading to a one dimensional spatial resolution. The usualslit-width used in this work amounts to 25µm. The mirrors inside the spectrograph aretoroidal shaped mirrors focusing and collimating the light onto the output plane with a highprecision. Furthermore the spectrograph contains three different, customer chosen gratingswith line densities of 150, 600 and 1200 l·mm−1. The gratings are mounted onto a rotatabletriangle holder which is driven automatically to choose either a specific wavelength or tochange the grating in use. The grating with a line density of 1200 l·mm−1, which is usuallyused for the investigations here, leads to an optical resolution of the whole spectrographsystem better than ∆λ = 0.15 nm FWHM.As shown in figure 3.8, the generated spectrum is recorded at the output of the spectrographby a scientific 2D CCD camera (PCO SensiCam QE). The resulting two dimensional CCDimage with a size of 1040 × 1376 pixels represents the measured wavelength distribution in

3.3. Optical diagonstic setup 55

the horizontal direction and the spatial resolution along the entrance slit of the spectro-graph in the vertical direction. Accordingly, the CCD camera with a physical pixel size of6.45×6.45µm leads to a spectral dispersion of 0.016 nm/pixel (using the 1200 l·mm−1 grating)and a spatial resolution of 4.98µm/pixel along the spectrograph entrance slit. Consideringall internal magnifications inside the spectrograph and CCD camera setup, the measurablespatial expansion along its entrance slit amounts to ≈ 6.85 mm. The dynamic range of thecamera is 12 bit, equivalent to 4096 counts/pixel, with a high linearity and a low noise ratio(≈ 50 counts) due to an active cooling of the CCD-chip down to TCCD < 259 K by means ofa Peltier element. The exposure time texp of the camera can be varied between 500 ns and3600 s. The moment of exposure is controlled by an external TTL trigger signal with a highaccuracy, thus it is suited for high speed measurements. The SensiCam CCD camera has autilisable sensitivity in the wavelength range from 280 nm to 1100 nm whereas the thresholdranges are hardly detectable. A characteristic sensitivity curve of the PCO SensiCam QE isgiven in the appendix of this work. The particular pixels of the CCD camera can be logicallycombined in the horizontal and vertical direction by means of the so-called“binning mode”togain higher count levels leading to a higher accuracy or lower exposure times for a constantmeasuring radiance.Both components of the spectroscopic setup, the spectrograph and the corresponding CCDcamera, are mounted on a motorised mechanical cross-table to adjust either the measuringposition in the image or the image focus point with a high precision.

3.3.2 2D-Imaging of electrode and arc attachment

To investigate the type of arc attachment on the HID electrodes and to measure electrodetemperatures two dimensionally, a photographic imaging setup is used as a second opticalmeasuring system besides the spectroscopy. A schematic drawing of the 2D photographicsystem is given in figure 3.10. The photographic system consists mainly of a digital CCD

| 3 |

investigatedHID-lamp

filtermechanicalshutter

zoom objectivedigital CCD camera

Figure 3.10: Schematic sketch of the 2D digital photography setup consisting of a PCO CCDcamera, a zoom objective, a mechanical shutter and a filter.

camera, a zoom objective, a mechanical shutter and an optical entrance filter.The scientific CCD camera of the photographic setup is also a PCO SensiCam which equalsthe CCD camera of the spectroscopic system in its configuration but contains a differentCCD chip. Thus, the photographic CCD camera is also equipped with an active Peltier

56 3. Experimental setup

cooling, a high speed triggering system and provides a logical combination of CCD pixels ina binning mode as previously discussed. However, the CCD chip of this camera has a smallerresolution of 1280 × 1024 pixels and a different, generally lower sensitivity curve which isalso given in the appendix. The corresponding pixel size of this 2/3 ′′ CCD chip amounts to6.7× 6.7µm and the exposure times texp of the photography camera is limited to a range of100 ns to 100 ms.A zoom objective (Navitar Zoom 1−6265 with 2x adapter tube Navitar Zoom 1−6030)is mounted directly onto the camera entrance to image the investigated HID electrode ontothe CCD chip with an arbitrary magnification factor. The specifically matched magnificationfactor of the zoom lens can be varied continuously between M = 1.4x and M = 9x. Thismagnification range in connection with the CCD chip size facilitates either digital images ofthe whole electrode length (maximal 4.85 mm) or certain excerpts of interest within the imagein the region of the electrode tip. The focus length is not adjustable at the zoom objective,it is given as a fixed parameter by the adjusted magnification and the tube length of theobjective. Hence, to adjust the focus length for gaining sharp images, the whole photographysystem can be moved forward against the HID-lamp or backwards by means of a micrometerscrew.The entrance of the zoom objective can be mechanically closed very fast by means of a highspeed shutter system (Vincent Associates - Uniblitz VS14) adjacent to the objectiveas shown in figure 3.10. This electro-mechanical shutter is necessary to avoid the so-called“smear-effect” occurring during high speed photography of a source with a high luminositylike an HID electrode. The CCD chip inside the SensiCam performs an electronic shutteringafter its exposure which is not perfect resulting in a postexposure of certain pixels duringcamera read-out time. Thus, the mechanical shutter in front of the objective is closed directlyafter camera exposure, it blocks the luminous flux of the imaged source and avoids therebypostexposure of CCD pixels. The exact triggering of this shutter combined with the CCDcamera is a complex requirement which will be discussed within the next subchapter.The last component of the 2D photography system presented in figure 3.10 is a filter holderwhich can be equipped with different optical filters. On the one hand optical filters are neededto avoid a damage of the CCD camera by very high luminous powers from the imaged sources.Therefore, within this work, the luminosity of arc attachment images at the electrodes in HID-lamps is reduced by means of so-called“neutral density filters”(ND). ND-filters attenuate theluminosity of the incoming light by a certain damping factor approximately independently ofthe wavelength. Thus, the image which is recorded by the CCD camera contains the originallyemitted wavelength distribution but with a reduced intensity. On the other hand, the holdercan be equipped with bandpass filters to record only a specific wavelength range with theCCD camera. For the measurements of the 2D electrode temperature in this work e.g. anarrow infrared (IR) interference filter (50 mm LOT-oriel, λIR−filter = 890 nm, ∆λIR−filter =10 nm) is used in front of the camera. This filter allows direct measurements of the infraredradiation from an HID electrode which can be converted to absolute temperatures by meansof an absolute calibration (cf. chapter 4.1.2).

3.3.3 Absolute calibration

For a measurement and evaluation of absolute temperature or density values, the introducedspectroscopic measuring system as well as the photography system with IR filter have to

3.4. Triggering for phase resolved measurements 57

be calibrated in absolute radiative units. Therefore, a tungsten ribbon lamp (Osram WI17/G Nr. 19) is mounted instead of an HID lamp at the same position within the opticalmeasuring system. The tungsten ribbon lamp acts as a constant radiation standard, itsabsolute spectral radiance values LTRL, in the units W/(nm·m2·sr), for a DC current ofIcal1 = 10 A and Ical2 = 13 A are known from a reference calibration by Osram, Munich.To calibrate the spectroscopic system, the calibrated area of the tungsten ribbon is imagedby the original optical measuring system onto the entrance slit of the spectrograph andrecorded at a specific wavelength by the CCD camera. The tungsten ribbon lamp is movedperpendicular to the direction of the spectrograph entrance slit to calibrate the whole CCDarea stepwise. After this reference measurement of the tungsten ribbon lamp, a calibrationmatrix can be calculated transforming the output count values of the CCD camera (SensiCamQE) to absolute radiance values. In the consequence, any measurement performed by thespectroscopic setup at the calibrated wavelength can be converted to absolute radiance valuesby a simple, piecewise matrix multiplication while taking into account the exposure timetexp. The advantage of this absolute calibration procedure by a reference radiation sourceis that all influences like losses by lenses, prisms etc. within the optical measuring systemare automatically considered and corrected during calibration. However, if anything of thespectroscopic measuring system is changed or other wavelengths have to be measured, a newabsolute calibration is required.The calibration of the photography system with the zoom objective and the specific infraredbandpass filter is performed comparable to the previously explained procedure. The tungstenribbon is imaged by the 2D IR CCD camera system to calibrate the middle of the CCD chipin absolute units whereas the inhomogeneity of the CCD chip sensitivity is considered bya flat-field correction. To perform this flat field correction, the homogeneously radiatingcalibrated area of the tungsten ribbon is magnified by means of the zoom objective ontothe whole CCD chip area. Afterwards, the relative difference between each measured CCDpixel value and the CCD average pixel value is stored into a flat field correction matrix. Ofcourse, the resulting 2D calibration matrix is only valid for one specific zoom factor of thephotography system.

3.4 Triggering for phase resolved measurements

To perform reliable electrical and optical measurements at specific operation points duringa switched-dc period of an HID-lamp, a time synchronisation of the whole measuring setupis needed. Therefore, a trigger signal is generated by detecting the voltage-zero crossing(VZC) of the lamp voltage from the amplifier monitor output. Due to its fast rise time, thevoltage-zero crossing can be detected easier than the current-zero crossing (CZC). The VZCis detected with an accuracy of ∆tCC = 2µs by a comparator circuit developed by Langen-scheidt in [23] as other methods resulted in high uncertainties or time jitters. The triggersignal determining the starting point of the measurements is delayed with an accuracy of∆tDG = 1.5 ns by means of a scientific pulse delay generator (Stanford Research Sys-tems DG535) to an arbitrary point within the switched-dc period of the investigated HID-lamp. Accordingly, successively triggered optical and electrical measurements are performedat different shifted points within the switched-dc cycle to obtain phase resolved results aspresented in chapter 5. However, due to camera read-out times and other retardations during

58 3. Experimental setup

the measuring procedure, time resolved measurements at different points within the samephase are impossible. Thus, all measurements presented within this work are phase resolvedand never time resolved. This means, phase resolved measurements are always recorded atmoments within different, successive periods and shifted phases whereas the HID-lamp op-erating behaviour is assumed to be stable over the short measuring time.A block diagram of the whole trigger setup used within this work is given in figure 3.11.

delay-

generator&

zero crossing

camera ready? ( )BUSY

Triggerdelayed

trigger

( )ub

voltage-

monitorcomparator-

circuit

digital-

camera

power-

amplifier

(ZR)( )TR

(D 1)TR

PCtdelay

texp

image

only in the photography setup

Figure 3.11: Schematic block diagram of the trigger setup including a comparator circuit, a cameraready (BUSY) interlock and a delay generator.

More details about the triggering and its specific problems and requirements are discussedin [23, 31]. It has to be mentioned that the triggering of the photography CCD camera andthe corresponding mechanical shutter is of high complexity as mechanical delay times of themoving shutter blades have to be considered to effectively avoid the smear-effect.

The whole optical and electrical measuring setup presented within this work is controlled by ameasuring computer. Therefore, a LabView program uses a commercial GPIB-communica-tion bus system to program and read out the stepping motors, the oscilloscope, the camerasand the delay generator stepwise. Accordingly, most of the presented measurements, espe-cially the space resolved ones, are recorded automatically to achieve short total measuringtimes with stable lamp operating conditions and to avoid uncertainties by wrong manualadjustments.

59

4. Measuring methods and data interpretation

After the measuring setup hardware was introduced in the previous chapter, a detailed dis-cussion of the measuring data processing and the determination of the electrode temperatureas well as particle densities in the gas-phase of HID-lamps is given in this chapter. The chap-ter starts with the determination of the electrode temperature by means of a well knownspectroscopy procedure and by an alternative, recently adopted 2D IR photography method.The second part of this chapter deals with the determination of particle densities within thegas-phase of operating HID-lamps by emission- and absorption spectroscopy. By a correlationof the determined electrode temperatures and corresponding densities of emitter-materials,a systematic investigation of the physical effects and disturbing impacts on the emitter-effectcan finally be performed in the results chapter 5.

4.1 Measurement of the electrode temperature and powerloss

As already mentioned several times, the electrode temperature of HID-lamps is a key pa-rameter which is defined by an energy flux equilibrium during lamp operation. The electrodetemperature determines the lamp lifetime, its efficacy and the power load capability of acertain HID-lamp. Regarding the investigated emitter-effect in HID-lamps, the electrodetemperature is the dominant parameter to evaluate its efficiency as an active emitter-effectshould reduce the electrode temperature significantly under stable lamp operating condi-tions.To avoid influences e.g. by mechanical contacts, a contactless, so-called “pyrometric” methodis the optimal in-situ measuring technique for the electrode temperature of HID-lamps. Thepyrometry principle uses the emitted thermal infrared (IR) radiation of a solid state materialto determine its temperature by means of the temperature depending Planck radiation law(cf. equation 2.44). Therefore, the surface emissivity ε(λ, T ) of the measured material, whichis also temperature dependent, has to be known in advance or determined by a numericaliteration procedure from the measuring results.Commercial pyrometers are available and can be used with some restrictions for the investi-gation of HID electrodes as it was done by Redwitz and Dabringhausen [19, 8]. There existtwo possible commercial realisations of pyrometers which both provide absolute tempera-ture values at the output: On the one hand so-called “1λ-pyrometers” measure the emittedthermal IR radiation only at one wavelength. Thus, to provide absolute temperature results,an absolute calibration of the whole system is needed. On the other hand, a more handy

60 4. Measuring methods and data interpretation

realisation of commercial pyrometers are so-called “multi-λ-pyrometers”. These pyrometersmeasure the IR radiation of the investigated surface at two or more different wavelengthsduring one measuring cycle. In the consequence, the nonlinear temperature dependency ofthe Planck function can be evaluated at different wavelength leading to absolute tempera-ture results without absolute calibration of the pyrometric measuring system. However, theuse of commercial pyrometers produces several problems and limitations for the intendedinvestigations which are discussed by Reinelt in [31]. Therefore, a spectroscopic and photo-graphic realisation of pyrometric temperature measurements will be described and used inthe following.

4.1.1 Spectroscopic electrode temperature measurement

A well established and proved spectroscopic electrode temperature measuring method wasdeveloped by Reinelt and Langenscheidt in [66, 22]. Therefore, a picture of the operatingHID electrode is imaged onto the entrance slit of the spectrograph by means of the opticalsetup described in chapter 3.3.1. Usually, the upside electrode is investigated within all opti-cal measurements of YAG-lamps and in the Bochum model lamp as the transmission of thelamp burner material is not influenced by dirt e.g. from the salt pool in YAG-lamps. Theelectrode picture is imaged with a parallel orientation to the electrode entrance slit as shownschematically in figure 4.1 (a). By means of this parallel electrode-slit orientation, the one

spatial axis spatial axis

projection ofplasma & electrode

spectrographentrance slit

(a)

spectrographentrance slit

(b)

Figure 4.1: Possible orientations of the imaged HID electrode and plasma onto the spectrographentrance slit: parallel to the slit orientation (a) and perpendicular (b)

dimensional spatial resolution of the spectrograph can be used to record the distribution ofthe electrode thermal radiation along its axial direction from the electrode tip to the bottom.As the thinnest measured electrodes have a diameter of dYAG = 360µm, the spectrographentrance slit is opened to a slit width of dslit = 100µm for all temperature measurements

4.1. Measurement of the electrode temperature and power loss 61

leading to a sufficient SNR without the risk of misleading results due to measurements per-formed partly besides the imaged electrode rod.The electrode thermal radiation has to be measured preferably in the far infrared wavelengthrange to determine its temperature by pyrometry with a high accuracy. On the other hand,the recording CCD camera adjacent to the spectrograph is limited in its infrared sensitivity.Thus, as a suitable compromise, the axial distribution of the electrode thermal radiation ismeasured at λT = 718 nm spectroscopically. Another essential precondition for the choice ofthe right IR measuring wavelength is the absence of any disturbing spectral line radiationfrom the arc discharge. This precondition is fulfilled by all investigated lamps within thiswork, no disturbing plasma line emission was found around the 718 nm measuring wavelength,but it has to be checked always again before investigating HID-lamps with other fillings. Toavoid disturbances by plasma line radiation with a high radiance around λ = 359 nm, whichcan be reflected within the spectrograph to the measuring wavelength by 2nd order angles,all radiation below λ = 600 nm is suppressed by an additional edge filter mounted at thespectrograph entrance slit.A measured and calibrated spectrum of the thermal radiation of a YAG electrode is ex-

wavelength / nmλ

posi

tion a

long s

pec

. en

tran

ce s

lit

/ m

m

705 710 715 720 725 730

0

1

2

3

4

5

0

0.5

1

1.5

2

x 102

abso

lute

rad

iance

L/

Wm

sr

-2 -1

λ718

,

cut-out at 718nm

6

position along spec. entrance slit / mm

1 2 3 4 5 60

electrode tip

Figure 4.2: Example of a spectroscopic measurement of the electrode broadband emission (left)from a YAG lamp electrode (NTD1, 450µm, 500mA, 1kHz switched-dc)) between λ = 705 nmand λ = 730 nm and a one dimensional cut-out at the reference wavelength λ = 718 nm plottedalong the electrode axis to determine its temperature distribution (right).

emplarily given in figure 4.2 (left). The spectrogram shows the spectral distribution in de-pendence on wavelength λ on the horizontal axis and the spatial radiance distribution alongthe spectrograph entrance slit in the vertical direction. Accordingly, the continuous thermalradiation of the measured electrode is shown spectrally resolved in the lower part of thespectrogram of figure 4.2 (left) whereas the upper part of the image represents the plasmaemission from the arc discharge adjacent to the electrode. A spatial distribution of the mea-sured radiance is cut out from the spectral image exactly at λT = 718 nm as marked withinthe image by a red arrow. This spatial distribution of the absolute radiance is plotted infigure 4.2 on the right-hand side, the spatial axis is also marked by a red arrow. The expo-nential course of the IR radiance Lλ718, starting at the left side in figure 4.2 (right), representsa typical qualitative distribution of the corresponding axial electrode temperature from itsbottom to the electrode tip. The maximum within this graph indicates the electrode tip. The

62 4. Measuring methods and data interpretation

right part of the example graph, after the electrode tip, is caused by undefined radiation fromthe adjacent arc plasma and is therefore cutted off before calculating electrode temperaturevalues.

In a last step, the extracted absolutely calibrated thermal radiation distribution of the elec-trode Lλ718 is converted into absolute temperature values by means of the Planck radiationlaw. Therefore, equation 2.44 is rearranged to determine the temperature values T while usingLλ718 as an input parameter according to

T =c2

λ·[ln

(ε(λ, T ) · c1

πλ5Lλ718

)+ 1

]−1

. (4.1)

Herein, the Planck-constants c1 and c2 are used similar to the definition given for equation2.43. The wavelength λ in equation 4.1 is not a variable value anymore as it is fixed toλ = 718 nm by the previously discussed measuring and data extraction principle.However, the surface emissivity ε(λ = 718 nm, T ) of tungsten still depends on the unknownelectrode temperature T . Thus, besides the absolute electrode temperature result, a valuefor the emissivity ε718(T ) has to be determined during data processing by an iteration loop.Therefore, an estimated temperature value within the expected result range is used as astarting parameter Ti=1 for ε718(T ), then a temperature result for the electrode is deter-mined according to equation 4.1 which is used in a further iteration step as a new startingparameter Ti=i+1 for ε and so on. Of course, this iteration procedure has to be performed

1500 2000 2500 3000 3500

0.38

0.39

0.4

0.41

0.42

0.43

0.44

surface temperature / K

rela

tive

emis

sivi

ty

ε

λ = 718nm

Figure 4.3: Relative tungsten emissivity ε at λ = 718 nm for different temperatures T = 1300−3700 K, taken from [67]

for every point of the varying electrode temperature distribution independently. The iter-ation for ε is very stable even for estimated starting temperatures far away from the realresult and converges usually with a sufficient accuracy of ∆T < 1 K within three iterationsteps. In principle, the iteration of ε718(T ) can be avoided by considering the temperaturedependence of ε718(T ) directly during calculation of the corresponding calibration matrix.

4.1. Measurement of the electrode temperature and power loss 63

Accordingly, an ε calculation during calibration combined with an evaluation of equation4.1 provides a calibration matrix which can directly convert the measured camera counts toabsolute electrode temperature values. This concept might be a useful implementation forfuture measurements.Correct values for the nonlinear surface emissivity ε(λ, T ) of tungsten in dependence onwavelength and surface temperature are given in literature. Nandelstaedt and Dabringhausenperformed an extensive literature enquiry [9, 8] concerning reliable values for the tungstenemissivity. This enquiry resulted in the application of values given by De Vos in [67] whichare the best suitable ones for the investigated temperature ranges of HID electrodes. Theused tungsten emissivity values ε718(T ) from De Vos for a fixed wavelength of λ = 718 nmare plotted in dependence on typical HID electrode temperatures in figure 4.3.It has to be mentioned that the accuracy of the measured electrode temperatures is mostlyinappropriate in the vicinity of the electrode tip. This problem is a general pyrometric prob-lem caused by uncertainties of the tungsten emissivity ε due to the hot and rough electrodesurface at its tip. Causes and solutions of this accuracy problem at the electrode tip willtherefore be discussed more generally at the end of the next chapter. However, the presentedpyrometric electrode temperature measurement by means of the spectrograph setup is rathercomplex and provides no information about the two dimensional temperature distribution onthe electrode surface. Especially in the case of a constricted arc attachment on the electrode,this measurement cannot distinguish whether the disturbing spot arc attachment is locatedwithin the measured area of the electrode or not. Hence, a second, alternative pyrometric 2Dtemperature measurement was adopted from Philips Lighting and further optimised withinthis PhD work as presented in the following.

4.1.2 2D electrode temperature measurement

The alternative two dimensional electrode temperature measurement is also performed py-rometrically but by means of the previously introduced photography setup equipped witha narrow optical bandpass filter in the infrared wavelength region. The concept is adoptedfrom a comparable measuring system by Philips Lighting and provides at the end a twodimensional image of the absolute electrode surface temperature. These temperature imagesallow a classification of the temperature distribution homogeneity and disturbing influencesby a constricted arc attachment which is not possible within the 1D results of the spectro-scopic realisation.

To record a 2D infrared image of the operating electrode, it is imaged onto the CCD cameraphotography system (cf. chapter 3.3.2) through an interference bandpass filter (AndoverCorporation 890FS10-50) with a diameter of d = 50 mm, a center wavelength λ = 890 nmand a FWHM of 10 nm. A measured filter curve of this IR-filter is given in the appendix.Again, the measured IR wavelength range has to be free of strong disturbing line radiationfrom the plasma arc which was approved by spectroscopic tests for all investigated HID-lamps. The resulting image of the infrared radiance of the HID electrode LIR890 is calibratedin absolute units by means of the tungsten ribbon lamp and a flat-field correction of theCCD sensor area.The rest of the 2D temperature measurement, the calculation of absolute temperatures fromthe infrared radiance values, is performed similar to the spectroscopic method. The absolute

64 4. Measuring methods and data interpretation

temperatures are determined for each pixel within the 2D image separately by evaluatingequation 4.1. However, the unknown surface emissivity of tungsten ε890(T ) is again calcu-lated by means of the previously described iteration procedure within this 2D temperaturemeasurement. Usually, it converges to an accuracy of ∆T < 1 K of the temperature resultsafter three iteration steps but its numerical calculation lasts a relatively long time of several10 s as it has to be performed in two dimensions now. Hence, for future applications of the2D electrode temperature measurements, a direct conversion from recorded CCD cameracounts to absolute temperature values by considering the tungsten emissivity ε already inthe calibration leads to a significant reduction of calculation times.A resulting two dimensional temperature image is exemplarily given for a 500µm YAG-lamp

0 1 2 3 4 5 6 70

1

2

3

position / mm

po

siti

on

/ m

m

evaluated stripe area

0

1000

2000

3000

abso

lute

tem

per

atu

re /

K

Figure 4.4: Example result of a 2D temperature measurement of an NTD1 YAG-lamp electrodein absolute units. Parameters: delectrode = 500µm, iRMS = 500 mA, f = 1 kHz, switched-dc. Theevaluated stripe area for an averaged one dimensional temperature distribution outside of thespot attachment disturbances is indicated by black lines.

(NTD1) electrode in figure 4.4. To obtain the electrode tip temperature Ttip or to calculatethe electrode power losses, a one dimensional temperature distribution along the axial elec-trode direction is extracted from the 2D image as indicated in figure 4.4 by black lines.Therefore, an average temperature distribution value is calculated from a 50µm thin stripearea (cf. figure 4.4) along the electrode axis. Due to the existing 2D temperature image,the stripe area can be positioned manually within the homogenous temperature distributionpart of the electrode surface outside of disturbances by spot mode attachments. Besides itssimplicity, this is the main advantage of the introduced 2D electrode temperature measure-ment compared to the spectroscopic pyrometry measurement. Furthermore, the type of arcattachment can be characterised directly from the 2D temperature image which is often thereason for unexpected electrode tip temperature results.

4.1. Measurement of the electrode temperature and power loss 65

4.1.3 Reproducibility and errors of the pyrometric temperature mea-surements

The presented spectroscopic and photographic electrode temperature measuring methodsexhibit a relatively high accuracy as their values are measured directly and no geometri-cal conversion, e.g. an inverse Abel transformation (cf. chapter 4.2.1) is required. The errorranges of the measurement are mainly determined by the accuracy of the absolute calibrationmultiplied by the accuracy of the CCD camera noise level in both cases. Also the accuracyof the values for the tungsten emissivity ε(λ, T ), given by De Vos [67], influences the totalerror of the absolute temperature result. However, a precise information about the error ofthese emissivity values cannot be obtained, but their influence on the total result is small(cf. figure 4.3).The uncertainty of the tungsten ribbon calibration lamp is declared by the calibration in-stitute and manufacturer Osram to a value of 1.6% - 2%. The PCO SensiCams used inthe spectroscopic setup and in the photography setup have a noise level of 50 counts leadingto a relative error of 1.2% for a full range image (4095 counts) and of 2.4% for a half rangeimage (2047 counts). Thus, the SNR and therewith the level of accuracy of the CCD camerasdecreases with shorter exposure times or reduced image luminance. Hence, a total error of5% has to be considered for the electrode temperature measurement with the spectrographsetup and also for the 2D photography setup.It is worth mentioning that the accuracy of the usually desired global electrode tip temper-ature result Ttip is further limited by influences of a constricted arc attachment. In conse-quence, the measured electrode tip temperature Ttip can be artificially high due to a measur-ing position at the spot arc attachment zone compared to the global tip temperature whichshould be observed (see figure 4.4 at the electrode tip). This fact leads to a higher measuringerror of the spectroscopic setup referring to the global electrode temperature as its exactmeasuring position along the electrode rod cannot be detected from the recorded spectralimage. However, the influence of this problem is further reduced by a fitting of theoreticallysimulated temperature distributions presented in the following chapter.

To check the error and the reproducibility of both absolute temperature measuring setups,reference temperature results at the tungsten ribbon lamp are measured and compared intable 4.1. Therefore, the described 2D electrode temperature measuring setup is first cali-brated in absolute radiance values by means of the tungsten ribbon lamp. Afterwards, thetungsten ribbon lamp is measured once again and its temperature is determined accordingto Planck’s law by means of the pyrometry principle which was explained in the previouschapter. Besides its absolute radiance values, the absolute tungsten ribbon temperatures of

current I Treal Tspec.−setup ∆Tspec.−setup T2D−setup ∆T2D−setup

10 A 2098 K 2076 K 1.04% 2061 K 1.76%

13 A 2497 K 2468 K 1.16% 2445 K 2.08%

Table 4.1: Reference measurements of the absolute tungsten ribbon temperature to prove theaccuracy of both electrode temperature measuring setups

this calibration lamp Treal are known for I = 10 A and I = 13 A and given in table 4.1.

66 4. Measuring methods and data interpretation

Thus, both measuring methods can be applied to the calibrated tungsten ribbon and theirdeviation from the known absolute temperature can be analysed and compared. Accordingly,as shown by the results in table 4.1, the spectroscopic temperature measuring setup has areal error of roughly 1% and the 2D photography setup of 2% which is much better than thetheoretically predicted accuracy of 5% in both cases.However, the comparability of absolute electrode temperature results obtained by the pre-sented 2D photography setup in Bochum to a similar method developed and applied byPhilips Lighting Eindhoven is verified by a second reference measurement. Therefore, theelectrode tip temperature of the same pure mercury YAG-lamp with an electrode diameterof 360µm is measured for different currents at a frequency of f = 120 Hz switched-dc inEindhoven and Bochum successively. The resulting 2D electrode temperature images as wellas the electrode tip temperatures measured by 2D photography and spectroscopy are plottedand compared to the Philips results in figure 4.5. The figure shows that the variation of

5.3%5.3%5.1%4.7%4.5%

3016 K3109 K3186 K3254 K3317 K

2.9%3.1%2.8%2.4%1.9%

2941 K3039 K3112 K3177 K3228 K

3107 K3208 K3278 K3334 K3381 K

ΔTtip Bo - Phspec

Ttip Bo, 2D

ΔT Bo - Phtip 2D

Ttip Bo, spec.

picture

Uni Bochum

Ttip Philips

picture

Philips

300 mA400 mA500 mA600 mA700 mACurrent

Figure 4.5: Comparison measurements of the electrode tip temperature Ttip of an Hg YAG-lampby spectroscopy and 2D photography in Bochum and by 2D photography within Philips LightingEindhoven. Parameters: dE = 360µm, i = 300− 700 mA, f = 120 Hz switched-dc

the measured absolute electrode tip temperatures Ttip has a maximum of 5.3%, whereas themaximal deviation between the comparable 2D measurements in Bochum and at PhilipsLighting Eindhoven amounts to only 3.1%. This excellent agreement of the Bochum andPhilips results shows additionally, that the influence of a varying emissivity ε on the finaltemperature results is small as the electrode temperature measurement within Philips is eval-uated with a fixed emissivity of ε = 0.3. The measurements in Bochum are performed laterthan the Philips measurements, thus wall blackening of the YAG-lamp can be the mainreason for the obtained differences in the absolute electrode tip temperature values. A lowerabsolute temperature value within all measurements in Bochum proves this assumption.In conclusion, the photographic 2D electrode temperature measurement is developed as analternative to the well established spectroscopic measurement. A high measuring accuracy

4.1. Measurement of the electrode temperature and power loss 67

of the 2D method is proved, thus it is preferred for all electrode temperature measurementspresented within this work.

4.1.4 Calculation of the electrode power loss

The electrode power loss Ploss, coupled into the electrode by the plasma, can be deducedcomparing the measured electrode temperature distributions with a simulation of the elec-trode heat balance which is briefly described in this chapter. Additionally, the real electrodetip temperature Ttip can be observed by this simulation with a reliable accuracy. As previ-ously mentioned, measuring results in the vicinity of the hot, rough electrode tip surface leadto wrong, artificially increased tip temperature values due to uncertainties of the tungstenemissivity ε(λ, T ).To simulate an electrode temperature profile Tsim(zi) which can be fitted to measured tem-perature distribution results Tmeas,i, equation 2.37 is evaluated numerically. According toNandelstadt and Dabringhausen [9, 8], a stationary electrode heat balance is assumed whichhas a rotational symmetry and no radial dependency of the electrode temperature T along theelectrode rod. Including theses simplifications, equation 2.37 can be written for a simulatedtemperature Tsim as

πR2E

d

dz

(κ(Tsim)

dTsim

dz

)= 2πREqrad(Tsim) (4.2)

withqrad(Tsim) = σSBεtot(Tsim) · (T 4

sim − T 4amb)

wherein Tamb is the ambient temperature adjacent to the electrode surface. Equation 4.2is a nonlinear partial differential equation of 2nd order. It can be solved by means of twocorresponding boundary conditions, namely Ttip for the electrode tip temperature and Tbot

for the electrode temperature at its bottom point. The resulting simulated electrode heatbalance is fitted to the measured temperature distribution by a numerical variation of Ttip

and Tbot. Therefore, a least square algorithm is evaluated by means of a Matlab computerprogram according to

ST =nν∑i=1

(Tmeas,i − Tsim(zi))2 → min. (4.3)

As a result of this numerical simulation, the values Ttip and Tbot are determined consistentlywith the measured electrode temperature distribution. An example for a simulated electrodetemperature profile of an NTD1 YAG-lamp electrode and the corresponding measuring datais given in figure 4.6. The simulation result plotted in figure 4.6 is only fitted to the shownmeasuring data within the region of confidence. At the bottom of the electrode (z < 1.1 mm),the electrode sealing and bending of the YAG burner leads to uncertain results, thus thisregion is excluded from the simulation fitting procedure. A similar problem arises often at theelectrode tip as it was already mentioned previously. Due to unknown values of the tungstenemissivity ε caused by the hot and rough electrode surface in the vicinity of the electrodetip, the measured electrode temperature results show a steep increase towards the electrodetip (z > 4.7 mm) as shown in figure 4.6. This temperature increase is artificial and does notexist in reality, thus, the electrode tip temperature Ttip provided by the simulation resultsis a much more reliable value. Accordingly, all measured electrode tip temperature resultspresented within this work are determined by simulation results to ensure a high accuracy.

68 4. Measuring methods and data interpretation

0 1 2 3 4 51600

1800

2000

2200

2400

axial position along electrode / mmz

TEMPERATURES:

Ttip = 2361.5 K

Tbot = 1615.2 K

POWER (Sim.):

Prad : 1.12 W

Pcond: 2.72 W

Ploss : 3.84 W

elec

trode

tem

per

ature

/ K

Tel

confidence region

T

T

-simulation-measurement

Figure 4.6: Example results of a measured and extracted 1D axial temperature distribution alongan NTD1 YAG-lamp electrode and corresponding simulation of the electrode heat balance in-cluding the numerical power simulation results. Parameters: delectrode = 500µm, iRMS = 500 mA,f = 1 kHz, switched-dc. The region of confidence is indicated by black lines.

For the measurement and simulation presented in figure 4.6, the numerical simulation resultsfor Ttip and Tbot are given in the graph.However, by evaluating the presented simulation results in figure 4.6 it is even possible todetermine the electrode power loss Ploss. According to equation 2.40, the power loss Ploss isgiven by the sum of the radiative power losses Prad and the conductive power losses throughthe electrode end Pcond. Both parts of the total power loss can be deduced from the simulationresults of the electrode heat balance. Therefore, equation 2.42 is evaluated according to

Prad = πR2EσSBεtot(Ttip,sim)(T 4

tip,sim − T 4amb) + 2πRE

∫ lE

0

σSBεtot(Tsim)(T 4sim − T 4

amb) dz (4.4)

and the conductive losses are calculated according to equation 2.41 as

Pcond = πR2Eκ(Tbot,sim)

dTsim

dz

∣∣∣∣z=0

. (4.5)

Finally, the numerical simulation of the electrode heat balance leads to the electrode tiptemperature Ttip and the total electrode power loss Ploss as given exemplarily in figure 4.6.The presented simulation of the electrode temperature distribution and power loss calculationis based on the assumption of a stationary electrode heat balance which is only realisedperfectly during DC lamp operation. Reinelt showed in [31] that this assumption is still validfor AC operation of thin and short HID electrodes especially in the case of a switched-dcoperation. The heating time constants of thin electrodes like in the investigated YAG-lampsare much shorter than the period times of the operating frequency up to several kHz, whereasin thick model lamp electrodes the heat capacity can have a major influence on the electrodeheat balance. This influence of the heat capacity is even more pronounced for sinusoidal lampoperating currents [31]. However, for the investigated HID-lamps and operating parameters in

4.2. Spectroscopic particle density measurements 69

this work, the influence of the electrode heat capacity is small and the previously introducedconcept of a stationary heat balance is always applied and simulated during data evaluation.The idea of a time dependent power loss calculation, introduced by Reinelt, will be discussedbriefly in the next chapter.

Time dependent power loss calculation

Compared to stable DC operation, the electrode temperature varies periodically during ACoperation due to different power loads during the anodic and cathodic electrode cycle. Ac-cordingly, for thicker electrodes like in the Bochum model lamp, the assumption of a station-ary heat balance is no longer valid, especially in the case of sinusoidal operation. Equation2.37 has to be complemented by a time dependent term considering the electrode heat ca-pacity according to

πR2E

∂

∂z

(κ(T )

∂T

∂z

)= 2πREσSBεtot(T )T 4 +R2

Eπρmcp(T )ω∂T

∂ϕ. (4.6)

Herein, ρm is the mass density of the tungsten material and cp is the tungsten heat capacity.Reinelt developed a mathematical method to solve equation 4.6 and to fit a simulated elec-trode temperature distribution to measured results. Therefore, he did not vary the tempera-ture parameter as shown before, but he fitted the axial position z to the corresponding Tel(z)value. By this method, Reinelt was able to determine even the time dependent heat capac-ity fraction of an HID electrode numerically. As the time dependency of the electrode heatbalance does not have a significant influence on the investigations in this work, a detaileddiscussion of the time dependent evaluation method is not further intended. Details can befound in [31].

4.2 Spectroscopic particle density measurements

Besides reliable electrode temperature results, precise measurements of the particle densitiesof emitter materials in the gas-phase in front of the electrode are needed to investigate theemitter-effect in HID-lamps. Two possible spectroscopic realisations of this particle densitymeasurement will be introduced in this chapter: A well established emission spectroscopymeasurement developed by Langenscheidt and Westermeier in [22, 25]. This method is basedon the determination of absolute spectral line emission coefficients leading to absolute den-sity values according to equation 2.16. On the other hand, a new broadband absorptionspectroscopy measurement (BBAS) will be introduced as an alternative to emission measure-ments. This investigation of absolute absorption profiles leads to some important advantagesconcerning its application to HID-lamps and its accuracy of the density results. As the BBASmethod forms the thematic focus of this PhD work, it will be discussed in more detail thanthe emission spectroscopy, here.

First of all, to record spectroscopic images of the arc plasma and the gas-phase withinthe HID-lamp, an image of the arc discharge adjacent to the electrode is imaged onto theentrance slit of the spectrograph. Usually, the arc discharge is imaged perpendicular to the

70 4. Measuring methods and data interpretation

spectrograph entrance slit as shown in figure 4.1(b). Thereby, the one dimensional spatialresolution of the spectrograph can be used to resolve the projection of the radial distributionof the cylindrical arc discharge along the entrance slit. To gain a high spectral resolution, theentrance slit of the spectrograph is opened to a slit-width of only dslit = 25µm for all densitymeasurements in contrast to spectroscopic electrode temperature measurements. By meansof the moveable cross table mounted underneath the spectroscopic setup, the spectroscopicmeasuring position is adjusted to a distance of 125µm in front of the electrode tip. Due to ge-ometric limitations of the imaging setup it is not possible to measure the plasma arc emissioncloser to the electrode tip without the risk of measuring the continuous radiation from theelectrode itself. These standard spectroscopic adjustments for density measurements mightbe varied for some cases within the HPS-lamp and also for a few YAG-lamp measurements,it will be mentioned individually in the corresponding result chapter.All spectroscopic images which are recorded for emission or absorption density measurementsby the spectrograph are processed similarly within the first steps: At first, a constant noiseratio of 50 counts is subtracted from the raw image. This value represents the characteristic,averaged noise level of the PCO SensiCam and is obtained by dark image measurements.Afterwards, the recorded spectral images are corrected by their exposure times. The refer-ence exposure time for this evaluation step is texp = 1 ms. Finally, the corrected spectralCCD images are calibrated in absolute radiance values according to the procedure given inchapter 3.3.3.

However, the recorded and calibrated spectral images with a 1D spatial resolution perpen-dicular to the discharge arc do not reflect the real radial distribution through the cylindricalarc. They reflect a one dimensional projection of a two dimensional, thin disc from thearc discharge integrated along the line of sight according to equation 2.21. But, if the arcdischarge has a cylindrical symmetry and the plasma can be treated as optically thin, therecorded one dimensional projection includes enough information to evaluate the real radialdistribution inside the arc discharge. The mathematical relation of this well known problemin arc discharge research is given by the so-called“Abel transformation”and will be discussedin the next chapter.

4.2.1 Inverse Abel transformation

The Abel transformation and its inverse give the mathematical relation of a cylinder sym-metric light emitting source f(r) = f(x, y), like an arc discharge in an HID-lamp, and itsone dimensional projection h(x). The projection represents an optical integration of severalinfinitesimal light source points along the line of sight s(y1, y2) perpendicular to the axis ofthe cylindrical source. In the case of an HID-lamp, the measured spectral intensity projec-tion Iλ(λ, x) from the spectrograph is correlated with the local emission coefficient at aninfinitesimal light source point ελ(λ, xi, yi) according to

Iλ(λ, x) =

∫ y2

y1

ελ(λ, x, y)dy. (4.7)

A schematic sketch of this relation including the definitions of the spatial directions is givenin figure 4.7.If the emitting source function f(r) has a cylindrical symmetry and a defined border,

4.2. Spectroscopic particle density measurements 71

x

y

f(r)

x

position alongspectrograph entrance slit

h(x)

s

y1

y2

1D-projection

(c)(b)(a)

Figure 4.7: Schematic sketch of the geometrical relation and of axis definitions for the Abeltransformation from a cylindric arc discharge (a) over a 2D disk cut-out with a radial distributionf(r) (b) to its measured one dimensional projection on the spectrograph entrance slit h(x) (c).

meaning that f(r > R) = 0 with R 6=∞, its parallel projection h(x) is determined by

h(x) =

∫ y=+√R2−x2

y=−√R2−x2

f(√x2 + y2)dy ∀x : 0 ≤ x ≤ R. (4.8)

By means of an appropriate substitution, this equation can be written as the so-called “Abeltransformation” according to

h(x) = 2

∫ R

x

f(r)r√

r2 − x2dr. (4.9)

This transformation possesses the unique inverse function

f(r) = − 1

π

∫ R

r

d h(x)

dx

1√x2 − r2

dx. (4.10)

Besides the problem of a singularity at x = r, this “inverse Abel transformation” is super-imposed by noise depending on the chosen number of supporting points for the measuredprojection h(x). Hence, this noise might lead to serious problem during numerical differen-tiation of equation 4.10.

However, the inverse Abel transformation is a standard problem within HID-lamp research,thus different evaluation methods have been developed to overcome the numerical troublesand uncertainties. Pretzler explained the most important ones of these known numer-ical methods in [68] and classified their advantages/ disadvantages concerning HID-lampresearch. Regarding its accuracy, the Fourier method is the most suitable one to calculateradial intensity distributions from their spectroscopically measured 1D projections. Accord-ingly, the numerical Fourier method is used within this work to evaluate radial resolved

72 4. Measuring methods and data interpretation

density results.For applying the Fourier method to a specific measurement, a series expansion for theunknown radial distribution f(r) is performed according to

f(r) =

Ng∑n=1

anfn(r) (4.11)

with the expansion functions

fn(r) = 1− (−1)n cos

(nπ

r

R

). (4.12)

An insertion of this expansion into equation 4.9 leads to

H(x) = 2

Ng∑n=1

an

∫ R

x

fn(r)r√

r2 − x2dr. (4.13)

The corresponding Fourier integrals

hn(x) = 2

∫ R

x

fn(r)r√

r2 − x2dr (4.14)

cannot be derived analytically, thus they are calculated numerically in advance and storedinto an allocation table.Furthermore, to determine the unknown Fourier coefficients an, the Fourier transforma-tion H(xk) is fitted at N supporting points to the measured data hmeas(xk) by a least squarealgorithm according to

ST =N∑k=1

(H(xk)− hmeas(xk))2 → min. (4.15)

The partial derivative of this constraint with respect to the searched coefficients an leadsto an equation system determining an which can be solved by means of the pre-computedFourier integrals (equation 4.14) and the measured input data hmeas(xk). Finally, by insert-ing the evaluated coefficients an into equation 4.11, the corresponding rotational symmetricdistribution f(r) to the measured projection hmeas(x) is obtained.

One advantage of the discussed numerical realisation of the inverse Abel transformationby the Fourier method is an automatic low pass filtering of the measured input datadue to the series expansion systematic. Thus, the Fourier method is particularly suit-able to evaluate measurements including a high noise level like spectroscopic measurementswith a CCD camera. However, a reliable result for f(r) is only observed if the preconditionf(r > R) = 0 respectively hmeas(x < −R) = 0 and hmeas(x > R) = 0 is met. Therefore,continuous radiation has to be subtracted from the measured spectrum and a zero-padding isperformed during numerical data processing adjacent to the measured distribution hmeas(x)before inverse Abel transformation. Furthermore, the output of the Fourier method tendsto oscillate artificially in the outer part of the result region which reduces the accuracy andhas to be considered during an interpretation of the results.A further precondition for the correct determination of the distribution f(r) is a careful

4.2. Spectroscopic particle density measurements 73

choice of the right symmetry axis in the middle of the measuring result hmeas(x). Therefore,the symmetry middle is not chosen by the maximum of the h(x) distribution but by an indi-vidual rating of all measuring points of h(x) concerning their weight to the total geometry.This approach was proposed by Pretzler in [68] and can be expressed mathematically as

xsym =

∑x(x · h(x))∑x(h(x))

(4.16)

4.2.2 Density measurement in YAG-lamps by emission spectroscopy

The easiest way to determine particle densities in YAG-lamps by spectroscopy is the spec-tral analysis of the radiated light from the investigated YAG-lamp itself. For this so-called“emission spectroscopy” method the spectral distribution of one or more characteristic emis-sion lines of the desired particle species, like Dy in this case, is measured and calibrated inabsolute radiance units. By means of a spectral integration of the emission line, a value forthe absolute line emission coefficient is determined leading to the absolute density of theobserved particle species according to equation 2.16.However, the measurement and interpretation of a spectral emission line only provides aninformation about the corresponding population density of the excited energy state nu emit-ting this specific spectral line. Usually, a knowledge about the groundstate density n0 oreven the total density n of a specific particle species is necessary to interpret e.g. the effi-ciency of the emitter-effect induced by Dy adjacent to the electrode surface. Therefore, anadditional measurement of the plasma temperature Tpl by means of emission spectroscopy isunavoidable. If the plasma temperature Tpl is known, the groundstate particle density n0 aswell as the total density n and any other excited level population can be calculated from themeasured population density of the excited energy state nu by the Boltzmann distributiongiven in equation 2.12.Thus, a brief description of the spectroscopic plasma temperature determination will be givennow followed by an explanation of the particle density calculation from measured absoluteline emission coefficients of Dy.

Determination of the plasma temperature Tpl by measuring the Hg temperature THg

To determine the groundstate or total density of species like Dy atoms within the YAG-lamp gas-phase from absolutely calibrated emission line measurements, a measurement ofthe plasma temperature Tpl is performed in advance. The high pressure plasma of a YAG-lamp arc discharge can be physically characterised by the local thermodynamic equilibrium(LTE, cf. chapter 2.2.3). Consequently, the plasma temperature Tpl is equal to all other heavyparticle temperatures and can be determined by measuring the temperature of any specificingredient in the gas-phase. Thus, the temperature of mercury THg is measured spectroscop-ically leading finally to the desired plasma temperature by the LTE assumption THg = Tpl

comparable to a method presented by Schoepp [69]. The measurement of the mercury tem-perature THg is preferred in this case as mercury is evaporated completely during YAG-lampoperation resulting in an unsaturated stable mercury pressure of pYAG,Hg = 1.98 MPa (calcu-lated theoretically by Philips Lighting) which equals almost the total YAG-lamp operating

74 4. Measuring methods and data interpretation

pressure. Hence, the operating pressure of mercury does not change for different YAG-lampoperating conditions offering the possibility of determining reliable plasma temperature re-sults Tpl with a reasonable accuracy.Mercury emits two very strong and characteristic spectral lines at λHg,1 = 576.96 nm andλHg,2 = 579.07 nm. Both Hg spectral lines are emitted by optical transitions between upperenergy states. Accordingly, the mercury lines λHg,1 and λHg,2 can be treated as optically thinwithin the measured lamp operating ranges as low population density of the lower energystate leads to a negligible small self absorption. Thus, to determine the plasma tempera-ture within a YAG-lamp, a spectral emission image is recorded at a center wavelength ofλspec,Hg = 579 nm 125µm in front of the upper electrode. The resulting image includes bothHg emission lines together which is important for the following Hg temperature determi-nation. The recorded spectrum is corrected and calibrated as discussed at the beginning ofthis chapter. A spectral excerpt along the wavelength axis in the centre of the discharge isextracted and plotted exemplarily in figure 4.8(a).It is clearly visible from figure 4.8(a) that the two measured Hg spectral lines overlap each

572 574 576 578 580 582 5840

5

10

15

x 103

wavelength λ / nm

spec

tral

rad

iance

L/

Wm

-2nm

-1sr

-1

integration limits

continuum background

00

1

2

3

4

5

6x 10

4

spatial position / mm

lin

e ra

dia

nce

Lλ,H

g/

Wm

-2sr

-1

-1-2 1 2

(a) (b)

Figure 4.8: Example of the spectrum emitted by the two measured Hg-lines λHg,1 = 576.96 nmand λHg,2 = 579.07 nm (a) and their integrated combined radial intensity profile (b). Parameters:NTD1 YAG-lamp, dE = 500µm, i = 800 mA switched-dc, f = 100 Hz, cathodic phase

other and cannot be treated as individual emission lines. Hence, to obtain a line radiationprofile for a further data evaluation, the intensity of both Hg emission lines λHg,1 = 576.96 nmand λHg,2 = 579.07 nm is integrated over both line profiles which is also indicated in figure4.8(a) by the green integration limits. The area below the Hg spectral lines, caused by su-perimposed continuum radiation, is subtracted during line integration as represented by thered continuum background line in figure 4.8(a). The mercury line profiles are integrated foreach spatial point individually, leading to a spatial distribution result of the Hg line radianceLλ,Hg which is exemplarily shown in figure 4.8(b).After this spectral line integration, the radial distribution of the Hg emission coefficientελ,Hg(r) for both lines can be determined from the spatial resolved line radiance Lλ,Hg(x) (cf.figure 4.8(b)) by an inverse Abel transformation as discussed in chapter 4.2.1. By assuminga Boltzmann distribution due to the LTE, the emission coefficient ελ,Hg can be written

4.2. Spectroscopic particle density measurements 75

according to equation 2.16, equation 2.9 and 2.12 as

ελ,Hg =h c0

4π

p0

kBTHgZHg

[gu,1Aul,1

λ1

exp

(− Eu,1

kBTHg

)+gu,2Aul,2

λ2

exp

(− Eu,2

kBTHg

)]. (4.17)

Herein, p0 is the pressure of the mercury vapour and ZHg(T ) is the partition function of theHg atoms, given by QHg(T > 7000K) ≈ 1. Finally, the mercury temperature and therewiththe equal plasma temperature THg = Tpl is determined by the temperature dependency of theHg emission coefficient ελ,Hg while assuming the previously discussed constant Hg pressureof p0 = 1.98 MPa. Therefore, THg is resolved in equation 4.17 by a numerical iteration ofits linear and nonlinear dependency. The constant emission line properties gu (statisticalweight), Aul (transition probability) and Eu (energy of the upper state) in equation 4.17 areobtained from [70] and listed in table 4.2. An example for the resulting radial distribution

λHg,1 = 576.96 nm λHg,2 = 579.07 nm

gu,1Aul,1 = 1.18 · 108 s−1 gu,2Aul,2 = 1.07 · 108 s−1

Eu,1 = 8.849 eV Eu,2 = 8.842 eV

Table 4.2: Constants for the two measured Hg spectral emission lines

of the plasma temperature Tpl corresponding to the intermediate result presented in figure4.8 is given in figure 4.9.The plasma temperature result shown exemplarily in figure 4.9 gives typical absolute values

-1 -0.5 0 0.5 15000

5500

6000

6500

7000

radial position r / mm

abso

lute

pla

sma

tem

per

atu

reT

pl/

K

electrodediameter

region of confidence

Figure 4.9: Example of a resulting radial distribution of the plasma temperature Tpl measured byHg emission lines and indicated region of confidence with a dimension of the electrode diameter.Parameters: NTD1 YAG-lamp, dE = 500µm, i = 800 mA switched-dc, f = 100 Hz, cathodicphase

of Tpl = 6000 − 7200 K for an i = 800 mA switched-dc operation of a YAG-lamp. However,

76 4. Measuring methods and data interpretation

the previously mentioned oscillation tendency of the inverse Abel transformation in theouter discharge parts is clearly visible within the graph. Hence, the radial resolved plasmatemperature results given in figure 4.9 are only reliable within an interval equal to theelectrode diameter, here dE = 500µm, around the center of the discharge. This limitationregion of confidence is marked within figure 4.9 and has to be considered during the followingdetermination of particle densities from emission spectroscopy.

Determination of the Dy-density

If the plasma temperature Tpl is known from a spectral Hg line measurement, the total orgroundstate particle density of Dy atoms adjacent to the electrode can be measured by meansof emission spectroscopy during a second measurement. Therefore, the spectral emission isrecorded using the known spectroscopic setup at a center wavelength of λspec,Dy = 696 nmat the same measuring position in the YAG-lamp where the Hg line measurement was per-formed. To avoid measuring errors due to changes in the YAG-lamp operation, the Hg linemeasurement and the Dy line measurement have to be performed with the same lamp oper-ating parameters and with a preferably short intermediate time of some 10 s. The recordedemission spectrum around λspec,Dy = 696 nm is processed and calibrated in absolute radianceunits as previously mentioned. An example of the measured spectrum for the Dy densitydetermination is given in figure 4.10.Intensive investigations of emission spectra from Dy containing YAG-lamps by Langen-

692 694 696 698 700 702

80

100

120

140

wavelength λ / nm

spec

tral

rad

ian

ce L

/W

m-2

nm

-1sr

-1

Dy 696

Dy 699

Figure 4.10: Example of a recorded emission spectrum around the middle wavelength λspec,Dy =697 nm including two Dy spectral lines. Parameters: NTD1 YAG-lamp, dE = 450µm, i = 800 mAswitched-dc, f = 100 Hz, cathodic phase

scheidt and Westermeier [22, 25] led to the two most suitable lines for a Dy density measure-ment and calculation: The two Dy spectral lines are located in the lamp emission spectrumat λDy,696 = 695.81 nm as well as λDy,699 = 699.13 nm and are marked within the exampleYAG-lamp emission spectrum in figure 4.10. The spectral line λDy,696 is a resonance line and

4.2. Spectroscopic particle density measurements 77

may be therefore an optically thick line which is influenced by self absorption. However, theemission of the alternative Dy line at λDy,699 is relatively weak (cf. figure 4.10) but it is not aresonance line offering the advantage that it can be treated as optically thin. Langenscheidtand Westermeier compared the shape of both Dy emission lines for a normalised amplitudeand varying lamp operating conditions. These investigations led to the conclusion that thespectral line λDy,696 does not show self absorption within the investigated lamp operatingranges and can also be treated as optically thin [22]. Accordingly, to have a good signalquality within the whole YAG-lamp operation ranges, a measurement of the stronger Dyspectral emission line λDy,696 = 695.81 nm is preferred and investigated for the Dy densitydetermination from emission spectroscopy within this work.A zoomed excerpt of the investigated Dy emission spectral line λDy,696 from the total recordedspectrum shown in figure 4.10 is given in figure 4.11. To determine the Dy density, the abso-

695.5 696

80

100

120

140

wavelength λ / nm

spec

tral

rad

ian

ce L

/W

m-2

nm

-1sr

-1

integrationlimits

cont. background

5% - line

Figure 4.11: Zoomed excerpt of the investigated Dy emission spectral line λDy,696 with integrationlimits, continuum background and 5%-line. Parameters: NTD1 YAG-lamp, dE = 450µm, i =800 mA switched-dc, f = 100 Hz, cathodic phase

lute spectral radiance of the spectral line profile λDy,696 is integrated within the integrationlimits marked in figure 4.11 and subsequently Abel inverted. The spectral integration is per-formed for each spatial position along the spectrograph entrance slit including a backgroundsubtraction as explained in detail for the Hg temperature measurement.However, for certain YAG-lamp operating parameters, the investigated Dy spectral line pro-file λDy,696 is influenced and artificially increased by an overlapping spectral line on the bluewing side as visible in figure 4.11. Therefore, the data processing MatLab program uses auto-matically the so-called “5%-line” which is also indicated in figure 4.11. The 5%-line indicatesa decision limit which is 5% higher than the continuum background: If the spectral radiancevalue L at the blue integration limit is higher than the 5%-line (as shown in figure 4.11),an artificial increase of the line profile is considered. To solve this problem, the measuredλDy,696 line profile is split in the middle and the blue wing of the line profile is exchangedby the mirrored red wing profile which is not influenced by other spectral lines. Thus, thetheoretical symmetry of the Dy line profile is used by means of a double evaluation of itsred wing leading to a more reliable line emission coefficient ε. Of course, the Dy spectral line

78 4. Measuring methods and data interpretation

profile λDy,696 is evaluated unchanged if the 5%-limit is not exceeded.Finally, as known from the Hg temperature measurement, the radially resolved line emissioncoefficient εDy,696(r) is obtained by means of an inverse Abel transformation. An intensivediscussion of the application limits and optimised parameters for the Abel transformation ofthe λDy,696 Dy emission line was performed by Westermeier in [25]. According to equation2.18, the population density of the upper energy level nu(r) can directly be evaluated fromεDy,696(r) by the relation

εul(r) =1

4π

h c0

λul

guAulnu(r). (4.18)

The Dy spectral line constants within this equation are determined from [70] and [55] andgiven in table 4.3.However, to investigate e.g. the emitter-effect of Dy, a knowledge of the groundstate density

λDy,696 = 695.81 nm λDyI = 394.47 nm

gu = 15 gu = 18

Aul = 8.766 · 104 s−1 Aul = 2.55 · 107 s−1

Eu = 1.7815 eV Eu = 3.1424 eV

Table 4.3: Constants for the Dy atom and Dy ion emission spectral lines

n0,Dy or absolute atom density nDy of dysprosium is much more important than a valuefor the density of an upper energy level nu(r). Therefore, the distribution of the previouslymeasured plasma temperature Tpl(r) is considered. Assuming a Boltzmann distribution ofthe upper energy levels, Tpl(r) is used to calculate the total atom density of Dy, nDy(r), fromthe spectroscopically measured excited state density nu(r) according to

nDy(r) = nu(r)Z(Tpl(r))

gu

[exp

(− Eu

kBTpl(r)

)]−1

. (4.19)

The partition function Z(Tpl(r)) within this equation is obtained by a sum over all knownspectral emission lines of Dy from [70] according to equation 2.14. Details of this partitionfunction calculation are given by Reinelt and Langenscheidt in [31, 22]. An example for aresulting radial distribution of the total Dy atom density within an NTD1 YAG-lamp corre-lated to the intermediate result from figure 4.11 is given in figure 4.12.It has to be mentioned again that the Dy atom density result shown in figure 4.12 is only

reliable within the indicated region of confidence limited by the diameter of the investigatedelectrode, herein dE = 450µm, due to the discussed numerical problems of the inverse Abeltransformation. However, for the interpretation of the emitter-effect, especially within phaseresolved measuring results, only an averaged value of the Dy atom density over the indicatedregion of confidence is considered and plotted.The presented Dy atom density determination by means of absolutely calibrated emissionspectroscopy can be used similarly to determine the density of Dy ions nDyI(r). Therefore,a Dy ion emission line at λDyI = 394.47 nm is investigated and evaluated according to thepresented procedure for the Dy atom density. The characteristic Dy ion line properties arealso given in table 4.3.

4.2. Spectroscopic particle density measurements 79

-1 -0.5 0 0.5 10

1

2

3

4

5

6x 10

15

radial position r / mm

tota

l D

y a

tom

den

sity

ND

y/

cm-3

electrodediameter

region of confidence

Figure 4.12: Example of a resulting radial distribution of the Dy atom density nDy measured byemission spectroscopy and indicated region of confidence. Parameters: NTD1 YAG-lamp, dE =450µm, i = 800 mA switched-dc, f = 100 Hz, cathodic phase

In conclusion, the particle density measurement by emission spectroscopy is a well estab-lished and reliable measuring method suitable for the determination of all particles whichcan be clearly identified by emission lines within the recorded HID-lamp spectrum. However,the emission spectroscopy method has some significant disadvantages: On the one hand, ameasurement of the particle densities in HID-lamps can only be performed at positions insidethe lamp where a sufficiently high luminance power is radiated. In the dark zones around theelectrode rod, for example, the emission spectroscopy measurement is usually not applicable.On the other hand, to determine the groundstate density or total density of atoms withinan HID-lamp, a preceding plasma temperature determination is unavoidable including nu-merical problems by a combination of results from two inverse Abel transformations. Thiscombination of results from two inverse Abel transformations reduces the accuracy of thefinal density result significantly. Also the need of an absolute calibration of the measuringsetup for emission spectroscopy leads to accuracy problems as the transmission of the YAGmaterial is temperature dependent and changes over the lamp lifetime, e.g. by wall blacken-ing. To overcome most of these disadvantages of the particle density evaluation by emissionspectroscopy, an alternative absorption spectroscopy method is developed within this workwhich will be presented in the following chapter.

4.2.3 Density measurement by absorption spectroscopy

In contrast to the previously discussed emission spectroscopy measurement, the concept ofabsorption spectroscopy uses an additional backlight source whose radiated light intensityis spectrally reduced while passing the gas inside the investigated HID-lamp. Thereby, anabsorption coefficient αlu of the measured HID-lamp particles can be determined which is

80 4. Measuring methods and data interpretation

directly correlated to their groundstate density if the absorption of a resonance line is con-sidered.To use the concept of absorption spectroscopy for particle density measurements in HID-lamps, the spectroscopic setup for emission measurements, shown in figure 3.8, is expandedby means of an additional backlight source with a powerful and almost stable luminance out-put. As discussed in chapter 3.3.1, an LED or a powerful UHP-lamp system is used withinthis work for providing the backlight signal. It is also shown in the sketch of the spectro-scopic setup in figure 3.8. This additional backlight system makes the density measurementindependent of emission from the investigated HID-lamp. Hence, particle densities can bemeasured by absorption at an arbitrary position within the HID-lamp which is an importantadvantage compared to the emission spectroscopy method.According to the definitions in figure 3.8, a cartesian coordinate system is introduced withits point of origin on the optical axis at a point in front of the investigated HID-lamp. Asy-axis, the optical axis is chosen whereas the x-axis represents the direction perpendicularto the vertical YAG-lamp arc discharge and is imaged along the spectrograph entrance slit.The z-axis characterises the axial distance to the electrode tip within YAG-lamp measure-ments. However, during absorption spectroscopy measurements in HPS-lamps, the definedcoordinate axis represent other orientations with respect to the HPS electrode due to itshorizontal operation: The x-axis is taken parallel to the electrode of HPS-lamps whereas thez-axis represents the rectangular distance to the HPS-lamp electrode.Assuming that the intensity of the backlight Iν(x, y = 0, z) is known while entering the in-vestigated HID-lamp and considering a light emission from the HID-lamp itself, representedby εν(x, y, z), the light intensity Iν(y) at a position y along the line-of-sight parallel to theoptical axis can be written according to the radiation transport equation 2.21 as

Iν(y) =

∫ ξ=y

ξ=0

εν(ξ) · exp

[−∫ η=y

η=ξ

α(ν, η)effdη

]dξ + Iν(0) · exp

[−∫ η=y

η=0

α(ν, η)effdη

],

or in a short abbreviated form

Iresν (y) = Iem,HIDν (y) + Iabs,UHPν (y). (4.20)

Herein εν(ξ) is the emission coefficient at the position (x, ξ, z) and α(ν, η)eff the effectiveabsorption coefficient at the frequency ν and the position (x, η, z) on the optical axis. Thecoordinates x and z perpendicular to the optical axis can be omitted in the subsequentconsiderations. The effective absorption coefficient αeff takes into account the reductionof the absorption coefficient α by induced emission. Within the investigated HID-lamps,induced emission of the lines in the visible spectrum can be neglected since the popula-tion density nu of the upper energy state is always much lower than that of the lowerenergy state nl of the specific optical transition (nu/nl << 1). Therefore it can be assumedαeff (x, y, z) = α(x, y, z).

To perform a spectroscopic absorption measurement, the resulting spectral light intensityIresν (x, y, z) emerging from the investigated HID-lamp is recorded by means of the spectro-graph setup comparable to the discussed emission spectroscopy measurements. In accordanceto equation 4.20, the measured spectral light signal Iresν includes the radiation emitted by theinvestigated arc discharge itself Iem,HIDν and the fraction which is emitted by the backlightsystem Iabs,UHPν passing the HID-lamp under investigation. Thus, the information of theabsorbing particle density inside the HID-lamp is contained exclusively within the Iabs,UHPν

4.2. Spectroscopic particle density measurements 81

part of the spectroscopically recorded light beam.Hence, two successive measurements are performed to separate the fraction Iabs,UHPν , provid-ing the aimed density information, from the spectroscopic images: At first, a spectroscopicmeasurement of the total resulting light beam Iresν including radiation from the UHP back-light source and from the measured HID-lamp is recorded. Afterwards, with a possibly shorttime delay, a reference measurement of the emission light from the investigated HID-lampIem,HIDν is performed by blanking out the light beam from the backlight source with a mirror.The attenuated UHP-backlight signal, carrying the measured particle density information,is obtained by a subtraction of both measurements according to

Iabs,UHPν (y) = Iresν (y)− Iem,HIDν (y). (4.21)

It has to be mentioned that the careful alignment and focussing of the UHP-backlight beamto the measuring point is one of the most challenging tasks while performing the describedabsorption spectroscopy measurement. On the one hand, a high backlight power for a goodSNR is needed in the vicinity of the measuring point but on the other hand, the radiation hasto be sufficiently homogenous within the measured spatial 1D area. Transparency problemsof the HID-lamp burner material, e.g. YAG or sapphire, leading to reduced UHP light beamintensities or even deflection of the backlight signal imply further difficulties while aligningthe UHP backlight system.However, to gain a higher measuring signal quality, always five to ten spectral images arerecorded successively for each measurement allowing averaging of the obtained absorptionsignals which increases the accuracy of the final results significantly. Besides a higher accu-racy, this procedure offers the possibility to remove measured spectra which are deterioratedby temporal changes of the UHP backlight source radiation like the discussed statisticalflickering effect.An example of a measured and evaluated absorption line profile Iabs,UHPν of a Dy absorption

625.5 626 626.5

2

2.2

2.4

2.6

2.8

3

3.2

wavelength λ / nm

light

inte

nsi

tyI νab

s,U

HP

/ a.

u.

Iν

abs,UHP

I

I

ν

ν

(0) meas.

(0) fitting0

0.1

0.2

0.3

0.4

opti

cal

dep

thτ(

λ)

625.5 626wavelength λ / nm

625.75

(a) (b)

integration limits

Figure 4.13: Example of a measured Dy absorption line profile Iabs,UHPν including measured UHP-backlight without absorption Iν(0)-measured and calculated fit Iν(0)-fitting (a) and correlatedoptical depth τ (b). Parameters: NTD1 YAG-lamp, dE = 450µm, i = 800 mA switched-dc,f = 100 Hz, cathodic phase

resonance line at λDy−abs,626 = 625.91 nm in an NTD1 YAG-lamp is given in figure 4.13(a).

82 4. Measuring methods and data interpretation

The image is taken as usual perpendicular to the electrode tip in a distance of 125µm andplotted exemplarily in dependence on wavelength for a spatial position in the discharge cen-ter in figure 4.13(a). The absorption line profile of the λDy−abs,626 Dy resonance line in thisfigure has a spectral width of ∆λ = 0.1 nm FWHM. On the one hand it reflects the appa-ratus profile of the spectroscopic setup, on the other hand the line width may be increasedby pressure and Stark broadening or by resonance collisions with Hg and Na. As it will beshown later in the data evaluation, the measured spectral absorption line profile in figure4.13(a) does not need an absolute calibration of the measuring system to determine particledensities which is unavoidable for emission spectroscopy measurements. This fact representsone of the main advantages of the introduced absorption spectroscopy measurement resultingin significantly lower influences of systematic measuring errors (cf. chapter 4.2.4).The intensity of the UHP-backlight signal Iν(0) which is needed for a Dy density calcula-tion from the absorption profile is not measured separately. Moreover, it is obtained by anextrapolation of the undisturbed course of Iν between the wings of the measured absorptiondip. The result of this extrapolation, Iν(0)-fitting, is also plotted in figure 4.13(a). However,to prove the reliability of the performed extrapolation procedure, the UHP backlight signalin dependence on wavelength is measured exemplarily without an HID-lamp in the spectro-scopic setup and plotted normalised into figure 4.13(a). The extrapolated signal Iν(0)-fittingand the measured one show an excellent agreement of both. The advantage of this fittingprocedure is a proportion Iν(y = 0)/Iabs,UHPν (y) which is determined independently of fluc-tuations of the UHP-backlight. This is not ensured by a direct measurement of Iν(y = 0).Moreover, the resulting proportion does not depend on changes in the optical properties ofthe path through the investigated HID-lamp, e.g. by temperature or wall blackening effects.Taking into account equation 4.20 and 4.21, the optical depth τ(ν, y) can be separated fromthe measured absorption dip profile by a logarithmic conversion:

τ(ν, y) =

∫ η=y

η=0

α(ν, η)dη = ln

(Iν(0)

Iabs,UHPν (y)

)(4.22)

Hence, the optical depth of the absorption profile τ(ν, y) can be calculated from the ab-sorption measurement result presented in figure 4.13(a) by a piecewise calculation of thelogarithm of the proportion Iν(y = 0)/Iabs,UHPν (y). An example for the optical depth τ(λ, y)corresponding to the absorption profile presented in figure 4.13(a) is given in figure 4.13(b).The spatial distribution of the correlated optical line depth τν(x) is obtained by a spectralintegration of the wavelength profile of the optical depth τ(λ, y) within the indicated integra-tion limits (cf. figure 4.13(b)). The integration is performed for all spatial positions as knownfrom the emission spectroscopy measurements and transformed from the wavelength λ rangeto the frequency ν range by multiplying the c0/λ

2 conversion factor. An example for thisevaluated spatial distribution of the optical line depth τν(x) =

∫ yη=0

αν(x, η)dη corresponding

to the measured optical depth profile τ(λ, y) in figure 4.13(b) is shown in figure 4.14.It is obvious from figure 4.14 that the distribution of the optical line depth τν(x) is repre-

sented by a characteristic increasing profile towards the middle of the arc discharge. However,the measured τν(x) distribution sometimes shows a dip around the symmetry axis, like infigure 4.14, which is a questionable result according to the discussion in chapter 4.2.1. Theproblem arises presumably due to a highly populated ground-state level in the outer partof the discharge dominating the absorption along the light beam path. As a consequence,the results of αL(r) determined by an inverse Abel transformation become erroneous on thearc axis. To solve this problem and to avoid numerical uncertainties, the measured signal ofτν(x) is corrected and smoothed before the following inverse Abel transformation according

4.2. Spectroscopic particle density measurements 83

-0.5 00

1

2

3

4

x 1010

spatial position / mm

op

tica

l li

ne

dep

thτ

ν

0.5

measuredcorrected

1-1

Figure 4.14: Example of a measured and corrected optical Dy line depth τν(x) at λDy−abs,626 =625.91 nm. Parameters: NTD1 YAG-lamp, dE = 450µm, i = 800 mA switched-dc, f = 100 Hz,cathodic phase

to the example given also in figure 4.14.Finally, with the definition of an integral line absorption coefficient according to equation2.19, the demanded particle density nl can be related to the measured line absorption τν(x, y)in figure 4.14:

τν(x, y) =

∫∆λ

τ(λ, x, y)c0

λ2dλ =

∫ η=y

η=0

αL(x, η)dη =e2

4πε0

· π

mec0

· flu

∫ η=y

η=0

nl(x, η)dη (4.23)

The path length ∆y of the light passing the investigated HID-lamp is determined by a pre-viously performed inverse Abel transformation of the spatial distribution τν(x) in the caseof YAG-lamp measurements (see figure 4.14) or by a geometrical estimation in the case ofHPS-lamp measurements. The oscillator strength flu, representing a measure for the absorp-tion probability, is taken from [55]. The specific resonance spectral lines and correspondingoscillator strength values used for absorption measurements and density calculations of Ba,Dy and Ce within this work are listed in table 4.4.

λDy−abs,626 = 625.91 nm λBa−abs,553 = 553.55 nm λCe−abs,577 = 577.36 nm

flu = 5.6 · 10−3 flu = 1.64 flu = 5.492 · 10−3

Table 4.4: Constants for the investigated Ba, Dy and Ce absorption spectral lines

In the end, the radial distribution of the ground-state density n0(r) of Dy or Ba can directlybe calculated from the absorption measurement by means of one single inverse Abel trans-formation. Thereby, numerical uncertainties caused by a second Abel inversion (cf. emissionspectroscopy measurements) are avoided leading to a much higher accuracy of the absorption

84 4. Measuring methods and data interpretation

density results. An example of the resulting radial distribution of the ground-state Dy den-sity is given in figure 4.15. Again the resulting Dy ground-state density presented within this

-1 -0.5 0 0.5 1

0.2

0.4

0.6

0.8

1.0

1.2x 10

15

radial position r / mmDy a

tom

gro

und-s

tate

den

sity

N0,D

y/

cm-3

electrodediameter

region of confidence

Figure 4.15: Example of a resulting radial distribution of the Dy atom ground-state density n0,Dy

measured by absorption spectroscopy and indicated region of confidence. Parameters: NTD1YAG-lamp, dE = 450µm, i = 800 mA switched-dc, f = 100 Hz, cathodic phase

figure is only reliable within the indicated region of confidence correlated to the electrodediameter. However, the density results in the outer parts of figure 4.15 are less influenced bynumerical problems than the results presented in figure 4.12 due to the reduced number ofinverse Abel transformations.

The presented absorption spectroscopy measurement of particle densities provides directlythe radial groundstate density distribution n0(r) of the specific species if a resonance lineis investigated. Therefore, all considered absorption lines for Ba, Dy and Ce listed in ta-ble 4.4 are resonance lines. However, for an interpretation e.g. of the emitter-effect of Dy,sometimes a value for the total atom density nDy is also needed which cannot be determinedfrom the absorption spectroscopy measurement. In the case of a Dy atom density measure-ment, the relation of the total density nDy to the ground-state density n0,Dy is determinedby the partition function according to equation 2.14 in dependence on the plasma tem-perature. This relation nDy/n0,Dy is calculated for typical HID-lamp plasma temperatures(Tpl = 6500 − 7500 K) and plotted in figure 4.16. It is clearly visible in figure 4.16 that therelation between the Dy ground-state and the corresponding total atom density of Dy is onlyweakly dependent on temperature within the typical HID-lamp operating ranges indicatedby the “region of interest” in figure 4.16. Thus, to avoid an additional plasma temperaturemeasurement which removes most of the absorption spectroscopy advantages, the total atomdensity nDy is always calculated from the measured Dy ground-state density n0,Dy by a con-stant factor multiplication of 4.4 within this work. However, if possible, a discussion of theDy ground-state density n0,Dy is preferred to interpret the Dy emitter-effect for avoidingthese uncertainties.

4.2. Spectroscopic particle density measurements 85

6500 6750 7000 7250 7500

4

4.5

5

plasma temperature Tpl

/ K

Dy a

tom

den

sity

rel

atio

n N

Dy

/ N

0,D

y

region ofinterest

Figure 4.16: Relation between the total Dy atom density nDy and the corresponding Dy ground-state atom density n0,Dy for typical plasma temperatures Tpl in HID-lamps

4.2.4 Reproducibility and errors of the spectroscopic Dy density mea-surements

Compared to the electrode temperature Tel measurements, presented in the first part of thischapter, a determination of particle densities n in the gas-phase of HID-lamps either byemission- or by absorption spectroscopy is more complex leading to generally higher errorranges and a lower reliability of the absolute results. But in contrast to Tel-measurements,the uncertainty problems for spectroscopic measurements arise mainly during the numericaldata processing and due to unappropriate spectral line data from literature.However, the optical CCD camera setups and their absolute calibration are comparable forboth types of measurement. Also in the case of spectroscopic measurements, the resultingsignal is recorded by a PCO SensiCam and the whole optical system is calibrated with ahigh accuracy by means of the tungsten ribbon lamp Wi17G from Osram. Thus, accordingto the discussion in chapter 4.1.3, the overall error for the optical measuring system can beestimated to roughly 5%. However, the transmission coefficient of the HID burner material,e.g. YAG or sapphire in this work, has a significant influence on the accuracy of the opticalemission spectroscopy measurement. Its temperature and wavelength dependent values areobtained by a precise reference measurement from Philips Lighting but they vary over lamplifetime due to wall blackening and crystallisation effects. On the other hand, the accuracyof the presented absorption spectroscopy measurement is not influenced by the HID-lamptransmission as it evaluates a relatively measured and extrapolated ratio.

During data processing of the emission spectroscopy measurement, the inverse Abel trans-formation is used twice to determine the radial distribution of the plasma temperature andfinally the radial distribution of the investigated total particle density n. Due to the previ-ously discussed numerical problems of the Abel transformation (evaluated by the Fouriermethod) a relatively high error source is introduced into the final results. Even if the maxi-

86 4. Measuring methods and data interpretation

mal error for the numerically evaluated inverse Abel transformation is estimated to 10% in[68], the oscillation tendency of the Abel inversion results determined from real, noisy mea-suring data like in figure 4.9 supposes a much higher real error of the absolute result valuesat least in the outer parts of the discharge. However, the particle density determination byabsorption spectroscopy is much more reliable since the measuring data is only transformedonce by the Abel inversion.

Another important aspect concerning the error estimation for the presented spectroscopymeasurements is the reliability of the spectral line data presented in the tables 4.2, 4.3 and4.4. An intensive study of different spectroscopy literature sources like [69, 55, 70, 71] shows,that the given absolute values for the transition probability Aul and the oscillator strengthflu differ sometimes more than one order of magnitude. The available spectroscopic datafor Ba and Dy is anyway less extensive and less accurate as for example spectral data forAr transitions because these elements are usually not within the focus of intensive research.Often it is unavoidable to use even theoretically determined spectral line data like [70] sinceexperimental reference values are not yet measured.

In conclusion, the uncertainties of the particle density determination by emission spec-troscopy measurements are significantly higher than the ones of the absorption method dueto the different data measuring and evaluation concept. Summing up the mentioned difficul-ties and keeping in mind a variety of performed density measurements, the minimal accuracyof density results determined by emission spectroscopy can be stated to a factor of 2. Thisis in agreement with the discussion of Langenscheidt [22] and Reinelt [31].The developed absorption spectroscopy measurement has the advantage to be independentof an absolute calibration and of a change of optical parameters like tube transmission. Ad-ditionally, it uses only one inverse Abel transformation leading to much more reliable densityvalues. Hence, the accuracy of the density results determined by absorption spectroscopy canbe estimated to 20%.However, for an investigation of the gas-phase emitter-effect within an HID-lamp, the ab-

1

1.5

2

2.5

3

3.5

4x 10

15

phase0 1/2π π 3/2π 2π

Dy

-ato

m d

ensi

ty/

cmN

Dy

-3

Dy

-ato

m d

ensi

ty/

cmN

Dy

-3 NDy from emission

at electrode

NDy from absorption

at electrode

NDy from absorption

in lamp middle

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6x 10

15

NDy from emission

at electrode

NDy from absorption

at electrode

NDy from absorption

in lamp middle

phase0 1/2π π 3/2π 2π

(a) (b)

Figure 4.17: Comparison of phase resolved total Dy atom density results nDy determined byemission- and absorption spectroscopy in an NTD1 (a) and an NTD2 (b) YAG-lamp. Parameters:values averaged over the electrode diameter, dE = 360µm, i = 800 mA switched-dc, f = 100 Hz

4.2. Spectroscopic particle density measurements 87

solute density values are less important than their relative changes according to the lampoperating conditions. Mostly, even the absolute density results determined by emission- orabsorption spectroscopy show a good agreement as presented exemplarily in figure 4.17. Theresults shown in figure 4.17(a) are determined within an NTD1 YAG-lamp whereas the re-sults in figure 4.17(b) are from an NTD2 YAG-lamp. The values are given phase resolvedand averaged over the region of confidence equal to the electrode diameter dE = 360µm fora f = 100 Hz switched-dc current at iRMS = 800 mA. The curves measured 125µm in frontof the electrode show a good agreement between the emission spectroscopy method and theabsorption spectroscopy method. They decrease during the anodic phase and increase duringthe cathodic phase of the investigated electrode which will be explained in detail later withinthis work. By means of the absorption spectroscopy method it is even possible to measurethe Dy atom density in the discharge middle which is also shown in figure 4.17. The Dyatom density in the middle does not show a significant change over the period cycle and isgenerally lower than the density values directly in front of the electrode. However, a compa-rable measurement of particle densities in the discharge middle is not possible by means ofthe emission spectroscopy method as Hg emission lines for the required plasma temperaturedetermination are not available.

88 4. Measuring methods and data interpretation

89

5. Measurements and results

After the measuring methods and data evaluation are introduced and discussed in detail, thischapter gives the main measuring results concerning the gas-phase emitter-effect in differentdischarge lamps and its interpretation. All results are obtained by means of the previouslydiscussed 2D pyrometry and the broadband absorption spectroscopy method whereas theemission spectroscopy method is partly used for a comparison.The chapter is divided into three main parts: At first, fundamental investigations at theBochum model lamp are presented. The model lamp results give a basic insight into thephysics of the emitter-effect of thorium and its impact on tungsten electrodes of HID-lampsfor low and high operating frequencies. Within the second part of this chapter, results ofthe emitter-effect of barium in the high-pressure sodium lamps are discussed. In these HPS-lamps the barium diffusion along the electrode shaft and its effect on the lamp lifetime isinvestigated in detail. Finally, in the third part of this chapter, results for the emitter-effectof dysprosium and cerium in YAG-lamps are given. The YAG-lamps are investigated withdifferent fillings and lamp operating parameters to study optimal working conditions for theemitter-effect and possible deteriorating side-effects.

5.1 Results at the Bochum model lamp

The Bochum model lamp is a very flexible research system, which reproduces a real HID-lamp in several parameters. The model lamp and its technical details are explained in chapter3.1.1. Within this chapter, tungsten electrodes with different amounts of thorium dopingare used in the model lamp to investigate fundamentals of the gas-phase emitter-effect.The basic physical aspects of the emitter-effect on tungsten surfaces were already givenin chapter 2.3.3. At first, experimental results of the cathodic tip temperature Ttip at DCoperation are compared to simulated Ttip results for different work functions according tothe black-box model introduced in chapter 2.3.4. This work was done by Bergner within theframework of a diploma thesis [58] and leads to absolute reference values for the reductionof the work function by a thorium layer on the tungsten electrode surface. Afterwards, inthe second part of this subchapter, the emitter-effect of model lamp electrodes is studied forhigher operating frequencies to compare its behaviour to real HID-lamp electrodes during HFoperation. This work was done within the framework of a Bachelor thesis [72] by Hoebing andleads to a complex correlation between adsorption, desorption and partial pressures of theemitter materials in HID-lamps concerning an interpretation of their electrode temperaturereduction.

90 5. Measurements and results

5.1.1 Determination of the work function by simulation and measure-ments

The DC cathode

The flexibility of the Bochum model lamp and its low complexity in geometry and gas mixtureoffers the unique possibility to investigate the emitter-effect by simulation and experiments.Therefore, a theoretical model is used which describes the boundary layer of the plasmaarc attachment on the cathodic electrode. The “black-box model” was already introduced inchapter 2.3.4 within this work leading to a theoretical description of the cathodic electrodebehaviour by means of “transfer functions”. The value of the effective tungsten electrodework function ϕ (cf. chapter 2.3.3) is an important parameter within the calculation of thesetransfer functions. From literature [59] it is known, that the work function of pure tung-sten amounts to ϕ = 4.55 eV. While using thorium as an emitter material in the Bochummodel lamp, the effective work function of the cathodic electrode can be significantly re-duced. Therefore, the corresponding theoretical electrode tip temperature Ttip to a certainarc current Iarc is calculated for different work function values by evaluating the transferfunctions. These theoretical electrode tip temperatures can be compared to measured valuesof pure tungsten and thoriated tungsten electrode tip temperatures to obtain the absolutereduction of the effective work function indirectly.

This concept was applied to the Bochum model lamp leading to absolute results of thework function reduction which will be given here. Figure 5.1 shows the first measured and

0 5 10 15 20

2000

2400

2800

3200

3600

arc current / Aiarc

cath

ode t

ip t

em

pera

ture

/ K

Tti

p

4.55eV

4.25eV

4.00eV

3.70eV

3.50eV

3.30eV

3.00eV

G18

BSD

Figure 5.1: Measured and simulated cathode tip temperatures Ttip for BSD and G18 electrodesin the Bochum model lamp in dependence on arc current Iarc for different work functions ϕ.Parameters: p = 0.1 MPa Ar, dE = 600µm, lE = 10 mm, larc = 10 mm, Iarc = 0 − 20 A DCcurrent

simulated results of the cathodic electrode tip temperature Ttip for a DC current inside theBochum model lamp. The measurement of Ttip is performed by means of the 2D pyrometric

5.1. Results at the Bochum model lamp 91

method for pure tungsten electrodes made of BSD material (pure tungsten, cf. chapter 3.1.1)with an electrode diameter of dE = 600µm, a length of lE = 10 mm and additionally for tho-riated electrodes of G18 material (1.8% of Th) with the same geometry (cf. chapter 3.1.1).The corresponding DC anode within the model lamp is chosen with an electrode diameterof dE = 2 mm to sustain high currents and the distance between the electrodes, respectivelythe arc discharge length, is adjusted to larc = 10 mm. The results of this Ttip measurementare plotted for different DC arc currents of Iarc = 2.5 − 12.5 A as dots (BSD) and crosses(G18) in figure 5.1. Concerning the simulation, it is difficult to fit the calculated values forTtip directly to the measured results by varying the work function ϕ within the simulatedtransfer functions. Accordingly, an array of simulated cathode tip temperature curves independence on the arc current is plotted for a set of different work functions in figure 5.1.Hence, a value for the effective work function of the investigated cathode can be obtainedby comparing the position of the Ttip measuring results in the figure to the work functionparameter of the neighbouring simulation result curve.It is clearly visible from the simulation results in figure 5.1, that the electrode tip tempera-ture Ttip increases non-linearly for an increasing arc current since the electrode heating powercoupled into the cathodic boundary layer rises. In the case of a work function of ϕ = 4.55 eV,which is the literature value for pure tungsten [59], the simulated Ttip starts at 2850 K fora current of 0.5 A and increases to 3700 K for 20 A whereas the corresponding cathode tiptemperature rises from 1850 K to 2750 K for a work function of ϕ = 3 eV. Thus, the elec-trode tip temperature Ttip decreases for a lower effective work function which represents themechanism of the emitter-effect in HID-lamps.The measuring results for Ttip of the pure tungsten BSD cathode, shown by red dots infigure 5.1, are located on the simulated curve for a work function ϕ = 4.55 eV with a goodagreement. Thus, the literature value for the work function of pure tungsten is confirmed bythis measurement which proves thereby a high accuracy of this comparison method for Ttip

simulation results with measurements. Accordingly, the electrode tip temperature results ofthe thoriated G18 tungsten cathode, which are expected to be significantly lower due to theemitter-effect of thorium, are also plotted into figure 5.1 as green crosses. By comparing theG18 Ttip-results within this figure to the array of simulated curves, an effective work functionof ϕG18 = 3 eV can be deduced from the correlation. This Ttip-result of the thoriated cathodeproves the existence of an emitter-effect induced by Th on the tungsten surface leading toan expected reduction of the effective work function. The obtained results are comparable toa reduced work function of ϕTh = 3.6 eV measured for AC operated electrodes by Luijks etal in [73]. Theoretical investigations of the work function reduction by means of a thoriummonolayer on a tungsten surface were performed in dependence on relative coverage by Altonin [74] and given in figure 5.2. The results in figure 5.2 show that the expected value for theeffective work function of a 50% Th coverage on W amounts to ϕ50% Th,theory = 3 eV which isin excellent agreement with the presented measuring and simulation outcome of this work.However, several tungsten literature sources like [59] specify the effective work function of athorium monolayer on tungsten surfaces to ϕTh,literature = 2.6 eV which is even ∆ϕ = 0.4 eVlower than the result determined in this work. But it has to be considered, that the citedliterature values are usually given for low tungsten surface temperatures whereas HID elec-trodes are operated at significantly higher temperatures slightly below their melting point.Thus, temperature influences like a higher desorption rate of Th from the hot HID electrodesurface might reduce the total efficiency of the emitter-effect leading to the obtained discrep-ancy of ∆ϕ = 0.4 eV between literature values and the measured results.Figure 5.3 shows the simulated dependence of the cathode tip temperature Ttip on the work

92 5. Measurements and results

wo

rk f

un

ctio

n/

eVφ

7

6

5

4

3

2

1

00 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

relative coverage Th monolayer on W

Th on W (Ref. 2)

Present theory

Topping model (Ref. 18)

Gyftopoulos andLevine model (Ref. 19)

Figure 5.2: Effective work function ϕ for a thorium (Th) monolayer on tungsten (W) dependingon the relative Th coverage. Graph taken from [74].

function ϕ for different arc currents between Iarc = 2.5 A and Iarc = 12.5 A. It is obvious fromthe graph that the cathode tip temperature increases almost linearly with the work function.This result is contrary to the nonlinear dependence of Ttip and Iarc shown in figure 5.1 dueto the Richardson-Dushman relation given by equation 2.50. Additionally, the measuredcathode tip temperatures for BSD and G18 electrodes are plotted in figure 5.3. From thedata presentation arrangement of figure 5.3 it can be observed that the effective work func-tions of BSD and G18 electrode materials vary slightly for different arc currents: In the caseof BSD material, the work function decreases for a decreasing arc current whereas the workfunction of the G18 electrodes increases for a decreasing Iarc. These contrary behaviours ona small scale might be a result of measuring uncertainties but it is also possible that theBSD electrodes have a reduced work function for lower arc currents due to an increasedSchottky reduction (cf. end of chapter 2.3.3). In contrast, the increasing work function ofthe thoriated G18 material for a decreasing arc current might be explained by changes ofthe thorium supporting ion current forming a different relative Th coverage of the tungstensurface which leads to an increased work function while exceeding the optimal coverage pointaccording to figure 5.2. But it might also be possible that the relative Th coverage increaseswith decreasing arc current by reason of the lower electrode temperature.

Besides the measured and simulated Ttip courses, the emitter-effect of thorium on tung-sten electrodes can also be investigated by means of the calculated electrode input powerPloss. Therefore, measuring results of the cathodic input power Ploss for BSD and G18 modellamp electrodes in dependence on arc current Iarc are given in figure 5.4. The input powerof the BSD electrode varies from Ploss = 11.3 W for Iarc = 2.5 A up to Ploss = 16.7 W forIarc = 12.5 A whereas the thoriated G18 electrode shows significantly lower input powersstarting at Ploss = 7.3 W for Iarc = 2.5 A and ending at Ploss = 8.3 W for Iarc = 12.5 A. Addi-tionally, the relative difference between Ploss of the BSD and G18 electrodes increases for anincreasing arc current significantly. This observation represents again an active emitter-effectof thorium on the G18 electrode surface as the voltage demand of the cathodic boundary

5.1. Results at the Bochum model lamp 93

3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.62200

2400

2600

2800

3000

3200

3400

3600

3800

work function / eVφ

cath

ode

tip t

emper

ature

/ K

Tti

p

12.5A10A7.5A5A2.5AG18BSD

Figure 5.3: Measured and simulated cathode tip temperatures Ttip for BSD and G18 electrodesin the Bochum model lamp in dependence on work function ϕ for different arc currents Iarc.Parameters: p = 0.1 MPa Ar, dE = 600µm, lE = 10 mm, larc = 10 mm, Iarc = 0 − 20 A DCcurrent

sheath increases for higher work functions of the electrode resulting in higher relative inputpowers for a higher arc current.In principle, the measured results of the cathodic loss power Ploss can also be compared tosimulates curves as it was done for Ttip in figure 5.1. However, the simulated curves for thecathodic input power Ploss, corresponding to the measurements presented in figure 5.4, differup to ∆Ploss = 4 W in the case of BSD material and Iarc = 12.5 A. This difference betweensimulation and measurement of the cathodic input power cannot be explained easily by theaccuracy of the research methods.Hence, Bergner showed the deviating Ploss simulation curves in [58] where he also discussedpossible solutions for this discrepancy problem in detail. In summary it can be stated thatthe bottom temperature of the electrode cannot be determined with a high precision from astandard 2D pyrometric electrode temperature measurement within the electrode tip region.The electrode bottom temperature for an investigated electrode length of lE = 10 mm is dis-tinctly higher than the previously assumed temperature of the cooling oil (300 K) in [8, 9].The excerpt of the 2D electrode temperature image, taken for the input power calculation,does not cover the whole electrode length lE = 10 mm due to the camera objective zoomratio (cf. chapter 4.1.2). Therefore, the electrode temperature distribution at the bottomcan only be derived from the extrapolated simulation data with a reduced accuracy. ButBergner showed in [58] that this uncertainty in the electrode bottom temperature cannotexplain the obtained difference of ∆Ploss = 4 W for the measured and simulated input poweralone. Moreover, the electrode holder and therewith the electrode bottom point seems to beheated additionally by a parasitic power flux from the plasma arc through the glass burneradjacent to the electrode holder. In the case of short electrodes with a length of lE = 10 mmas they are investigated here, the problem is even more pronounced than in comparable mea-surements with longer electrodes of lE = 20 mm presented in [8, 9]. The simulation does not

94 5. Measurements and results

2 4 6 8 10 12 14

8

10

12

14

16

arc current / Aiarc

cath

odic

pow

er l

oss

/W

Plo

ss

BSDG18

Figure 5.4: Measured cathode power loss Ploss for BSD and G18 electrodes in the Bochum modellamp in dependence on arc currents Iarc. Parameters: p = 0.1 MPa Ar, dE = 600µm, lE = 10 mm,larc = 10 mm, Iarc = 0− 20 A DC current

take into account these parasitic power fluxes and leads therefore to the discussed differencesin the result values. Including this correction, the discrepancy between the measured andsimulated results for Ploss is reduced to ∆Ploss = 0.2 − 1.3 W. As Bergner discussed also in[58], the additional influence of the so-called “ohmic heating” of the electrode, caused by itsohmic resistance for the current density inside the material, cannot be neglected for the elec-trode input power determination Ploss. In contrast, the presented results for the cathode tiptemperature Ttip are only weakly influenced by the ohmic heating, therefore the previouslyperformed determination of work function values from measured Ttip results, as presented infigure 5.1, is much more reliable.

To investigate the influence of the thorium amount on the emitter-effect, the previouslypresented measurements are repeated with G7 electrodes which include a lower amount(0.7%) of Th in the electrode material compared to the G18 (1.8% Th) electrodes. Resultsof this comparative measurement of the electrode tip temperature Ttip for the G7 cathodeare presented in figure 5.5. In general, the Ttip results of G7 electrodes shown in this figuredo not differ distinctly from the ones presented for G18 electrodes in figure 5.1. From a moreprecise view, the electrode tip temperatures are slightly higher for all operating currents ofG7 electrodes than the comparable G18 results. The difference of ∆Ttip = 40 K in the caseof Iarc = 2.5 A and ∆Ttip = 20 K for all other operating currents indicate a slightly weakeremitter-effect for the G7 electrodes with a lower amount of Th as it was expected previously.However, by comparing the measuring results in figure 5.1 and 5.5 it can be stated that theamount of thorium inside the model lamp electrodes does not show a significant differencebetween G7 and G18 material regarding the Th emitter-effect.

5.1. Results at the Bochum model lamp 95

0 5 10 15 20

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4.55eV

4.25eV

4.00eV

3.70eV

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3.30eV

3.00eV

G7

BSD

arc current / Aiarc

cath

od

e t

ip t

em

pera

ture

/ K

Tti

p

Figure 5.5: Measured and simulated cathode tip temperatures Ttip for BSD and G7 electrodesin the Bochum model lamp in dependence on arc current Iarc for different work functions ϕ.Parameters: p = 0.1 MPa Ar, dE = 600µm, lE = 10 mm, larc = 10 mm, Iarc = 0 − 20 A DCcurrent

The DC anode

As the emitter-effect of thorium is proved and characterised for DC cathodes now, additionalelectrode tip temperature measurements for the anodic DC electrode are performed for com-parison reasons. The resulting Ttip values for the anode in dependence on DC arc currentIarc are given for thin electrodes with a diameter of dE = 600µm in figure 5.6(a) and forthicker electrodes with dE = 2 mm in figure 5.6(b). The anode tip temperature results of thethinner electrode in figure 5.6(a) may be compared to the cathodic results given in figure 5.1regarding the electrode geometry. With an anode tip temperature of Ttip = 3600 K alreadyfor an arc current of Iarc = 2.5 A, it is obvious from figure 5.6(a) that the anode made ofBSD and G18 material is heated much more than the comparable cathode at the same DCcurrent value presented in figure 5.1. In a first estimation of the anodic boundary behaviour,the heating power flux Ploss from the plasma arc discharge into the electrode increases lin-early with an increasing arc current according to the model given by Dabringhausen [8].In accordance with this linearly dependent heating power flux into the anode, the anodictip temperature Ttip of the thin electrode (dE = 600µm) reaches critical values slightly be-neath its melting point already for an arc current of Iarc = 2.5 A (cf. figure 5.6(a)). However,differences of the anode tip temperature Ttip between the tungsten BSD material and thethoriated G18 material are only marginally in the case of thin electrodes with dE = 600µmas given in figure 5.6(a). Thus, an active emitter-effect of thorium on the anodic electrodesurface cannot be detected for dE = 600µm electrodes within the Bochum model lamp.To gain anodic Ttip measuring results for higher arc currents of Iarc = 2.5− 12.5 A, a thickeranodic electrode with dE = 2 mm is used which sustains the thermal stress resulting fromthe discussed strong anodic heating power flux for higher arc currents. The results of thismeasurement are given in figure 5.6(b). Herein, the anode made of thoriated G18 materialshows even higher Ttip values than the pure tungsten BSD anode, which is contrary to the

96 5. Measurements and results

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arc current / Aiarc

ano

de

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tem

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ano

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tem

per

atu

re/

KT

tip

2 4 6 8 10 12 141200

1600

2000

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3200BSDG18

(a) = 600 mdE μ (b) = 2 mmdE

Figure 5.6: Measured anode tip temperatures Ttip for BSD and G18 electrodes in the Bochummodel lamp in dependence on arc current Iarc for two different electrode diameters dE. Parameters:p = 0.1 MPa Ar, (a) dE = 600µm, (b) dE = 2 mm, lE = 10 mm, larc = 10 mm, Iarc = 0− 20 ADC current

active emitter-effect of Th on the cathode in figure 5.1. The higher anodic temperature ofG18 electrodes might be explained partly by electrode tip structure effects or by a higherspotwise thermionic emission of electrons due to Th in the W material. The emitted elec-trons are heated in the anodic boundary layer and accelerated back to the anode due to theelectric field. This additional heating flux by means of returning hot electrons emitted fromthe anode itself might lead to the increased Ttip values of the G18 anode with respect to theBSD results presented in figure 5.6. The locally enhanced emission is obviously not sufficientto reduce the average work function of the anode, as it is indicated by comparing Ttip(I) ofthe G18 electrodes and BSD electrodes.Concluding the anodic Ttip measuring results for thin and thick electrodes of G18 and BSDmaterial given in figure 5.6(a) and (b), an active anodic emitter-effect of thorium can beclearly excluded for DC operation of the Bochum model lamp.

Interpretation of the Th emitter-effect in the Bochum model lamp

The comparison of simulation and measurements within the preceding subchapters shows adistinct reduction of the effective tungsten work function from ϕ = 4.55 eV to ϕ = 3 eV bymeans of a thorium monolayer inducing an active emitter-effect. However, this emitter-effectis only present on the cathode for a DC current operation.The emitter-effect of thorium on the tungsten surface of the cathode can even be proved

by a reduced electrode tip temperature up to an arc current of Iarc = 12.5 A and a corre-lated maximal electrode temperature of Ttip = 2620 K in figure 5.1. Keeping in mind thisconclusion and taking into account the results presented by Brueche and Mahl in figure 5.7,some important information about the thorium transport to the electrode surface can bededuced: In figure 5.7 Brueche and Mahl investigated the electron emission current densityjem in dependence on surface temperature for pure tungsten, pure thorium and thoriatedtungsten surfaces. It is obvious from figure 5.7 that a monolayer of thorium on tungsten,

5.1. Results at the Bochum model lamp 97

1500 2000 2500 3000

surface temperature / KT

elec

tro

n e

mis

sio

n c

urr

ent

den

sity

/A

cmj e

m

-2

102

101

100

10-1

10-2

10-3

10-4

10-5

10-6

(mon

olay

er)

pure

tung

sten

thor

ium

thor

ium

ontu

ngst

en

0.9

0.80.6

0.50.40.2

0.1

Figure 5.7: Electron emission current density in dependence on surface temperature for puretungsten, pure thorium and a thorium monolayer on tungsten. Graph taken from [75]

inducing a high jem, starts to vanish above a surface temperature of T = 1750 K. This effectis caused by an increasing desorption rate of Th on W for an increasing surface temperaturedue to the limited adsorption energy of thorium. For a surface temperature of T = 2500 K,the stable Th monolayer on the tungsten surface is completely removed according to theresults of Brueche and Mahl given in figure 5.7.However, as previously mentioned, the cathodic emitter-effect of Th on W surfaces can evenbe proved for HID-lamp electrodes with a tip temperature of Ttip = 2620 K. Thus, a thoriumsupporting flux mechanism from the gas-phase must exist, compensating the high Th desorp-tion rate above T = 2500 K as shown in figure 5.7. Due to the results of Brueche and Mahl,the thorium for the emitter-effect cannot be supported only thermodynamically by diffusionout of the G18 electrode body material to its surface. Accordingly, the thorium evaporatedfrom the electrode surface has to be ionised within the plasma boundary sheath adjacent tothe electrode and transported back to the cathode surface by a significant Th ion currentflux. This general assumption of an emitter material transport by means of an ion currentcan explain the existence of a cathodic emitter-effect by thorium even for Ttip = 2620 K (cf.figure 5.1) in spite of the Th desorption context given by Brueche and Mahl in figure 5.7.Within the scope of his diploma thesis, Bergner tried to find evidence for the deduced as-sumption of a thorium ion current onto the cathode by measuring Th ion emission lines inthe emission spectrum adjacent to the cathode surface. However, he did not succeed withhis attempt since the measurement of ion emission lines with low intensities at usually lowwavelengths by means of spectroscopy is quite difficult. At the current state of research, theassumption of an emitter material support by means of an ion current is the only known,logical explanation for the presented thorium desorption and surface temperature correla-tion. Further evidence for the existence of an emitter ion current was already found in YAG

98 5. Measurements and results

HID-lamps by Langenscheidt and Westermeier in [26].

5.1. Results at the Bochum model lamp 99

5.1.2 Thoriated electrodes with low and high operating frequencies

The existence of the anodic emitter-effect

Another interesting aspect which can be investigated by means of the Bochum model lampis the existence of an anodic emitter-effect for high operating frequencies. From YAG-lampmeasurements by Reinelt [31] it is known that for certain operating conditions, an activeemitter-effect of dysprosium, indicated by a decreasing Ttip, can also be found during the an-odic phase of an electrode. A measuring result of the electrode tip temperature Ttip includingan indication for the existence of this so-called “anodic emitter-effect” [76] in YAG-lamps isgiven exemplarily in figure 5.8(a). The graph in figure 5.8(a), taken from [31], shows phase

Phase

0 1/2¼ ¼ 3/2¼ 2¼

Tip

tem

peratu

re

K/

3020

3040

3060

3080

3100

3120

3140

3160

3180

25Hz

50Hz

100Hz

2kHz

(a) YAG-lamp, pure DyPhase

0 1/2� � 3/2� 2�

Tip

tem

perau

re

K/

3000

3020

3040

3060

3080

3100

3120

3140

3160

10Hz

25Hz

500Hz

10kHz

(b) Model lamp

Figure 5.8: Electrode tip temperature for various frequencies a) in a YAG-lamp with pure Dyfilling and b) in the model lamp. Parameter: a) dE = 0.36 mm, electrode length lE = 5 mm,i = 0.8 A, taken from [31] b) BSD electrode material, dE = 1 mm, electrode length lE = 20mm, i = 3 A, argon at p = 0.26 MPa, taken from [33]

resolved Ttip results of the upper electrode within a YAG-lamp which is exclusively filledwith Dy salts (cf. chapter 3.1.3) for frequencies of f = 25 Hz - 2 kHz. It is clearly visible inthis figure that the electrode is heated within the anodic phase, represented by the first halfperiod, whereas it cools down in the cathodic phase. The electrode tip temperature variationis more pronounced for low operating frequencies of f = 25 Hz than for higher frequencies off = 2 kHz due to the time constants of its heat capacity. This qualitative electrode heatingand cooling behaviour is expected within YAG-lamps at a current of iarc = 800 mA switched-dc and known from former research results [22, 25].However, an important observation found within the results shown in figure 5.8(a) is thetendency of a decreasing electrode tip temperature Ttip for an increasing lamp operatingfrequency. The emitter-effect of Dy in YAG-lamps is active during the cathodic phase whichwas proven by Langenscheidt [22], but its cooling effect on the cathode is generally constant.Thus, a cathodic emitter-effect should not show a frequency dependence as visible in figure5.8(a). The observed tip temperature reduction for an increasing frequency can be explainedby an anodic emitter-effect [76]: As shown in the previous chapter, the emitter material, in

100 5. Measurements and results

this case Dy, is transported to the tungsten electrode surface mainly be means of an ioncurrent. Accordingly, the Dy transport by an ion flux is interrupted in front of the anode dueto its positive electric field. Moreover, the Dy ions are pushed away during the anodic phaseof the electrode and the Dy monolayer on its surface disappears due to a fast desorption rateof Dy atoms. However, because of their mass and high adsorption energy, Dy atoms on thetungsten electrode surface have an inertia keeping them on their position obtained duringthe cathodic phase for a short time within the beginning of the anodic phase. If now the du-ration time of the anodic phase is low due to a high lamp operating frequency, the Dy atomsforming a monolayer on the electrode surface have too much inertia to be desorbed from theanode. In the consequence, the sustaining Dy monolayer reduces also the work function forthe entering electrons during the anodic phase resulting in a lower anodic heating power fluxfrom the arc plasma. This assumption of an anodic emitter-effect can explain the frequencydependency of Ttip observed in a Dy YAG-lamp which is shown in figure 5.8(a).Reinelt also found in [31] that the frequency dependency of Ttip is opposite in the model lampoperated with pure tungsten BSD electrodes as shown in figure 5.8(b). It is clearly visiblein figure 5.8(b) that the electrode tip temperature rises with an increasing lamp operatingfrequency from f = 10 Hz to f = 5 kHz. This result is in agreement with the discussed an-odic emitter-effect since an emitter material is generally absent in the model lamp operatedwith BSD electrodes. Hence, a cathodic and an anodic emitter-effect cannot be observed inthe model lamp equipped with pure tungsten electrodes which is another indication for itsexistence in the Dy YAG-lamp.It has to be mentioned that Ttip plotted in figure 5.8(b) decreases during the anodic phase andincreases during the cathodic phase which is also opposite compared to the YAG-lamp resultsin figure 5.8(a). This electrode tip temperature behaviour is typical for low current/electrodediameter ratios and for high work function electrodes. The turning point, characterised byan equal and then higher anodic heating power flux with respect to the cathodic heating, isobviously located at a higher model lamp current than i = 3 A.

The anodic emitter-effect of Th in the Bochum model lamp

To prove the existence of the anodic emitter-effect also for thorium within the Bochum modellamp, several frequency depending, phase resolved electrode tip temperature measurementsby means of 2D pyrometry were performed by Hoebing within the framework of a Bache-lor thesis [72]. Therefore, results from model lamp electrodes made of pure tungsten BSDmaterial are compared to results obtained with G18 electrodes containing 1.8% thorium (cf.chapter 3.1.1).A first result of these phase resolved investigations for BSD electrodes driven with a switched-dc current of iarc = 6 A is given in figure 5.9. The investigated BSD model lamp electrodeshave a diameter of dE = 1 mm and a length of lE = 20 mm. The model lamp is filled withargon at a pressure of p = 0.26 MPa. To assure their comparability, all model lamp inves-tigations presented in the following use the same electrode geometry and noble gas fillingparameters. It is visible in figure 5.9 that the electrode tip temperature Ttip for a low fre-quency operation of f = 10 Hz and f = 25 Hz shows a phase modulation whereas in thehigher frequency operation of f = 100 Hz - 10 kHz Ttip stays almost constant over a phasecycle. This observation is in agreement with former research results and is caused by the timeconstants of the electrode heat capacity with respect to the duration time of an operation

5.1. Results at the Bochum model lamp 101

elec

trode

tip t

emper

ature

T/

Kti

p

3650

3600

3550

35000 π 2π

phase

10 Hz25 Hz100 Hz500 Hz

1 kHz2 kHz5 kHz10 kHz

Figure 5.9: Phase resolved electrode tip temperature results Ttip for tungsten BSD electrodesin dependence on frequency for f = 10 Hz - 10 kHz. The first half period represents the anodicphase, the second half period represents the cathodic phase. Parameters: lE = 20 mm, dE = 1 mm,iarc = 6 A switched-dc, p = 0.26 MPa Argon

phase cycle. In the case of f = 10 Hz it is obvious that the electrode is heated during theanodic phase, represented by the first half period, and cools down during the cathodic phasewith a temperature deviation of ∆Ttip = 45 K. Accordingly, the previously discussed, currentdepending turning point, represented by an equal heating of the anode and the cathode, ispassed for an iarc = 6 A operation of the model lamp for this specific electrode geometry.As previously expected for BSD electrodes, the electrode tip temperature presented in fig-ure 5.9 shows an increase for a rising operating frequency from Ttip = 3525 K in the caseof f = 25 Hz operation up to Ttip = 3645 K for f = 10 kHz. This is the typical electrodetemperature behaviour for an HID-lamp without emitter material content and in agreementwith the model lamp results from Reinelt shown in figure 5.8(b). However, an unexpectedcorrelation can be found in figure 5.9 between the Ttip values of f = 1 kHz and f = 2 kHz op-eration: In spite of the general tendency of a rising electrode tip temperature for an increasingoperating frequency, the f = 1 kHz-operation is characterised by Ttip = 3580 K whereas thef = 2 kHz-operation shows a significantly lower Ttip value of 3560 K. This correlation of theTtip results for an f = 1 kHz and an f = 2 kHz operation is also present within all followingmeasuring results and cannot be explained easily by a measuring error or changes of thearc attachment on the electrode. Obviously, the f = 2 kHz operation of this certain modellamp configuration forms an optimal energetic operation point represented by an additionalreduction of the electrode tip temperature.

To determine the specific turning point, represented by a higher cathodic heating for lowerarc currents, the model lamp measurement in figure 5.9 is repeated for several lower operatingcurrents. For an arc current of iarc = 5 A, this turning point is not yet under-run according to[72]. Thus, a result of the phase resolved electrode tip temperature for a switched-dc currentof iarc = 4 A is given in figure 5.10. The Ttip-results in figure 5.10 show a decreasing temper-ature during the anodic phase and an increasing temperature during the cathodic phase for

102 5. Measurements and results

elec

trode

tip t

emper

ature

T/

Kti

p

3540

0 π 2π

phase

3520

3500

3480

3460

3440

3420

3400

10 Hz25 Hz100 Hz500 Hz

1 kHz2 kHz5 kHz10 kHz

Figure 5.10: Phase resolved electrode tip temperature results Ttip for tungsten BSD electrodesin dependence on frequency for f = 10 Hz - 10 kHz. Parameters: lE = 20 mm, dE = 1 mm,iarc = 4 A switched-dc, p = 0.26 MPa Argon

a low frequency operation of f = 10 − 25 Hz. In this case, even for the f = 100 Hz and thef = 500 Hz operation, a slight phase modulation can be observed from figure 5.10. Accord-ingly, the turning point in electrode heating is under-run for an iarc = 4 A operation of themodel lamp with these conditions: The cathode needs an additional heating power flux fromthe plasma to emit a sufficiently high electron flux according to the Richardson-Dushmancorrelation (equation 2.50) whereas the electron flux from the plasma, scaling linearly withthe arc current, heats the electrode less intense while entering during the anodic phase.The general tendency of an increasing electrode temperature for increasing operating frequen-cies, found in figure 5.9, can also be approved by the results in figure 5.10. Of course, due tothe lower operating arc current, the electrode tip temperature range of Ttip = 3340− 3445 Kmeasured for iarc = 4 A in figure 5.10 is generally lower than the range of Ttip = 3520−3645 Kmeasured in the case of iarc = 6 A operation, which is presented in figure 5.9.Electrode tip temperature measurements of BSD electrodes performed for lower operatingcurrents, e.g. iarc = 3 A, show uncertainties and switches of the absolute result values due toan unstable arc attachment during the measurements [72]. The discussed, generally observedTtip tendencies can also be approved by these measurements, but a clear interpretation isdifficult since Ttip values obtained for different types of arc attachment cannot be compareddirectly to each other. For comparison, Hoebing also investigated BSD electrodes with ashorter length of lE = 10 mm in the Bochum model lamp. However, the global tempera-ture of lE = 10 mm model lamp electrodes is slightly higher than the temperature valuesfor lE = 20 mm electrodes as their electron emitting surface is geometrically reduced. Butcontrary or at least additional observations of the general Ttip dependence on the operatingfrequency for BSD model lamp electrodes could not be derived from these short electrodemeasurements.

To determine the emitter-effect of thorium especially within the anodic phase, compara-ble measurements of the electrode tip temperature are performed for thoriated G18 model

5.1. Results at the Bochum model lamp 103

lamp electrodes (1.8% Th in the W material). The phase resolved Ttip measuring results for

elec

trode

tip t

emper

ature

T/

Kti

p

3500

0 π 2π

phase

3400

3300

3200

3100

10 Hz25 Hz100 Hz500 Hz

1 kHz2 kHz5 kHz10 kHz

Figure 5.11: Phase resolved electrode tip temperature results Ttip for thoriated G18 electrodesin dependence on frequency for f = 10 Hz - 10 kHz. Parameters: lE = 20 mm, dE = 1 mm,iarc = 6 A switched-dc, p = 0.26 MPa Argon

G18 electrodes driven at iarc = 6 A switched-dc are given in figure 5.11. The G18 electrodesin use have the same geometric parameters as the previously discussed BSD electrodes, thusthe results given in figure 5.11 are directly comparable to the corresponding BSD electroderesults shown in figure 5.9. With a temperature range of Ttip = 3090−3470 K, the G18 resultsare generally ∆Ttip = 175 − 430 K lower than the corresponding BSD electrode results dueto an active emitter-effect of thorium.Again, Ttip shows a phase modulation for f = 25− 100 Hz as the electrode is heated withinthe anodic phase and cools down within the cathodic phase for an iarc = 6 A operation ofG18 electrodes. But the temperature deviation, determined as ∆Ttip = 45 K for BSD elec-trodes, is much more pronounced for the electrode tip temperature results of G18 electrodesgiven in figure 5.11. Herein ∆Ttip = 220 K can be observed for the Ttip phase modulation ofthoriated electrodes which is a first indication for the absence of an anodic emitter-effect:Obviously, the electrode is cooled down significantly in the cathodic phase due to the ca-thodic emitter-effect but it is heated up during the anodic phase with a strong heating powerflux comparable to BSD electrodes presented in figure 5.9. Because of this high anodic heat-ing power, which is obviously not reduced significantly by an anodic emitter-effect, the Ttip

variation over phase amounts to a high value of ∆Ttip = 220 K in the case of G18 electrodes.However, a second observation indicating the absence of a Th emitter-effect during the an-odic phase is the general tendency of an increasing electrode tip temperature Ttip for a risingoperating frequency as shown in figure 5.11. The electrode temperature increase of G18electrodes from a low frequency operation (f = 10 − 25 Hz) to a high frequency operation(f = 5 − 10 kHz) is in the same order of magnitude as determined for the pure tungstenBSD electrodes. Accordingly, the presumed existence of an anodic emitter-effect of thoriumwithin the Bochum model lamp cannot be confirmed by these Ttip measurements.

104 5. Measurements and results

However, the turning point of the anodic and cathodic electrode heating power flux shouldalso be investigated for the G18 electrode material. Additionally, a possible current depen-dency of the anodic emitter-effect for thoriated electrodes in the model lamp should beexcluded by Ttip measurements for different arc currents. Therefore, Hoebing measured theelectrode tip temperatures for G18 electrodes also for iarc = 5, 4, 3 A in [72] similarly tothe measurement presented in figure 5.11. A result of these additional Ttip measurements of

elec

trode

tip t

emper

ature

T/

Kti

p

3400

0 π 2π

phase

3300

3200

3100

3000

2900

2800

10 Hz25 Hz100 Hz500 Hz

1 kHz2 kHz5 kHz10 kHz

Figure 5.12: Phase resolved electrode tip temperature results Ttip for thoriated G18 electrodesin dependence on frequency for f = 10 Hz - 10 kHz. Parameters: lE = 20 mm, dE = 1 mm,iarc = 3 A switched-dc, p = 0.26 MPa Argon

thoriated G18 electrodes for iarc = 3 A is given in figure 5.12. As previously mentioned, thelamp operating conditions, indicated by the type of arc attachment, are more unstable forlower arc currents. This effect is even more pronounced in the case of thoriated G18 lampelectrodes due to their inhomogenous thorium coverage and locally reduced work function.Thus, within the results shown in figure 5.12, a significant jump of ∆Ttip = 330 K betweenthe electrode tip temperature values for low operating frequency and the ones for high oper-ating frequencies can be observed, caused by a mode change of the arc attachment: For a lowfrequency operation of f = 10−100 Hz, the discharge arc attaches in the spot mode which ischaracterised by the indicated lower global tip temperatures (cf. figure 5.12) whereas duringhigh frequency operation (f = 500 Hz - 10 kHz) the arc attaches in the diffuse mode leadingto generally higher Ttip values. Nevertheless, the general trend of an increasing electrode tiptemperature for an increasing operating frequency is also visible in figure 5.12 while keepingin mind the discussed causes for the jump of the Ttip values between f = 100 Hz operationand f = 500 Hz operation.From the phase resolved Ttip course at f = 25 Hz it is hardly visible that the G18 electrodecools down during the anodic phase and heats up during the cathodic phase. This behaviouris more significant while zooming out the specified value ranges. However, the turning pointbetween a higher anodic and cathodic heating power of a thoriated G18 electrode, includingan emitter-effect, is reached at a lower arc current of iarc = 3 A compared to the BSD elec-trodes without emitter-effect. This fact is also approved by additional 4 A Ttip measurementsof G18 electrodes from Hoebing in [72].

5.1. Results at the Bochum model lamp 105

In general it can be stated from the model lamp Ttip measurements of BSD and G18 elec-trodes that an active emitter-effect during the cathodic phase is clearly proven by meansof an electrode temperature reduction. However, the existence of an anodic emitter-effect inthe model lamp equipped with thoriated electrodes could not be approved.

The anodic emitter-effect of Th in YAG-lamps

To investigate a possible existence of the anodic emitter-effect induced by thorium in YAG-lamps, additional phase resolved Ttip measurements in dependence on frequency are per-formed comparable to the measurements with Dy emitters given in figure 5.8(a). Figure 5.13

elec

trode

tip t

emper

ature

T/

Kti

p

3000

0 π 2π

phase

2900

2800

2700

10 Hz25 Hz100 Hz

500 Hz1 kHz2 kHz

Figure 5.13: Phase resolved electrode tip temperature results Ttip for electrodes of YAG-lampswith thorium filling in dependence on frequency for f = 10 Hz - 2 kHz. Parameters: dE = 360µm,iarc = 800 mA switched-dc

shows the electrode tip temperature results for iarc = 800 mA switched-dc current obtainedfrom a YAG-lamp (dE = 360µm) including thorium emitter material (ThI4) at a frequencyof f = 10 Hz - 2 kHz. Due to the occurrence of acoustic resonances in the small, closed YAG-lamp discharge volume, a measurement at an operating frequency above f = 2 kHz mightlead to a lamp destruction and can therefore not be performed [31]. Contrary to the usualelectrode tip temperature measurements at the upper electrode, the lower lamp electrode hasto be investigated in the Th containing YAG-lamp in figure 5.13 since the upper electrodeis deteriorated by an unusually strong wall blackening of the YAG material.However, it is clearly visible in figure 5.13 that the general electrode tip temperature Ttip forthe Th containing YAG-lamp driven at its nominal current (iarc = 800 mA) is lower than theelectrode temperatures determined from model lamp measurements in figure 5.9 and figure5.11. The most important result in figure 5.13 is the tendency of a decreasing electrode tiptemperature for an increasing operating frequency which is already known from YAG-lampsfilled with Dy emitters. Hence, in spite of the Ttip results obtained for thoriated G18 elec-

106 5. Measurements and results

trodes in the model lamp, the Th emitter within the YAG-lamp shows obviously the typicalbehaviour for an existing anodic emitter-effect explained at the beginning of this chapter.Due to their inertia and high adsorption energy, the Th emitter atoms on the tungstenelectrode surface can survive the anodic phase for higher operating frequencies as clearlyindicated by the frequency dependent Ttip-reduction in figure 5.13.A final reference measurement of the electrode tip temperature in the same Th contain-

elec

trode

tip t

emper

ature

T/

Kti

p

2600

0 π 2π

phase

2550

2500

2450

2400

2350

2300

10 Hz25 Hz100 Hz

500 Hz1 kHz2 kHz

Figure 5.14: Phase resolved electrode tip temperature results Ttip for electrodes of YAG-lampswith thorium filling in dependence on frequency for f = 10 Hz - 2 kHz. Parameters: dE = 360µm,iarc = 500 mA switched-dc

ing YAG-lamp for a lower arc current iarc = 500 mA, given in figure 5.14, shows the sametendency: The electrode tip temperature Ttip for iarc = 500 mA is generally lower than the800 mA results given in figure 5.13 due to the lower arc current. But a decreasing Ttip for anincreasing operation frequency is clearly visible in the 500 mA measurement, too. Thus, theexistence of an emitter-effect of Th during the anodic phase of a YAG-lamp electrode canfinally be assumed and proved as in the case of a Dy emitter (cf. figure 5.8(a)).

While comparing the absence of the anodic emitter-effect by Th in the model lamp to itsclear existence within Th containing YAG-lamps, some general characteristics of the emitter-effect can be deduced: The main transport mechanism of emitter material like Dy or Th tothe tungsten electrode surface is represented by an ion flux towards the cathode. However,the efficiency of the emitter-effect on the electrode surface is obviously determined by a com-plex correlation between the supporting ion flux, the specific adsorption rate of the emittermaterial and its desorption rate which are both strongly dependent on the electrode surfacetemperature. Moreover, the relative coverage of an emitter monolayer forms a certain mini-mal work function (cf. figure 5.2), which has to be considered additionally while interpretinginvestigations of the emitter-effect in HID-lamps.In the case of the presented Ttip measurements for an emitter-effect of thorium within themodel lamp and comparable YAG-lamps, the surrounding gas pressure and volume has ob-viously a significant influence on the Th desorption rate from the anode. In the model lampwith a relatively low gas pressure of p = 0.26 MPa and a huge gas volume (glass burner,

5.1. Results at the Bochum model lamp 107

connection pipes, electrode feedthrough etc.), the thorium emitted from the G18 electrodedisappears directly and does not establish a considerably high partial pressure. In the conse-quence, the desorption rate of the Th monolayer during the anodic phase of the model lampis so high that an anodic emitter-effect is not sustained. In contrast, the Th content withinthe YAG-lamp with a small volume and a high gas pressure of p = 1.98 MPa (roughly tentimes higher than in the model lamp) forms a significant partial pressure. Thereby, the des-orption rate of Th during the anodic phase of the YAG-lamp electrode is significantly reducedleading to a pronounced anodic emitter-effect, which was proven by the Ttip measurements.

108 5. Measurements and results

5.2. Results at the high-pressure sodium lamps 109

5.2 Results at the high-pressure sodium lamps

After some fundamentals concerning the emitter-effect in HID-lamps were investigated inadvance, within this chapter, investigations of the barium emitter density and the bariumdiffusion along the electrodes in the HPS-lamps are presented. In the HPS-lamps, the emittermaterial Ba is stored inside a tungsten coil which is mounted on the electrode rod as describedin detail in chapter 3.1.2. The barium diffuses along the electrode surface from the coil tothe electrode tip where it is consumed for the emitter-effect.However, in some commercially used HPS-lamps with the same emitter diffusion process[64], the lifetime does not meet the specified requirements due to a fast emitter consumptionfrom the storage coil. As a result, the arc attachment inside the HPS-lamp moves from itsspecified position at the electrode tip directly to the tungsten coil, where it consumes the restof the emitter material and destroys the lamp rapidly by overheating. Thus, investigationsof this diffusion process around the electrode rod in HPS-lamps are needed to gain a betterinsight into the diffusion parameters and their physical influences and to have absolute datafor a theoretical modelling of the Ba diffusion.The following subchapter is divided into three main parts: The first part characterises the Badiffusion process by emission-, LED- and UHP-absorption measurements. Within the secondpart, the influence of hydrogen on the Ba density is investigated in the getter/ dispenserHPS-lamp. Finally, in the third part, results of the electrode temperature measurements inthe HPS-lamps are shown and discussed for rising frequencies.

5.2.1 The Ba diffusion process in the HPS-lamp

This subchapter shows the investigation results of the barium diffusion from its storage inthe tungsten coil to the electrode tip. Several measuring methods for emission and absorptionspectroscopy are discussed to find the most suitable one. The main lamp parameter withinthese investigations is the distance between the tungsten coil around the electrode rod andthe electrode tip dcoil−tip. The distance dcoil−tip can be varied by use of different HPS-lampswhich are equal in all other physical and geometrical details.

Ba emission results

At first, a spectroscopic investigation of the λ = 553 nm barium line emission was madewithin the HPS-lamp to gain an overview over the distribution of the barium density andthe possibilities to measure it. It is performed by use of the emission spectroscopy setupdescribed in chapter 3.3.1 without a backlight system. The HPS-lamp is investigated bymeasuring the light emission during lamp operation with a slit orientation perpendicularto the electrode. The outcome is a distribution of radiance transverse to the arc axis. Thetransversal emission distribution of the barium resonance line at λ = 553 nm was recordedat different distances from the electrode tip. The resulting diagram is given in figure 5.15.Figure 5.15 shows the two main characteristics of the barium distribution adjacent to the

electrode: The first significant result is two maxima of the λ = 553 nm Ba line which areseparated by a gap in the middle. It reflects the electrode diameter with an accumulation

110 5. Measurements and results

−3 −2 −1 0 1 2 30

0.5

1

radial position / mm

Ba

553n

m li

ne e

mis

sion

/ a.

u.

el tip −250 µm

el tip −125 µm

el tip

el tip +125 µm

el tip +250 µm

el tip +375 µm

Figure 5.15: Radially resolved measurement of relative Ba line-emission at the Ba resonance lineλ = 553 nm in front of and behind the electrode tip, HPS-lamp 6,2 (Gobin old), del = 900µm,dcoil−tip = 1.4 mm, operated with power controlled Philips standard HPS-lamp driver at P = 140W, f = 200 kHz triangular current

of barium at the edges and a lower concentration in the center. Furthermore, the two Baemission peaks at the edges of the electrode are even narrower than the electrode diameterrepresenting a high Ba accumulation close to the electrode surface but only a little Ba inthe gas-phase around the electrode. The thin Ba accumulation zone around the electrode isa first indicator that the Ba diffusion process from the coil to the electrode tip is dominatedby surface diffusion. It is also found in the graph that the maximum intensity peaks areunsymmetric caused by a higher particle concentration on the top side of the lamp due tothe buoyancy flow around the electrode in its horizontal burning position.The dependency of the Ba line intensity on the distance from the electrode tip is the secondsignificant result of the first Ba measurement presented in figure 5.15. At the electrode tip,the recorded signals are already sufficiently high for a quantitative analysis. They becomeeven higher if the distance of the slit position from the electrode tip is increased. In contrast,the signal measured between the electrode tip and and the tungsten coil decreases rapidlywith increasing distance from the electrode tip so that a quantitative analysis becomes ratherdifficult.As shown in figure 5.15, it is possible to measure Ba densities by an analysis of the Ba emissioncoefficient ε553 in front of the electrode tip and further inside the arc plasma (assumingthat the plasma temperature Tpl has been determined). But a measurement adjacent tothe electrode shaft, which has to be performed to investigate the Ba diffusion, is nearlyimpossible due to a weak light emission. Since the region between the tungsten coil and theelectrode tip is the interesting one for the Ba diffusion, further measurements can only beperformed using an absorption technique. The additional backlight source within the conceptof absorption spectroscopy makes the investigated position inside the HPS-lamp independentof Ba self-emission and is therefore a reasonable further step in measuring technology.

5.2. Results at the high-pressure sodium lamps 111

General aspects of the Ba absorption spectroscopy

The following results are gained by absorption measurements with a backlight system. Theyprovide a better understanding of the barium diffusion process from the tungsten coil tothe electrode tip than measurements by emission spectroscopy which are difficult or evenimpossible in this region.The spectroscopic setup described in chapter 3.3.1 offers a one dimensional resolution alongthe axis of the slit of the spectrograph, which can be used to achieve a spatial distributionof the barium density. Thus, the absorption measurement can be performed either perpen-dicular to the electrode from which a radial distribution of the absorption coefficient can bededuced, or parallel to the electrode with results yielding distributions along the electrodeaxis. In both cases, the second measuring dimension can be included by moving the spectro-graph and performing successive measurements. For offering a better understanding of bothmeasuring orientations, they are presented in the drawing in figure 5.16.

(a) (b)

Figure 5.16: Orientation of the spectrograph entrance slit (arrow) to the HPS-electrode withtungsten coil perpendicular for radial measurements (a) and parallel for axial measurements (b).The +/- arrow indicates the moving possibility of the spectrograph for measuring a seconddimension.

For measurements with the spatial resolution perpendicular to the electrode, cf. figure5.16(a), the mathematical relation between the measured parallel projection of the absorptionsignal h(x) and the radial absorption coefficient α(r) is given by the inverse Abel transforma-tion in chapter 4.2.1. However, the absorption measurement perpendicular to the electrodebetween the tungsten coil and the electrode tip in the HPS-lamp has a very special geometrywhich is sketched in figure 5.17. The backlight passing the HPS-lamp is shadowed by theelectrode rod in the spatial middle of the measurement so that a projection of a cylindricaldischarge with a maximum in the middle cannot be assumed.

The electrode radius in figure 5.17 is Ri. Within the electrode radius (r ≤ Ri) the backlightis blanked out by the electrode shadow so that Iν(x)abs = 0 for −Ri ≤ x ≤ Ri, correspondingto αL(−Ri ≤ x ≤ Ri) → ∞. Between x = Ri and x = Ra where Ra is the radius of thesapphire tube, a measurement may be possible. In figure 5.18 an example of a measuringresult for h(x) is given with an LED as backlight source. Only in the vicinity of the electrode

112 5. Measurements and results

ΔxRi

r = Ra

tungstenelectrode

sapphire-tube

backlight

0

spectrograph slit position x

y

Δy

Figure 5.17: Sketch of the special geometry of an absorption measurement perpendicular to thetube axis in a HPS-lamp between the tungsten coil and the electrode tip. The electrode rod inthe middle, shadowing the backlight, has to be considered for inverse Abel transformation.

−1 −0.5 0 0.5 10

0.2

0.4

0.6

0.8

1

spatial position x / mm

Ba

line

abso

rptio

n / a

.u.

Figure 5.18: Radially resolved measurement of relative Ba line-absorption at the Ba resonanceline λ = 553 nm, d = 70µm behind the electrode tip, HPS-lamp 3b3 (Gobin new), del = 900µm,dcoil−tip = 1.0 mm, operated with power controlled Philips standard HPS-lamp driver at P = 140W, f = 200 kHz triangular current

surface, within an interval of ∆x = 100− 200µm, a distinct absorption was measured. Thesignal is stronger on the upper side of the electrode than on the lower side, presumablycaused by a buoyancy flow around the hot electrode. This outcome shows that a rotational

5.2. Results at the high-pressure sodium lamps 113

symmetry is lacking. Additionally, the measuring outcome is quite noisy as it is shown infigure 5.18. Therefore a standard procedure by which αL(r) could be evaluated from inverseAbel transformation is not applicable in this case.Taking into account that an absorption is only present in a thin layer, an estimate of αL canbe made according to

αL = − 1

π

∆h

∆x

∆x√x2 − r2

(5.1)

where ∆x is the width of the measuring signal in figure 5.18 (see also equation 4.10). With

x2 −R2i = (Ri + ∆x)2 −R2

i ≈ 2Ri∆x

−∆h

∆x∆x ≈ hmax

where hmax is the maximum value of h(x), the equation for αL can be converted to

αL =1

π

hmax√2Ri∆x

. (5.2)

The information that the absorption is limited to a thin layer adjacent to the electrode isquite useful. But the accuracy by which radially resolved Ba-densities can be measured withthis method is rather poor mainly by reason of a low signal to noise ratio SNR. Therefore,another measuring and evaluation procedure for the Ba diffusion process was developed toget a better signal to noise ratio and higher accuracy.The main aim of the whole HPS-lamp investigations is the measurement of the barium diffu-sion from the tungsten coil along the electrode shaft to its tip. Thus, the radial distributionis not the actual aim. It is set aside for gaining a higher measuring accuracy and better rep-resentation of the diffusion along the electrode within the HPS-lamp. Accordingly, instead ofa perpendicular, a parallel orientation of the spectrograph slit with respect to the electrodeis chosen (cf. figure 5.16(b)). Thereby the barium absorption signal can be measured with ahigher quality close to the electrode on its topside and spatially resolved from the end of thetungsten coil to the electrode tip. The final accuracy is additionally increased by averagingover (mostly five) spatial intervals parallel to the electrode axis since this information issufficient for analysing the diffusion process of barium.The length ∆y of the optical path of the backlight through the layer of barium vapour isassumed to be constant while keeping the same measuring distance to the electrode. It isestimated by a radial projection of the outcome in figure 5.18. With this new developedevaluation method, the inverse Abel transformation, being quite sensitive against measuringerrors, can be avoided with the advantage of more accurate results for the barium density.Additionally, the radial distribution of barium adjacent to the electrode can be measured bymoving the spectrograph entrance slit to another distance of the electrode. Of course, the esti-mation of the optical absorption path length ∆y has to be recalculated for this measurement.Depending on the thickness of the Ba-layer around the electrode and the measuring distanceto the electrode surface, the values for ∆y vary between ∆y = 0µm and ∆y = 470µm. Infigure 5.19 a schematic drawing of the principle arrangement is given by which the densitydistribution parallel to the electrode is measured.

114 5. Measurements and results

displacementof spectrographslit

spatialintegration

orientation ofspectrograph slit

tungsten coilHPS-electrode

Δy

Figure 5.19: Schematic drawing of the final absorption measuring and evaluation method adjacentto the electrode in the HPS-lamp with slit orientation parallel to the electrode, calculation ofspatial intervals and spectrograph movement for radial distribution.

Ba results by LED-backlight absorption spectroscopy

For the first absorption measurements which are presented in this chapter, the LED systemis used as backlight source. The LED-absorption measurement presented in figure 5.20 is

-2 -1.5 -1 -0.5 0 0.50

5

10

15

x 1013

position along electrode from tip / mm

Ba

atom

den

sity

/ c

m-3

Figure 5.20: Measured Ba atom density results along the electrode rod from tip to coil for HPS-lamp 3b3 (dcoil−tip = 1 mm), measured by absorption spectroscopy with LED-backlight, lampis operated with the Philips standard HPS-lamp driver at P = 140 W, f = 200 kHz triangularcurrent

performed at the HPS-lamp 3b3 which has a coil to electrode tip distance of dcoil−tip = 1 mmand an electrode diameter of d = 0.9 mm. The lamp is operated by means of the Philips

5.2. Results at the high-pressure sodium lamps 115

standard driver which is power controlled at P = 140 W, f = 200 kHz triangular current.The first Ba density measurement, presented in figure 5.20, is performed with an orientationof the spectrograph slit perpendicular to the electrode and produces thereby a transversaldistribution. But the transversal distribution has a very poor SNR as discussed in the previ-ous chapter. Moreover it is limited to a thin layer adjacent to the electrode surface as shownin figure 5.18. Therefore, only a raw analysis of the measuring result is possible. Inserting themaximum value hmax gained by the absorption measurement and the thickness of the layer∆x into equation 5.2 an average value of the barium density in the layer was determined. Torealise a position dependent measurement as it is shown in the sketched half cut electrodein figure 5.20, eight successive measurements are performed by moving the spectrographstepwise along the electrode. In principle, the scanning procedure offers the possibility of atwo dimensional measurement of the barium density adjacent to the electrode. But it wassubstituted in the subsequent measurements by the parallel measuring procedure presentedin the previous chapter to achieve a higher SNR.The course of the Ba density shown in figure 5.20 does not match with the general idea ofdiffusion process. In the case of stable lamp operation, the barium is stored in the tungstencoil around the electrode, which operates as a barium source, and the barium diffuses alongthe electrode to its tip where it is vaporized in the attachment zone of the arc. Thus, incontrast to the first results given in figure 5.20, one would expect a decreasing Ba density ofbarium towards the electrode tip.But the measurement of the HPS-lamp 3b3 which was performed shortly after its first startshows the opposite behavior, namely an increasing barium density from the coil to the elec-trode tip. This unexpected effect seems to reflect a contamination of the electrode duringthe lamp production. During their assembling, the electrodes are dived into a solution of abarium compound to fill the tungsten coil. Even after a cleaning process of the electrodeshaft some parts of the barium seem to stay on the electrode surface and increase its densityin front of the electrode surface during the first lamp burning hours. This might induce theunexpected result in figure 5.20.

To remove any initial effect caused by a barium contamination of the electrode shaft, theinvestigated HPS-lamps in the following have all been burned in for at least 20 hours. Thisprocedure eliminates the initial Ba contamination as it is shown by the measuring resultspresented in figure 5.21. Herein, a typical diffusion process can be recognised by the decreas-ing barium density towards the electrode tip as it was expected before.However, as it is visible by comparing the results in figure 5.20 with the ones in figure 5.21,

the overall Ba density is more than an order of magnitude lower in the burned-in lamp thanin the new, contaminated lamps. It results in a worse signal quality for the measurement inburned-in lamps and makes the radial resolved measurements even more difficult. Thus, themeasuring method has to be changed from the perpendicular to a parallel orientation of thespectrograph entrance slit with respect to the electrode as it was previously discussed andshown in figure 5.19.The results given in figure 5.21 are determined in the 2b3 HPS-lamp, which has a coil toelectrode distance of dcoil−tip = 0.6 mm and an electrode diameter of d = 0.9 mm. As itwas expected, the barium density decreases from the tungsten coil to the electrode tip andincreases by measuring closer to the electrode. The results confirm the assumption of a Badiffusion process from the tungsten coil as Ba source to the electrode tip as Ba drain. Addi-tionally, the high Ba density which is only present closely adjacent to the electrode is a firstproof for a strong surface diffusion effect. However, the presented results in figure 5.21 are the

116 5. Measurements and results

-0.4 -0.2 0 0.2 0.4 0.61

1.5

2

2.5

3

3.5x 10

12

position along electrode from coil / mm

Ba

atom

den

sity

/ c

m-3

62 μm

187 μm

312 μm

562 μm

Distance xfrom electrode

x

Figure 5.21: Measured Ba atom density results along the electrode rod from tip to coil for HPS-lamp 2b3 (dcoil−tip = 0.6 mm), measured by absorption spectroscopy with LED-backlight, lampis operated with the Philips standard HPS-lamp driver at P = 140 W, f = 200 kHz triangularcurrent

first Ba absorption results which are reliable also from a theoretical point of view regarding astandard diffusion process. Moreover, the signal quality gained by the parallel orientation ofthe spectrograph entrance slit and the lamp electrode is sufficient for an analysis and furtherinterpretation of the data. Accordingly, this data is used by our project partner PhilipsLighting, NL as input and optimisation parameters for a first simulation of the Ba diffusionin the HPS-lamps.

For a further increase of the measuring accuracy and to measure also lower barium den-sities, a change of the backlight system to one with a much higher light output power isunavoidable. Figure 5.22 shows the logarithmic comparison of the absolute spectral lightintensity of the operated 5 W green LED and the filtered 120 W UHP-lamp system, cf. chap-ter 3.3.1. The spectrum is taken in the vicinity of the Ba resonance-line at λBa = 553 nmwhere the spectral power of the backlight system determines the accuracy and sensitivityof the Ba absorption measurement. As it can be found in figure 5.22, the output power ofthe UHP-backlight system in the desired wavelength interval is a factor of 100 higher thanthe corresponding output power of the high-power LED. Moreover, the spectrally resolvedlight output of the UHP-lamp around λBa = 553 nm is very smooth so that the requiredfitting procedure discussed in chapter 4.2.3 is easily applicable. Overall, the advantages ofthe UHP-backlight system for Ba density absorption measurements are so significant thatthe measurements presented in the following chapter are exclusively performed by use of thistechnique with a high accuracy.

5.2. Results at the high-pressure sodium lamps 117

545 550 555 56010

4

105

106

107

λ / nm

inte

nsity

/ a.

u.

UHPLED

Figure 5.22: Comparison of the spectral backlight output power of the 5 W green LED andthe filtered 120 W UHP-lamp system, the spectrum is taken in the vicinity of the measured Baresonance-line at λBa = 553 nm

Ba results by UHP-backlight absorption spectroscopy

62 μm

187 μm

312 μm

437 μm

Distance xfrom electrode

-1 -0.5 0 0.5 10

5

10

15x 10

11

position along electrode from coil / mm

Ba

atom

den

sity

/ c

m-3

x

Figure 5.23: Measured Ba atom density results along the electrode rod from tip to coil forHPS-lamp 3b1 (dcoil−tip = 1.0 mm), measured phase averaged by absorption spectroscopy withUHP-backlight, lamp is operated at iRMS = 2 A, f = 400 Hz switched-dc current

118 5. Measurements and results

Figure 5.23 shows the first axially resolved results determined by use of the improved UHP-baklight system. The investigated HPS-lamp is the 3b1, which has a coil to electrode tipdistance of dcoil−tip = 1.0 mm and an electrode diameter of d = 0.9 mm. Thus, the geomet-rical lamp data is comparable to the results given in figure 5.20. The distinctly lower Badensity in this measurement, compared to figure 5.20, is a further proof for the initial ac-cumulation of barium caused by a contamination of the electrode shaft with Ba during itsmanufacturing process. The lamp in figure 5.23 is measured after a burn in time of 20 hours.The lamp 3b1 in this measurement is not operated by the standard Philips driver but bythe scientific power amplifier (cf. chapter 3.2) at a switched-dc current of iRMS = 2 A witha frequency of f = 400 Hz. The tendencies of the course of the barium density within themeasurement of the 3b1 lamp (figure 5.23) are the same as in the 2b3 measurement withLED-absorption in figure 5.21. Again, a typical distribution for a diffusion process is visible,characterised by the decrease of Ba towards the tip. Moreover, the layer of barium is verythin around the electrode as it is demonstrated by the strong decrease of the density withincreasing distance x to the electrode surface.

To have a comparison to the measurement in figure 5.23, the same measurement with exactlythe same electrical properties and measuring method is performed once again on the lamp3b3 which also has a coil to electrode tip distance of dcoil−tip = 1.0 mm and an electrodediameter of d = 0.9 mm. The results can be used to analyse the reproducibility of the Badiffusion process within the HPS-lamp and to verify the accuracy of the developed UHP-absorption measuring method.A comparison between figure 5.23 and 5.24 confirms that the barium density is in both cases

-1 -0.5 0 0.5 12

4

6

8

10

12x 10

11

62 μm

187 μm

312 μm

437 μm

Distance xfrom electrode

position along electrode from coil / mm

Ba

atom

den

sity

/ c

m-3

x

Figure 5.24: Measured Ba atom density results along the electrode rod from tip to coil forHPS-lamp 3b3 (dcoil−tip = 1.0 mm), measured phase averaged by absorption spectroscopy withUHP-backlight, lamp is operated at iRMS = 2 A, f = 400 Hz switched-dc current

5.2. Results at the high-pressure sodium lamps 119

in the same order of magnitude and reflects the same spatial course being characteristic fora diffusion process. In figure 5.24, the differences between the measurements at differentdistances x from the electrode surface are not so significant as in figure 5.23. On the otherhand, the maximum density of barium measured in the lamp 3b1 in figure 5.23 is higherthan that in the lamp 3b3 in figure 5.24. However, the presented Ba density results showminor differences between two comparable HPS-lamp measurements but they are still in theacceptable range of production tolerances between the lamps 3b1 and 3b3.

The comparative measuring results confirm that a reliable absorption measurement of thebarium density in a HPS-lamp is possible if a powerful UHP-lamp is used as backlight source.They all represent a distribution being typical for a diffusion process along the electrode shaft.Moreover, the UHP absorption measuring method developed and implemented within theHPS-lamp project can now be adopted to a whole set of barium density measurements withdifferent operating parameters or even for special analysis. An example is presented in thenext section.

Due to the geometric limits of optical particle density measurements, the Ba surface dif-fusion along HPS-lamp electrodes can only be investigated by means of measurements byinterpreting its effect on the Ba density in the adjacent gas-phase. However, modeling of theBa surface diffusion process by Philips led to different results: Representing the Ba diffusionas a migration of a monolayer along the electrode surface, an increase of the barium densityfrom the tungsten coil to the electrode tip is expected by theory. This discrepancy betweenmeasuring results and theoretical results will be discussed in the consecutive chapter.

The barium diffusion process in measurements and theory

As it was already mentioned, besides the absorption measurements in HPS-lamps at theRuhr-University Bochum, the barium diffusion process was modelled theoretically by PhilipsLighting. The theoretical results are based on a simulation of the Ba density, assuming thatit diffuses as a monolayer along the electrode surface from the tungsten coil to the electrodetip. They lead to a different spatial dependency of the Ba density compared to the presentedmeasurements in the gas-phase close to the electrode surface. Theoretical results predict alow density of barium in front of the surface of the tungsten coil as source and a higherdensity at the electrode tip, where the barium is consumed within the arc attachment zone.This behaviour is opposite to the measurements shown in figure 5.21, 5.23 and 5.24.

The reason for this discrepancy between measurement and theory may be an ionisation ofbarium in the vicinity of the electrode tip which is not taken into account within the absorp-tion measurements. The applied and discussed absorption measuring method at λ = 553 nmonly detects barium groundstate atoms but not excited states of barium atoms and also notBa ions. Thus, a high plasma temperature Tpl resulting in a high excitation and ionisationdegree of Ba at the plasma arc attachment on the electrode tip would explain a significantreduction of the Ba atom density. Therefore, a switch-off experiment is performed to checkthis assumption by measuring the Ba atom density along the electrode during lamp operationas usual and tdelay = 50µs after switching off of the HPS-lamp. The time constant for therelaxation of plasma composition, such as excitation and ionisation, is assumed to be in the

120 5. Measurements and results

sub microsecond range. In contrast, the relaxation of the Ba diffusion process lasts longerand should be in the order of some hundred microseconds. Thus, the experiment shouldshow a rising Ba atom density at the electrode tip after switch-off if the barium ions andexcited state atoms contribute significantly to the barium density during lamp operation.The corresponding measuring results are given in figure 5.25.As shown in figure 5.25, the absolute barium density decreases very fast after switch-off and

10.50

4

6

8

10

12x 10

11

position along electrode from coil / mm

Ba

atom

den

sity

/ cm

−3

operationswitch−off

Figure 5.25: Measured Ba atom density results along the electrode rod from tip to coil for HPS-lamp 3b1 (dcoil−tip = 1.0 mm), measured during operation (solid, red line) and tdelay = 50µs afterswitch-off with a camera exposure time of texp = 500µs (dotted, blue line), lamp is operatedwith the Philips standard HPS-lamp driver at P = 140 W, f = 200 kHz triangular current

a diffusion process is hardly visible after tdelay = 50µs. However, the switch-off experimentdoes not yield results which clearly confirm or refuse the assumption of a high barium iondensity as the reason for the discrepancy between measuring results and theoretical mod-elling. Additionally, optical emission spectroscopy has been performed to identify barium ionsat the electrode tip. As the investigation of strong barium ion spectral lines at λ = 490 nm,λ = 493 nm and λ = 614 nm [70] did not lead to any signal, strong ionisation of barium atthe electrode can be excluded.

Besides ions, also a large number of atoms in excited states might influence the measurementsof the barium density and the interpretation of the Ba diffusion process. The absorption spec-troscopy setup can only measure groundstate atoms at the resonance line λBa = 553 nm. Butthe fraction of excited states can be estimated if the partition function of barium atomsis calculated assuming a Boltzmann distribution (cf. chapter 2.2). As the Ba temperatureTBa is not measured separately within the absorption measurement, the ratio of ground-stateatoms n0 to all atoms n (excited and ground-state) is determined and shown in table 5.1for typical plasma temperatures in high-pressure sodium lamps [64]. The calculated ratioindicates that the existence of excited state atoms cannot be neglected but their influence isnot strong enough to change the generally measured diffusion characteristic.

5.2. Results at the high-pressure sodium lamps 121

TBa/K 4500 4750 5000 5250 5500n0/N 0.48 0.44 0.40 0.36 0.33

Table 5.1: Ratio of ground-state Ba atoms n0 to the number of all atoms (excited states andground-state) n for different Ba temperatures TBa

According to the results of the described switch-off experiment and the calculation of theexcited states, other reasons for the difference between the simulated Ba surface diffusionand the measured density results have to be considered. The explanation might be foundin the different position for measuring and modelling: The theoretical modelling of Philipsconsiders exclusively a barium surface diffusion whereas the presented density measurementswithin this work are made in the gas-phase in a distance of at least 50µm from the electrodesurface. Thus, the absorption measurements can only reflect the density of Ba evaporatedfrom the surface which depends not only on the surface diffusion process but also on theelectrode temperature. Accordingly, the measurements of the Ba density in the gas-phaseadjacent to the electrode surface can only be used to investigate and characterise the Basurface diffusion indirectly from its impact on the Ba density in the gas-phase. A direct in-vestigation of the Ba surface diffusion by spectroscopic measurements is much more complexor even impossible.

However, the difference between the measured and simulated Ba distribution along the elec-trode cannot be completely clarified within this work. An analysis of the inner wall depositwithin an aged HPS-lamp has been performed by Philips Light Labs to analyse the dis-tribution of evaporated barium in the electrode region. But this analysis shows a relativelyconstant Ba deposition along the wall. Thus, even the additional investigations do not lead toa clarification whether the density of barium on the electrode surface increases or decreasesbetween the tungsten coil and the electrode tip in a HPS-lamp.

5.2.2 The influence of hydrogen in HPS-lamps

Results at the HPS getter/ dispenser lamp

Within this chapter, the developed and tested UHP absorption measuring setup is used toanalyse the barium content of some specially designed HPS-lamps which have a tunable bar-ium pressure. As it was already mentioned in chapter 3.1.2, the dispenser/ getter HPS-lampshave an extended outer vessel which can be filled with and purged by hydrogen by meansof an external heating which activates a hydrogen dispenser or a hydrogen getter. A higherhydrogen pressure in the outer vessel increases the heat conductivity between the burnerand the surrounding atmosphere so that the temperature of the burner is reduced. Thiseffect can be recognized by a lower lamp voltage due to a lower cold-spot temperature andlower partial pressure of sodium inside the sapphire burner. Thus, the falling lamp voltageof the HPS-lamp is a good indicator for a higher hydrogen concentration between the outerand the inner tube. Additionally, hydrogen diffuses through the hot Nb feedthrough of theburner into the discharge tube. Thermodynamic calculations performed by Philips indicatethat traces of hydrogen within the burner may lift the barium density in the HPS-lamp. As

122 5. Measurements and results

hydrogen pollution up to a certain amount is an unavoidable effect during lamp production,its theoretically predicted influence on the Ba content is investigated by UHP absorptionmeasurements adjacent to the HPS electrodes and discussed within the following results.

0 0.5 1

0.5

1

1.5

x 1012

position along electrode from coil /mm

Ba

atom

den

sity

/ cm

−3

getter on

getter off 30min

disp. on 10min

disp. on 30min

disp. on 40min

Figure 5.26: Ba atom density results along the electrode rod from tip to coil for the dispenser/getter HPS-lamp at several times of the getter and dispenser heating cycle, measured phaseaveraged by absorption spectroscopy with UHP-backlight, lamp is operated at iRMS = 2 A, f =400 Hz switched-dc current

To test the correlation between an increase of the hydrogen in the outer bulb and the concen-tration of barium in the sapphire burner, several independent UHP absorption measurementsare performed at the HPS-lamp electrodes. The first measurement, shown in figure 5.26, re-flects in principle the expected behaviour: While heating the getter, the density of barium isstabilised within a timescale of roughly 30 minutes. When the dispenser is heated, a responseis again recognised after 30 minutes by a higher concentration of barium. But the effect ofthe external hydrogen supply on the barium density is much lower than it was expected fromthe theoretical thermodynamic simulations. The results of the calculations claim an increaseof the barium density by a factor of 5− 10, but within the measurement in figure 5.26 onlya doubled concentration is found.Consecutive measurements within the same dispenser/ getter HPS-lamp show even less Badensity variations than presented in figure 5.26. The expected correlation between a higherhydrogen pressure (when the dispenser is heated) and an increase of the barium density,which is only slightly visible in figure 5.26, is absent during all further measurements of thislamp. The reason for this unexpected lamp behaviour might be found in a wrong operationor a wrongly preadjusted parameter range of the getter and dispenser heating. This assump-tion has been proved by subsequent analysis of the dispenser and getter hydrogen contentby Philips. The chemical analysis show that the hydrogen content in the dispenser is moreor less exhausted while the getter is filled with the rest of the hydrogen. Within this firstHPS-lamp dispenser/ getter measurements, the external heating seems to be operated incor-rectly. Obviously, the dispenser and the getter got activated at the same time and exchanged

5.2. Results at the high-pressure sodium lamps 123

the hydrogen content directly between each other without diffusing into the inner sapphireburner. Accordingly, the initial Ba density measurements, which are exemplarily shown infigure 5.26, are repeated with a new HPS-lamp. The new HPS-lamp is equipped with a freshhydrogen dispenser and a new hydrogen getter.

After replacing the spoiled getter and dispenser part in the vessel extension of the spe-cial HPS-lamp, the barium density measurement is repeated several times in dependency ofthe hydrogen pressure. As it is more important to see the time development of the hydrogenpressure (monitored by the lamp voltage) and of the barium density, the following measure-ments are made time-resolved. Thus, only a mean value of the barium density is plotted,determined by an integral measurement between the tungsten coil and the electrode tip. Inaddition, to have comparable measuring results, the measurements of the getter/ dispenserHPS-lamp in figure 5.27, 5.28 and 5.29 are made by executing the same standard procedure:At first, the getter is activated by heating it to a temperature above Tgetter = 300 ◦C, af-terwards the lamp is operated without external heating to reach an equilibrium. A slightlyrising lamp voltage is expected during this process step. Then, the dispenser is heated toa temperature around Tdispenser = 170 − 190 ◦C. Thereby, it is activated and supplies anincreased pressure of hydrogen in the outer bulb. Within this process step, an immediatereduction of the lamp voltage Ulamp is expected. But, Ulamp has to be kept always above45 Volts by regulating the external dispenser heating to avoid the risk of a driver securityswitch-off. Finally, the getter is heated once again to turn the lamp back into its initial stateand to measure the time constant of this process.Figure 5.27 shows the result of a first time resolved Ba density measurement in the dis-

40

50

60

lam

p v

olt

age

Ula

mp

/V

0 50 100 150 200 250 300 350

1

2

x 1012

lamp operation time tlamp

/ min

-3

Ba density

Ulamp

getter on getter ondispenser on

Figure 5.27: Time development of the averaged Ba atom density results along the electrodeand lamp voltage for the dispenser/ getter HPS-lamp, measured phase averaged by absorptionspectroscopy with UHP-backlight. The lamp is operated with the Philips standard HPS-lampdriver at P = 140 W, f = 200 kHz triangular current

penser/ getter HPS-lamp while executing the described heating procedure. The dependency

124 5. Measurements and results

of the barium density on the hydrogen pressure, monitored by the lamp voltage, is clearlyvisible. The time constant of the barium density reduction when the getter is heated isdistinctly longer than the time constant of its increase when the dispenser is heated. Thereaction of the lamp voltage, showing the dependence of the hydrogen pressure on heatingthe getter or dispenser, is very sensitive in this measurement and takes place only with ashort time delay. But, as can be seen in figure 5.27, the reduction/ increase of the bariumdensity is again in the range of a factor of 2 and not of a factor of 5 − 10 as it is expectedfrom theory. On the other hand, the getter-on phases are chosen presumably too short sinceat its end the barium density doesn’t achieve a constant value. Thus, the measurement wasrepeated with the same conditions but longer time scales for the getter-on phase in figure5.28.The results in figure 5.28 show a measurement with the same procedure as used in figure

lam

p v

olt

age

Ula

mp

/V

lamp operation time tlamp

/ min-3

Ba density

Ulamp

getter on getter ondispenser on

0 50 100 150 200 250 300 350 40059

60

61

62

63

64

65

66

0 50 100 150 200 250 300 350 4004

5

6

7

8

9

10

11x 10

11

Figure 5.28: Time development of the averaged Ba atom density results along the electrodeand lamp voltage for the dispenser/ getter HPS-lamp and longer getter-on phases, measuredphase averaged by absorption spectroscopy with UHP-backlight, lamp is operated with the Philipsstandard HPS-lamp driver at P = 140 W, f = 200 kHz triangular current

5.27 but with longer time scales for the getter-on phases to reach expected equilibrium val-ues of the barium density. However, it is clearly visible that within the second measurementthe special HPS-lamp does not operate in the same parameter range as in the first mea-surement presented in figure 5.27. The achievable lamp voltage reduction is only roughly∆Ulamp,2 = 5 V compared to ∆Ulamp,1 = 15 V in the measurement in figure 5.27. Addition-ally, it is not stable during the heating process of the dispenser. The barium density, whichis much lower than in the previous measurement, shows at least the right reaction on thegetter- and dispenser-phases. But the variation of the barium density in the HPS-burnerstarts sometimes earlier than that of the lamp voltage which is contrary to the logical cor-relation. Moreover, the time constants of the barium density at the beginning of the twogetter-on phases in figure 5.28 are different.To verify the unexpected results in figure 5.28, a third measurement is performed at the

5.2. Results at the high-pressure sodium lamps 125

HPS-lamp, leading to the same problem: The voltage decrease and correlated Ba density in-crease is even worse. Since an interpretation of the bad results of the third dispenser/ getterHPS-lamp measurement will only mislead, it is not shown in a separate graph here.

Considering the results of the last two Ba density measurements in the dispenser/ getterHPS-lamp, the lamp seems to have a problem comparable to the measurement in figure 5.26.Obviously it is not only an operating fault of the external heating as presumed in the begin-ning because the dispenser part in the measurement in figure 5.28 is again empty or unableto provide hydrogen due to other physical reasons. The cause for the high consumption ofhydrogen during a measurement cycle as shown in figure 5.27 might be found in the chemicalbehaviour of the Nb feedthrough of the sapphire burner. According to thermodynamic ef-fects, the niobium material has a high storage capacity of hydrogen and is presumably filledtemporarily with a huge amount of the hydrogen during the dispenser-on phase. Afterwards,the hydrogen stored in the niobium is absorbed by the getter and is no longer available forfurther measurements. This effect can presumably pump the complete amount of hydrogenfrom the dispenser to the getter during one single measuring procedure, which is much fasterthan previously expected.

Thus, the lamp is rebuild a third time and equipped with new dispenser and getter partsto perform at least one reliable measurement which offers absolute timeconstants for thehydrogen diffusion into the burner and for the Ba increase triggered by the hydrogen con-tent. To allow a higher number of consecutive measurements, the amount of hydrogen in thedispenser is doubled in the rebuild HPS-lamp. The new HPS-lamp is operated and measuredwith the same parameters and procedures as in figure 5.27 and 5.28. The results are givenin figure 5.29.The results in figure 5.29 show an initial reduction of the averaged Ba density as it is ex-

pected during the first process step when the getter is heated. In contrast to the measurementin figure 5.27, the lamp voltage is not rising during the getter heating phase which indicatesno significant reduction of the hydrogen pressure in the first step. However, after heating thedispenser, the lamp voltage drops significantly down, indicating an increase of the hydrogenpressure. It induces also an increase of the barium density by a factor of four. Together withthe measurement in figure 5.27, this is a second proof for the correlation of the hydrogenpressure and the barium density in the HPS-lamp. In the last measuring cycle, after switch-off of the dispenser heating, the lamp voltage rises quickly to the initial level. This indicatesan unexpected self-gettering of the hydrogen but it clearly induces the expected reductionof the barium density.

After switch-off of the dispenser heating, the Ba density in figure 5.29 decreases with acertain time constant. To measure this time constant more precisely, the measurement in fig-ure 5.29 is repeated with longer intervals between the heating cycles and with a planned quickdecrease of the hydrogen pressure by heating the getter immediately after the dispenser-onphase. The results of this second measurement at the rebuild HPS-lamp are shown in figure5.30.As already found in the previous measurements, the Ba density decreases during the first

heating of the hydrogen getter due to its correlation with the reduced hydrogen pressure.The initial lamp voltage Ulamp in the measurement presented in figure 5.30 is clearly higherthan in the measurement given in figure 5.29 which indicates a change in the lamp operatingconditions. Also the response of the lamp voltage to a heating of the hydrogen dispenser is

126 5. Measurements and results

50 100 150 200 250 300 350 40045

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-3

lam

p v

olt

age

U/

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mp

dis

pen

ser/

get

ter

tem

p. /

°C

Ba density

Ulamp

Figure 5.29: Time development of the averaged Ba atom density results along the electrode,the lamp voltage and the dispenser/ getter temperature for the dispenser/ getter HPS-lamp,measured phase averaged by absorption spectroscopy with UHP-backlight. The lamp is operatedwith the Philips standard HPS-lamp driver at P = 140 W, f = 200 kHz triangular current

Ba

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Ulamp

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/ min

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6

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10

100

200

300

400

500

dispenser

getter

Figure 5.30: Second time development of the averaged Ba atom density results along the electrode,the lamp voltage and the dispenser/ getter temperature for the dispenser/ getter HPS-lamp,measured phase averaged by absorption spectroscopy with UHP-backlight, lamp is operated withthe Philips standard HPS-lamp driver at P = 140 W, f = 200 kHz triangular current

different, the voltage decreases only by ∆Vlamp = 10 V in the second measurement comparedto ∆Vlamp = 16 V in the previous measurement shown in figure 5.29. Even by rising the tem-

5.2. Results at the high-pressure sodium lamps 127

perature of the hydrogen dispenser from Tdispenser = 155 ◦C to Tdispenser = 250 ◦C, the initiallamp voltage reduction and thereby the hydrogen pressure of the first measurement cannotbe reached. Accordingly, the second measurement shown in figure 5.30 is interrupted after300 minutes as it is not possible to reproduce the necessary hydrogen pressure conditions fora determination of the correlated timeconstants of the barium density.Obviously, the hydrogen in the dispenser of the rebuild HPS-lamp is again consumed duringthe first measurement. This is a surprising result since the rebuild HPS-lamp is equippedwith a hydrogen dispenser having a doubled storage capacity. However, it is again not pos-sible to measure the exact time constant for the hydrogen diffusion into the sapphire tubeand the delay time for the coupling between the hydrogen pressure within the envelope andthe Ba density within the tube.

Summarising finally the results of the hydrogen influence on the Ba density, the mea-surements on the dispenser/ getter HPS-lamp show a clear correlation between the hy-drogen pressure and the Ba density around the lamp electrodes which was the main aimof the presented investigations. Even a rough estimation of the timeconstant of the hydro-gen diffusion into the sapphire vessel and the timeconstant of the influence of the hydro-gen pressure on the Ba density can be found in the measurements presented in figure 5.27and 5.29: The lamp voltage decrease, correlated with a rising hydrogen density, starts ap-proximately tdelay,hydrogen−dispenser = 12 minutes after the beginning of the dispenser heatingwhereas the Ba density increase starts tdelay,barium−dispenser = 16 minutes after the dispenserheating. The hydrogen time delay of tdelay,hydrogen−dispenser = 12 minutes depends on sev-eral effects, for example on the effectiveness of the heat coupling between the heating ovenand the dispenser material and is therefore not very meaningful. But the time difference oftdelay,hydrogen−barium = 4 minutes between the hydrogen and the barium time delay offers adirect information of the timeconstant of the Ba density response on the H increase. Furthermeasurements for the investigation of the hydrogen pollution are only useful if special HPS-lamps can be fabricated in which the complicated and up to now unreproducible hydrogenabsorption and desorption process in the lamp can be controlled. However, it was not possi-ble and favoured within the research cooperation project of the Ruhr-University, AEPT andPhilips Lighting.

5.2.3 Electrode temperature measurements in the HPS-lamps

Finally, within this last HPS subchapter, electrode temperature measurements in some ofthe investigated HPS-lamps will be given for high and low operating frequencies. The pre-sented measurements are performed with the spectroscopic IR-measurement at λ = 718 nm(cf. chapter 4.1) and complete the emitter-effect investigations of barium in the HPS-lamps.The electrode temperature measurements are only done and presented exemplarily on someHPS-lamps due to the dominant importance of the barium density measurements. Anotherreason is the problematic interpretation of the results in the region of the tungsten coil.

Figure 5.31 shows the results of the electrode temperature measurements for the lamp 6,2,which is the oldest HPS-research lamp investigated in Bochum. The lamp 6,2 has a tip tocoil distance of dcoil−tip = 1.4 mm and an electrode diameter of d = 0.9 mm. The results are

128 5. Measurements and results

0 10.5−0.5−11600

1700

1800

1900

2000

2100

position along electrode from coil / mm

elec

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f = 0.1 Hz

f = 1 Hz

f = 10 Hz

(a) middle of anodic phase

0 0.5 1−0.5−11600

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position along electrode from coil / mm

elec

trod

e te

mpe

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re /

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f=0,1Hz

f=1Hz

f=10Hz

(b) middle of cathodic phase

Figure 5.31: Electrode temperature distribution in the HPS-lamp 6, 2 for various low frequenciesa) in the middle of the anodic phase and b) in the middle of the cathodic phase. Parameter:operating current iRMS = 2 A, switched-dc

given for a low frequency operation from f = 0.1 Hz to f = 10 Hz and an operating currentof iRMS = 2 A switched-dc. Within this low frequency range, it is possible to measure a phaseresolved temperature distribution along the electrode rod with the spectroscopic setup. Re-sults for the middle of the anodic phase are given on the left side of figure 5.31 and theresults for the middle of the cathodic phase on the right side. One can see that within theanodic phase the temperature at the electrode and close to the tip is significantly higher fora low frequency operation whereas within the cathodic phase, the course of the temperaturealong the electrode is only weakly dependent on frequency. For f = 0.1 Hz and f = 1 Hzthe electrode temperature is higher within the anodic phase than within the cathodic phase.But for f = 10 Hz the temperature in the cathodic phase passes that in the anodic phase.

Figure 5.32 shows the corresponding electrode temperature measurement for higher oper-ating frequencies. It is done at the same lamp and with the same parameters as the lowfrequency measurement in figure 5.31. Due to the short time periods for high frequencies,it is not possible to distinguish between anodic and cathodic phase of the electrode in themeasurement. Therefore, the results in figure 5.32 represent a phase averaged value. Againa decrease of the electrode temperature with increasing frequency is observed in agreementwith the results given in figure 5.31. The temperature decrease slows down with increasingfrequency.

The reason for the observed dependency might again be found in the impact of the emitter-effect in the anodic phase as already discussed in chapter 3.1.1. For low frequency operation,the Ba density at the arc attachment area induces an emitter-effect only in the cathodicphase and disappears within the anodic phase. But for higher frequencies, the relaxationtime of the barium monolayer is higher than the anodic half period. In this case, the bariumlayer can survive on the electrode surface even in the anodic phase. Thus, an emitter-effect ofbarium might also be possible in the anodic phase for higher frequencies. This assumption ofan “anodic emitter-effect” may explain the observed reduction of the electrode temperature

5.2. Results at the high-pressure sodium lamps 129

0 0.5−0.5−1 1

1600

1700

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1900

position along electrode from coil / mm

elec

trod

e te

mpe

ratu

re /

K

f=100Hz

f=500Hz

f=1kHz

f=10kHz

f=20kHz

Figure 5.32: Electrode temperature distribution in the HPS-lamp 6, 2 for various high frequenciesaveraged over phase. Parameter: operating current iRMS = 2 A, switched-dc

with increasing frequency.

The temperature ripples, occuring at a distance of 1.4 mm from the electrode tip in the mea-surements in figure 5.31 and 5.32, are caused by the windings of the tungsten coil around theelectrode in the HPS-lamp. Reminding the discussion in chapter 4.1, the emissivity ε of thetungsten material, which is necessary for a temperature calculation from the measurement,is not well defined on the windings. Thus, the temperature calculated for the windings ofthe tungsten coil and their interspaces are only estimates. Accordingly, if a more detailedinterpretation is desired, the coil temperatures which control the supply of barium for thediffusion process should be determined by modelling and simulation of the heat conductionfrom the hot electrode rod to the coil and between the windings.

Another explanation for the decreasing electrode temperature with increasing operating fre-quency presented in figure 5.31 and 5.32 might be found in the quality of the switched-dclamp current with frequency. It is known that the rectangle shape of the lamp current sig-nal of the power amplifier is deteriorated at higher frequencies leading to a reduction of thephase averaged lamp input power (cf. chapter 3.2). This effect could explain a lower electrodetemperature for higher operating frequencies in the setup. To prove or to disprove this as-sumption, the frequency depending electrode temperature measurement is repeated for highfrequencies in the HPS-lamps 3b1 (dcoil−tip = 1.0 mm) and 4b1 (dcoil−tip = 1.4 mm). Duringthese repeated measurements, which are shown in figure 5.33 and 5.34, the phase averagedinput power into the lamp is measured by the oscilloscope and adjusted to Pin,el = 140 Wfor each frequency separately.

The results in figure 5.33 for the lamp 3b1 and in figure 5.34 for the lamp 4b1 show again thesame frequency dependence of the electrode temperature distribution as the previous mea-

130 5. Measurements and results

10.50−0.5

1900

2000

2100

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2300

position along electrode from coil / mm

elec

trod

e te

mpe

ratu

re /

K

f=500Hz

f=1kHz

f=10kHz

f=20kHz

Figure 5.33: Electrode temperature distribution in the HPS-lamp 3b1 (dcoil−tip = 1.0 mm) forvarious high frequencies averaged over phase. Parameter: power controlled operation at Pin,el =140 W, switched-dc

1.40.50−0.5−1 1

1700

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position along electrode from coil / mm

elec

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e te

mpe

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K

f=500Hz

f=1kHz

f=10kHz

f=20kHz

Figure 5.34: Electrode temperature distribution in the HPS-lamp 4b1 (dcoil−tip = 1.4 mm) forvarious high frequencies averaged over phase. Parameter: power controlled operation at Pin,el =140 W, switched-dc

surements: While rising the frequency from f = 500 Hz to f = 20 kHz, the averaged electrodetemperature distribution decreases approximately by ∆T = 25 K in both cases. But the ab-solute temperature reduction for a rising frequency is much lower in the power controlledmeasurements than in the current controlled measurements. This result is due to a higherpower input for higher frequencies compared to the measurements in figure 5.31 and 5.32

5.2. Results at the high-pressure sodium lamps 131

since the effective power reduction by a distorted switched-dc signal at higher frequencies iscompensated. However, the tendency of a lower electrode temperature for a higher operatingfrequency as it was found for Th-electrodes in the Th containing YAG-lamps (cf. chapter5.1.2) is also present in the HPS-lamp with Ba emitter and thus confirms the assumptionof an anodic emitter-effect for a high frequency operation. The reduction of the electrodetemperature of the HPS-lamp with increasing operating frequency is the first indication forthe extension of the emitter effect onto the anodic phase of the electrode for high operatingfrequencies.

132 5. Measurements and results

5.3. Results at the YAG-lamps 133

5.3 Results at the YAG-lamps

This final results chapter gives an overview about the possibilities and limitations of thedeveloped broadband absorption measurement concerning the investigation of the gas-phaseemitter-effect. The gas-phase emitter-effect of dysprosium is determined in ceramic HID-lamps made of YAG (yttrium aluminium garnet) material for research purposes (cf. chapter3.1.3) and compared to the emitter-effect of other rare earth materials like cerium. As al-ready described in chapter 3.1.3, commercial HID-lamps are usually filled with a variety ofdifferent salt ingredients to adjust e.g. the power input into the arc, the spectral light out-put and physical parameters like the electrode temperature. These complex lamp fillings inHID-lamps have a distinct, mostly negative influence on the efficiency of the emitter-effectrepresented by a change of the electrode temperature for different salt mixtures. An examplefor the influence of different salt mixtures on the electrode tip temperature in YAG-lampsfilled with Dy emitters is shown in figure 5.35.

2700

2800

2900

3000

3100

3200

phase

elec

trod

e tip

tem

pera

ture

Ttip

/ K

0 π 2π

pure Hg

Dy

NT

NTD1

NTD2

Figure 5.35: Phase resolved electrode tip temperature for YAG-lamps with different salt-fillings(pure Hg, Dy, NT, NTD1, NTD2). Parameters: dE = 450µm, electrode length lE = 5 mm,iRMS = 800 mA switched-dc, f = 100 Hz

Figure 5.35 shows the phase resolved tip temperature of YAG-lamps which have differentsalt fillings, an electrode diameter of dE = 450µm and are equal in all other parameters. Thelamps are all operated with a current of iRMS = 800 mA, f = 100 Hz switched-dc. It is clearlyvisible that the YAG-lamp with a pure Hg buffer gas filling has a high electrode tip tempera-ture of Ttip,Hg ≈ 3180 K whereas the YAG-lamp which is seeded exclusively by Dy has a muchlower tip temperature of Ttip,Dy ≈ 2750 K. This reduction of the electrode tip temperatureof ∆Ttip = 430 K while keeping the same operating conditions of the lamp is a proof forthe emitter-effect induced by a dysprosium filling in the YAG-lamp. The YAG-lamps NTD1and NTD2, which are filled with certain amounts of dysprosium, thallium and sodium (cf.chapter 3.1.3), have tip temperatures of Ttip,NTD1 ≈ 2980 K and Ttip,NTD2 ≈ 3090 K locatedin between the temperature results of pure Hg and pure Dy YAG-lamps. Thus, salt additions

134 5. Measurements and results

like thallium and sodium have a poisoning effect and reduce the emitter-effect of Dy, result-ing in a higher Ttip. The amount of Dy in the salt mixture of the NTD1-lamp is higher thanin the NTD2-lamp, accordingly, the tip temperature of the NTD1-lamp is lower than thatof the NTD2-lamp representing different intensities of poisoning by Na and Tl. For a com-parison, the electrode tip temperature of a YAG-lamp without Dy but filled with Na and Tlis included in figure 5.35. The electrode temperature of this NT-lamp Ttip,NT is in the rangeof the electrode temperature Ttip,Hg of the pure Hg lamp showing that the emitter-effect isonly induced by an addition of dysprosium in these YAG-lamps.

However, the results which are exemplarily shown in figure 5.35 demonstrate that the saltmixture within a YAG-lamp has a distinct influence on the emitter-effect. At least in com-mercial lamps it is unavoidable to use different salt ingredients beside the emitter materialto adjust the lamp spectrum and other physical parameters. Accordingly, it is important toinvestigate the emitter-effect of dysprosium in YAG-lamps with simple, pure fillings but alsoto investigate poisoning effects of other ingredients in YAG-lamps with more complex saltfillings comparable to commercially used HID-lamps.The following chapter gives results of Dy density measurements by absorption spectroscopyand Ttip measurements in pure Dy YAG-lamps to investigate advantages, disadvantages andoptimal amount of Dy-fillings. Afterwards, results of measurements in YAG-lamps with morecomplex salt fillings are presented and discussed with main focus on the poisoning effect ofthallium. In the end, the broadband absorption measurement is applied to commercial PCA-lamps, made out of translucent ceramic materials and finally the absorption measurement inYAG-lamps is combined with emission measurements to determine the plasma temperatureTpl. The presented measurement of the plasma temperature Tpl might become very impor-tant for future HID-lamp research in Hg-free lamps as it is completely independent of themercury content within the HID-lamp.

5.3.1 Pure Dy-lamps - variation of the Dy-amount of the salt mixture

To investigate the emitter-effect of dysprosium and influencing effects of other salt ingre-dients systematically in ceramic YAG-lamps, it is useful to start with the easiest availablesalt filling in YAG-lamps: pure Dy. Within this chapter, three different YAG-lamps withan electrode diameter of dE = 360µm and a pure filling of 1 mg, 2 mg, and 4 mg Dy areinvestigated concerning the amount of Dy in the gas-phase and its effect on the electrodetemperature. One disadvantage of the pure Dy YAG-lamps has to be mentioned in advanceto be considered while interpreting the following results: The YAG wall material is a trans-parent ceramic as discussed in chapter 3.1.3 but it is extremely attacked by the pure Dy saltfilling. This wall attack of Dy on the YAG material results in a reduced transmission of thelamps right from the first operation and becomes even worse over the lamp lifetime. In prin-ciple, one advantage of the absorption spectroscopy measurement is that it is not influencedby the transmission of the measured lamp (cf. chapter 4.2.3) as long as the measured UHPbacklight signal is strong enough to be detected. However, the wall attack of dysprosiumeven leads to a crystallisation of the YAG-material which scatters the measured backlightwithin the absorption measurement and thereby reduces the accuracy of the results. Thisproblem becomes more pronounced with an increasing Dy amount in the YAG-lamp.

5.3. Results at the YAG-lamps 135

Figure 5.36 shows the first Dy density results measured by absorption spectroscopy in YAG-

2

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16x 10

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1mg Dy, 50 Hz

1mg Dy, 100 Hz

2mg Dy, 50 Hz

2mg Dy, 100 Hz

4mg Dy, 50 Hz

4mg Dy, 100 Hz

(a) i = 500 mA (b) i = 800 mA

Figure 5.36: Phase resolved Dy atom groundstate density 125µm in front of the electrode forYAG-lamps with 1 mg, 2 mg and 4 mg pure Dy salt fillings. Parameters: dE = 360µm, electrodelength lE = 5 mm, iRMS = 500−800 mA switched-dc, low frequency operation at f = 50−100 Hz

lamps with a 1− 4 mg pure Dy filling for (a) iRMS = 500 mA and (b) iRMS = 800 mA at lowoperating frequencies (f = 50− 100 Hz). For both currents, the Dy density curves show theexpected phase modulation in accordance to the previously determined emitter supply by aDy ion current due to cataphoresis: The Dy density decreases to nDy,500mA = 0−2 ·1014 cm−3

for iRMS = 500 mA and to nDy,800mA = 0.5−1·1015 cm−3 for iRMS = 800 mA during the anodicphase (always represented by the first half period) since the transport by a Dy ion flux is re-versed by the inverted electric field in front of the anode. In contrast, the Dy density increasesduring the cathodic phase to nDy,500mA = 10−14 ·1014 cm−3 and nDy,800mA = 3−4 ·1015 cm−3

due to the supply by a Dy ion flux from the plasma boundary sheath. Because of the lowerpower input into the whole YAG-lamp, the Dy atom density values are generally lower forthe iRMS = 500 mA current than for the iRMS = 800 mA current as clearly visible in figure5.36. Of course it has to be considered during interpretation, that the Dy atom density re-sults measured by means of absorption spectroscopy always reflect the Dy ion flux and theDy atom coverage of the electrode surface indirectly and with a certain delay. According tothe conclusion of the previous HPS-lamp chapter, a measurement directly on the electrodesurface is impossible by spectroscopy and the performed Dy atom density measurement canonly show the impact of the cataphoretic Dy ion flux to the electrode indirectly. Hence, theincrease or decrease of the Dy atom density always starts with a certain time delay aftercurrent zero crossing (CZC) of the switched-dc current in figure 5.36.However, the Dy atom density results given in figure 5.36 (a) and (b) for the 4 mg Dy YAG-lamp do not show the expected values with regard to the other results as they are significantlybelow the density values for the 1 mg and 2 mg Dy YAG-lamp. This artificially low Dy atomdensity within the 4 mg Dy YAG-lamp is the result of a high measuring inaccuracy due tothe discussed YAG material erosion leading to scattering of the measuring backlight beam.Hence, the Dy atom density results within the 4 mg Dy YAG-lamp are not reliable and willtherefore be left out within the following result graphs.The Dy density curves shown in figure 5.36(a) for a lower current and (b) for a higher current

136 5. Measurements and results

do obviously not reach a saturated equilibrium value, indicating that the final cathodic Dydensity might increase further in the case of lower operating frequencies. However, especiallyfor the iRMS = 800 mA operation in figure 5.36(b), which is the YAG-lamp nominal current,the Dy atom density values are almost equal during the anodic and cathodic phase for thedifferent YAG-lamp types. Minor deviations between the single Dy density results can beexplained by the YAG erosion problem which is more pronounced for the 2 mg Dy lamp.This result shows that the partial pressure of the Dy atoms within the YAG-lamp gas-phasemight already be saturated for a 1 mg Dy salt filling.

In comparison to the Dy atom density for low frequency operation, figure 5.37 shows cor-responding Dy atom density results for a high frequency operation (f = 500 Hz - 1 kHz) ofthe 1 mg and the 2 mg YAG-lamp for low (500 mA) and high (800 mA) currents. The Dydensity results of the 4 mg Dy YAG-lamp are left out within figure 5.37 due to the discussedaccuracy problems. It is clearly visible in figure 5.37(a) and especially in figure 5.37(b) that

(a) i = 500 mA (b) i = 800 mA

Figure 5.37: Phase resolved Dy atom groundstate density 125µm in front of the electrode forYAG-lamps with 1 mg and 2 mg pure Dy salt fillings. Parameters: dE = 360µm, electrode lengthlE = 5 mm, iRMS = 500− 800 mA switched-dc, high frequency operation at f = 500 Hz−1 kHz

the modulation of the Dy ion density and resulting atom density over one phase cycle isalmost disappeared. The inertia of the measured Dy particles is obviously too high to followthe electric field movement within these operating frequency ranges. This phenomenon wasalready discussed in the context of the anodic emitter-effect within chapter 5.1.2. Moreover,the measuring results plotted in figure 5.37 represent averaged values over the anodic andcathodic phase indicated by horizontal lines as the exposure time of the PCO SensiCamCCD camera adjacent to the spectrograph (cf. chapter 3.3) cannot be reduced further todetermine a higher phase resolution with a sufficient accuracy.Also in the case of HF frequency operation, the general Dy atom density nDy,500mA =0.5 − 2 · 1014 cm−3 for a low operating current of iRMS = 500 mA in figure 5.37(a) is lowerthan the Dy atom density nDy,800mA = 2− 6 · 1014 cm−3 for the nominal YAG-lamp currentof iRMS = 800 mA in figure 5.37(b). The absolute values of the Dy density over the wholephase cycle in figure 5.37(a) and (b) are similar to the corresponding absolute density valuesduring the anodic phase for a low frequency operation presented in figure 5.36. Obviously, the

5.3. Results at the YAG-lamps 137

formation of a Dy monolayer due to a cataphoretic Dy ion current during the cathodic phaseand the depletion of the monolayer by Dy desorption during the anodic phase, as visible forLF operation, is less pronounced during HF lamp operation in figure 5.37. This observationof an almost balanced Dy atom density adjacent to the electrode surface is in agreement withthe assumption of an anodic emitter-effect which was observed for high operating frequenciesof Th YAG-lamps in chapter 5.1.2.

To confirm the assumption of a Dy ion and atom movement due to the phase cycle inthe vicinity of the electrodes for LF operation and an almost balanced density for HF oper-ation, the phase resolved Dy atom density in the middle between the electrodes is measuredas a reference value. This measurement of particle densities in the lamp middle is exclusivelyavailable by means of the established absorption spectroscopy measurement since an emis-sion spectroscopy measurement is not possible in the middle of HID-lamps due to the lackof Hg spectral emission lines.A measuring result of the Dy atom density in the middle of the 1 mg and 2 mg Dy YAG-lamp

(a) i = 500 mA (b) i = 800 mA

Figure 5.38: Phase resolved Dy atom groundstate density in the discharge middle between theelectrodes for YAG-lamps with 1 mg and 2 mg pure Dy salt fillings. Parameters: dE = 360µm,electrode length lE = 5 mm, iRMS = 500−800 mA switched-dc, low and high frequency operationat f = 100 Hz−1 kHz

for low and high current values (iRMS = 500 − 800 mA) is given in figure 5.38. The graphsfor the lower current (a) and the higher current (b) in figure 5.38 both include a measuringresult for the low frequency operation of f = 100 Hz and a result for high frequency operationof f = 1 kHz. It is visible within this figure that the modulation of the Dy atom density isdisappeared in the lamp middle especially for the case of low frequency operation in contrastto the graphs given in figure 5.36. Thus, the movement of Dy towards and backwards to theelectrodes, forming a Dy monolayer for the emitter-effect, is only present adjacent to theelectrode surface.The absolute values nDy,500mA = 3 − 5 · 1014 cm−3 for iRMS = 500 mA and nDy,800mA =1 − 1.3 · 1015 cm−3 for iRMS = 800 mA of the Dy atom density in the middle of the YAG-lamps, presented in figure 5.38, are almost equal for the 1 mg Dy and the 2 mg Dy salt filling.This observation is another indication for the previously discussed assumption of a saturated

138 5. Measurements and results

Dy partial pressure in the gas-phase already for a 1 mg Dy salt filling. Moreover, the absolutevalues shown in figure 5.38 are comparable to the Dy atom density results during the anodicphase for LF operation presented in figure 5.36 and are roughly two times higher than thealmost constant HF density results presented in figure 5.37. It can be concluded that theDy atom density in YAG-lamps shows a movement with the electric field exclusively for lowoperating frequencies and only close to the electrode surface in the vicinity of the plasmaboundary sheaths. For high operating frequencies of f ≥ 500 Hz and at positions within theplasma bulk far away from the electrode sheaths, the Dy atom density is almost in agree-ment with absolute values in the range of the results given in figure 5.38 and does not showa dependence on the operating electric field.

To investigate the effect of the observed Dy atom densities and their phase modulationsadjacent to the electrodes on the emitter-effect, the measured phase resolved electrode tiptemperatures Ttip for (a) iRMS = 500 mA and (b) iRMS = 800 mA at low operating frequencies(f = 1−10 Hz) are given in figure 5.39. It is clearly visible in figure 5.39 that the electrode is

(a) i = 500 mA (b) i = 800 mA

Figure 5.39: Phase resolved electrode tip temperature Ttip for YAG-lamps with 1 mg, 2 mg and4 mg pure Dy salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm, iRMS =500− 800 mA switched-dc, low frequency operation at f = 1− 10 Hz

heated during the anodic phase and cools down during the cathodic phase for iRMS = 800 mAas well as for iRMS = 500 mA operation. In the case of iRMS = 500 mA, the electrode tip tem-perature deviation is ∆Ttip ≈ 500 K for the 1 Hz operation and ∆Ttip ≈ 100 K for the 10Hzoperation. In contrast, in the case of iRMS = 800 mA, the electrode tip temperature deviationis ∆Ttip ≈ 700 K for the 1 Hz operation and ∆Ttip ≈ 200 K for the 10Hz operation. For af = 100 Hz operation, which is not plotted in figure 5.39, the electrode tip temperature is al-most constant with Ttip,100Hz = 2565 K at a current of iRMS = 500 mA and Ttip,100Hz = 2870 Kat iRMS = 800 mA. The turning point where the plasma in the cathodic phase heats theelectrode stronger than within the anodic phase is obviously positioned below a current ofiRMS = 500 mA. This turning point is moved to lower currents by an active emitter-effectdue to its additional temperature reduction within the cathodic phase in accordance to theresults of chapter 5.1.2. This observation is comparable to results of Reinelt in [31].Within certain error margins, the electrode tip temperature results presented in figure 5.39

5.3. Results at the YAG-lamps 139

for different amounts of Dy salt filling are equal to each other, reflecting the almost equalDy density results in front of the electrode given in figure 5.36 and 5.37. Obviously, theemitter-effect of Dy is already sufficiently active for a 1 mg Dy salt filling whereas the elec-trode tip temperature results of the 2 mg Dy YAG-lamp are even slightly higher. However,especially in the case of the 4 mg Dy YAG-lamp, the YAG material is attacked leading touncertain, lower Ttip results. Therefore, deviations between the electrode tip temperaturesfor 1, 2, 4 mg Dy YAG-lamps should not be overinterpreted. It can be concluded from figure5.36 that the Dy emitter-effect acts almost equal within the YAG-lamps with different Dyfillings indicated by equal Ttip values.

For comparing the low frequency Ttip results (cf. figure 5.39) and for investigating the anodicemitter-effect of Dy in YAG-lamps, the correlated electrode tip temperature values for ahigh frequency operation of f = 500 Hz - 1 kHz are given phase resolved in figure 5.40. It is

2250

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e tip

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pera

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/ K

0 π 2π

1mg Dy, 500 Hz

1mg Dy, 1 kHz

2mg Dy, 500 Hz

2mg Dy, 1 kHz

4mg Dy, 500 Hz

4mg Dy, 1 kHz

(a) i = 500 mA (b) i = 800 mA

Figure 5.40: Phase resolved electrode tip temperature Ttip for YAG-lamps with 1 mg, 2 mg and4 mg pure Dy salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm, iRMS =500− 800 mA switched-dc, high frequency operation at f = 500 Hz−1 kHz

clearly visible within this figure that the electrode tip temperature for HF operation is almostconstant over phase which was expected due to the time constants of the heat capacity ofthe tungsten electrode. Moreover, the electrode tip temperature Ttip decreases with increas-ing lamp operating frequency for an iRMS = 500 mA current and also for iRMS = 800 mAcurrent as visible in figure 5.40(a) and (b), respectively. The decreasing tip temperature foran increasing lamp operating frequency is even more pronounced while comparing the LFresults in figure 5.39 to the HF results in figure 5.40 and proves the existence of an anodicemitter-effect in YAG-lamps also for the Dy emitter material.It has to be mentioned that the direct comparison of absolute Ttip values for 1, 2, 4 mg DyYAG-lamps is again doubtful due to the erosion of the YAG material. Hence, the deviationsbetween the Ttip results of different YAG-lamps presented in figure 5.40 might be caused bythis measuring error whereas the frequency dependency of the Ttip results obtained from onesingle YAG-lamp are directly comparable.

To prove the determined assumption of a saturated Dy atom partial pressure already for

140 5. Measurements and results

a 1 mg Dy filling, which was originally not expected, several additional investigations areperformed. A deviation of the electrical input power into the 1, 2, 4 mg Dy YAG-lamp is con-sidered and the Dy atom densities are additionally checked by the so-called “∆λ-method”.The almost equal Dy atom densities in the 1 − 4 mg Dy YAG-lamps obtained by the mea-surement in figure 5.36 and 5.37 might be caused by a significantly differing ionisation degreewithin the different lamp types. To check this possible influence, the Dy ion density in frontof the YAG-lamp electrode is measured by means of emission spectroscopy and given infigure 5.41 for iRMS = 500 mA (a) and iRMS = 800 mA (b). This Dy ion measurement cannotbe substituted by an absorption spectroscopy method since the absorption strength of anyavailable Dy Ion line is too weak. This is one of the main disadvantages concerning the pre-sented BBAS method with UHP backlight.The observed Dy ion densities for iRMS = 500 mA in figure 5.41(a) and for iRMS = 800 mA

(a) i = 500 mA (b) i = 800 mA

Figure 5.41: Phase resolved Dy ion density 125µm in front of the electrode for YAG-lamps with1 mg and 2 mg pure Dy salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 500− 800 mA switched-dc, low and high frequency operation at f = 100 Hz−1 kHz

in figure 5.41(b) are almost equal in the case of 1 mg Dy filling and 2 mg Dy filling. Even forthe 4 mg YAG-lamp the Dy ion density results are in the same order of magnitude, but theirabsolute values vary due to the discussed bad YAG material quality and they are thereforeomitted in figure 5.41. However, the Dy ion results in figure 5.41 prove that the ionisationdegree within the different YAG-lamps does not differ significantly. Moreover, the assump-tion of a generally saturated Dy partial pressure, obtained by atom density measurements,is not disproved by the effect of the Dy ion density.While comparing the Dy ion densities in figure 5.41 to the absolute Dy atom densities in figure5.36 and 5.37, the conversion factor of 4.3 (cf. chapter 4.2.3) between the shown atom ground-state density within the figures and the total atom density has to be considered. Includingthis conversion factor, the ionisation degree amounts to roughly nDy,ion/nDy,atom = 18% forthe 500 mA current and nDy,ion/nDy,atom = 12% for the 800 mA current. It corresponds tothe decrease of the ionisation degree with increasing vapour pressure at constant plasmatemperature described by the Saha-equation 2.13 according to

5.3. Results at the YAG-lamps 141

(ni(Dy)

na(Dy) + ni(Dy)

)2

=kBTplSDy(Tpl)

kBTplSDy(Tpl) + p(Dy)(5.3)

with

SDy(Tpl) =2Ze

Za

exp

(−Ei(Dy)

kBTpl

). (5.4)

Another possible explanation, leading to comparable Dy atom density results in 1, 2, 4 mgDy containing YAG-lamps, might be a differences in the electrical power consumption of thedifferent YAG-lamp types. A YAG-lamp with a significantly higher electrical power consump-tion would have a higher cold-spot temperature of the liquid salt pool leading to enhancedDy atom densities. Therefore, the electrical power input PRMS in dependence on frequencyis measured for all three YAG-lamps and plotted in figure 5.42(a) for iRMS = 500 mA and infigure 5.42(b) for iRMS = 800 mA. Of course, the accuracy of the power input results PRMS

does not depend on the optical quality of the YAG material, thus the results for the 4 mgDy YAG-lamp are directly comparable to the other YAG-lamps.It is visible in figure 5.42(a) and figure 5.42(b) that the different power input values are

100

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operating frequency (log) / Hz

elec

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t PR

MS /

W

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2mg Dy

4mg Dy

(b) i = 800 mA

Figure 5.42: Electrical lamp input power PRMS for YAG-lamps with 1 mg, 2 mg and 4 mg pureDy salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm, iRMS = 500 − 800 mAswitched-dc, frequency depending from f = 1 Hz to f = 1 kHz

comparable for all three YAG-lamps with a deviation of ∆PRMS = 3.6% for iRMS = 500 mAand of ∆PRMS = 2.9% for iRMS = 800 mA. The influence of these small PRMS variations onthe cold-spot temperature and the correlated Dy atom density within the different YAG-lamps can be neglected.It has to be mentioned, that the observed general decrease of the electrical power input intothe YAG-lamps for an increasing operating frequency (cf. figure 5.42) is caused by the usedpower amplifier. It was already discussed in chapter 3.2 that the slope of the switched-dccurrent signal provided by the driving power amplifier is distorted for operating frequenciesin the range of kHz.

Finally, to prove the unexpected, almost equal Dy atom density results within the 1 mg

142 5. Measurements and results

and 2 mg Dy YAG-lamps in figure 5.36 and 5.37, a so-called “∆λ measurement” of the Dyatom density is considered: According to [56] and [77], the distance ∆λ between the twomaxima of a self absorbing emission spectral line, in this case λDy = 404 nm, is proportionalto the particle density, here nDy ∝ ∆λDy,404. Different to the spectroscopy measurementof absolute Dy density values, the relative ∆λ-method is not influenced by transmissionproblems of the YAG material or by numerical problems during calibration or absolute dataprocessing. However, the exact distance of the two maxima is often difficult to determine dueto the shape of the emission line. Thus, a ∆λ-measurement is only able to prove or disprovegeneral trends found by absolute emission- or absorption spectroscopy and can therefore beconsidered in addition.A result of the ∆λ-measurement determined from the self absorbing Dy atom resonance line

(a) i = 500 mA (b) i = 800 mA

Figure 5.43: ∆λ-method for Dy density 125µm in front of the electrode in YAG-lamps with 1 mg,2 mg and 4 mg pure Dy salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 500− 800 mA switched-dc, low and high frequency f = 1 Hz-f = 1 kHz

at λDy,self−abs = 404.6 nm for iRMS = 500 mA and iRMS = 800 mA at low and high operatingfrequencies is shown in figure 5.43. The results of the ∆λ-method presented in figure 5.43confirm the previously determined Dy atom density results. The results within this figurealso indicate a decreasing Dy atom density during the anodic phase and an increasing Dyatom density during the cathodic phase as found in figure 5.36. The general Dy densitywithin the 1 mg and 2 mg Dy YAG-lamps is almost equal, which is additionally proven bythe roughly similar courses of ∆λ404.6 in figure 5.43. Furthermore, figure 5.43 shows thateven the Dy density within the 4 mg lamp is comparable to both other lamps. The presentedresult for the 4 mg lamp can only be obtained by means of the ∆λ-method as it does notdepend on YAG transmission problems due to erosion. It confirms the overall assumption ofa saturated Dy partial pressure within the YAG-lamps already for a 1 mg Dy salt filling.

In summary it can be stated from the measurements in pure Dy YAG-lamps that the emitter-effect of Dy is clearly visible, which is in agreement with former results from Reinelt [31]and Langenscheidt [22]. Also the existence of an anodic emitter-effect of Dy can be provedby means of a measured decreasing Ttip for an increasing operating frequency of pure Dylamps. Moreover it can be determined from Dy atom density measurements by absorption

5.3. Results at the YAG-lamps 143

spectroscopy, that the partial pressure of Dy within the gas-phase of the YAG-lamp is al-ready saturated for a salt filling containing 1 mg of DyI3. Thus, a further addition of Dy intothe YAG-lamp salt pool (e.g. 2 mg or 4 mg) does not lead to a higher Dy vapour density ora more pronounced Dy emitter-effect on the lamp electrodes. However, a higher amount ofDy inside the YAG-lamp salt pool leads obviously to an increased YAG erosion problem bymeans of a liquid capillary effect.However, investigations of the emitter-effect within pure Dy YAG-lamps do not reflect theconditions of real, commercial HID-lamps. Commercial HID-lamps usually contain a complexmixture of salt additions leading to a desired lamp emission spectrum with a high efficacyand a high CRI. The emitter-effect of Dy is of course influenced and often hampered by thesecomplex salt mixtures within commercial HID-lamps. Thus, the Dy emitter-effect will be in-vestigated for NTD YAG-lamps within the next subchapter. Besides Dy, NTD YAG-lampscontain sodium and thallium (cf. table 3.2) to adjust an acceptable light emission spectrum.These NTD lamps reflect the most simple types of commercially used HID-lamps and willtherefore be investigated concerning the Dy emitter-effect within this work.

144 5. Measurements and results

5.3.2 Results in the NTD-lamps

The investigation of the Dy emitter-effect in NTD lamps containing dysprosium, sodium andthallium salts is an intermediate step towards measurements in real, commercial HID-lampswhich usually contain a variety of complex salt ingredients. Due to multiple overlappingemission and absorption spectral lines in commercial HID-lamps, which cannot be evalu-ated separately, an investigation of a certain effect like the Dy emitter-effect by absolutespectroscopy methods is usually impossible. Therefore, NTD-lamps, which are investigatedwithin this chapter, are only filled with the two most important HID-lamp salts, sodium andthallium, in addition to the Dy emitter salt, which was investigated separately in pure DyYAG-lamps within the previous chapter. However, the additional sodium and thallium saltswithin an NTD YAG-lamp already show a significant influence on the efficiency of the Dyemitter-effect, which will be investigated in detail here. While comparing measuring resultsof pure Dy lamps with 1 mg Dy iodide content to results of NTD-lamps it has to be takeninto account that the dysprosium iodide content of the NTD1 and NTD2 lamps amounts to2.11 mg and 1.17 mg, respectively.

The best classifications and conclusions concerning the efficiency of the emitter-effect can beobserved by investigating the electrode tip temperatures Ttip of operating HID electrodes.Thus, a measurement of the phase resolved Ttip distribution is performed for NTD1-lamps

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3000

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elec

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e tip

tem

pera

ture

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/ K

0 π 2π

NTD1, 1 Hz

NTD1, 10 Hz

NTD1, 100 Hz

NTD2, 1 Hz

NTD2, 10 Hz

(a) i = 500 mA

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ture

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/ K

0 π 2π

NTD1, 1 Hz

NTD1, 10 Hz

NTD1, 100 Hz

NTD2, 1 Hz

NTD2, 10 Hz

NTD2, 100 Hz

(b) i = 800 mA

Figure 5.44: Phase resolved electrode tip temperature Ttip for YAG-lamps with NTD1 (10%Dy) and NTD2 (5% Dy) salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 500− 800 mA switched-dc, low frequency operation at f = 1− 100 Hz

containing 10% Dy within their salt mixture and for NTD2-lamps containing 5% of Dyemitter material. The results of this electrode tip temperature measurement for low oper-ating frequencies (f = 1 − 100 Hz) are shown for iRMS = 500 mA in figure 5.44(a) and foriRMS = 800 mA in figure 5.44(b). The electrode geometry of the investigated NTD-lampsand their electrical driving parameters are similar to the pure Dy YAG-lamps of the pre-vious chapter, thus Ttip values in figure 5.44 are directly comparable to the pure Dy-lampresults given in figure 5.39.It is clearly visible that the electrode tip temperatures for NTD1 and NTD2-lamps in figure

5.3. Results at the YAG-lamps 145

5.44 are ∆Ttip,Dy−NTD ≈ 200 K higher than the values for the pure Dy YAG-lamps in thecase of iRMS = 500 mA operation and ∆Ttip,Dy−NTD ≈ 150−200 K higher for iRMS = 800 mA,respectively. However, the Ttip results of NTD-lamp electrodes are still significantly lowerthan values for pure Hg lamp electrodes: For YAG-lamp electrodes with similar geometricproperties, Ttip of pure Hg lamps amounts to Ttip,Hg = 3253 K averaged over phase for aniRMS = 500 mA, f = 100 Hz switched-dc operation which is ∆Ttip,NTD−Hg ≈ 300 − 500 Khigher than the NTD-lamp results shown in figure 5.44(a). For a high current operationof iRMS = 800 mA, the pure Hg lamp results of Ttip,Hg = 3437 K are still ∆Ttip,NTD−Hg ≈200 − 300 K higher than the comparable Ttip values presented in figure 5.44(b). In conclu-sion, the emitter-effect of Dy is still active in NTD-lamps containing sodium and thallium,represented by a reduced Ttip with respect to pure Hg YAG-lamps, but its efficiency is sig-nificantly lower than in pure Dy lamps due to these additional salt ingredients.Additionally, it is visible in figure 5.44(a) that the electrode tip temperatures for the NTD1-lamp are always lower than that for the NTD2-lamp. This difference is almost removedin the case of iRMS = 800 mA presented in figure 5.44(b). However, these Ttip differencesbetween NTD1 and NTD2 YAG-lamps are mainly caused by changes of the type of arcattachment which occur for iRMS = 500 mA operation. The Ttip values for the NTD1-lampin figure 5.44(a) are obtained from a spot mode arc attachment, whereas the results fromthe NTD2-lamp are obtained from a diffuse arc attachment with a characteristically higherglobal electrode tip temperature. Thus, deviations of the electrode tip temperature deter-mined from figure 5.44 cannot be correlated to the different amounts of Dy in the NTD saltmixtures. Moreover, figure 5.44(b) shows that the Ttip results of an NTD1 and an NTD2YAG-lamp are almost equal at a current of iRMS = 800 mA which produces a stable diffusearc attachment.

The general trends of Ttip in NTD YAG-lamps can also be confirmed by comparable mea-surements at high operating frequencies of f = 500 Hz and 1 kHz presented in figure 5.45.Also in the case of HF operation, the electrode tip temperatures of NTD1 and NTD2-lamps

(a) i = 500 mA (b) i = 800 mA

Figure 5.45: Phase resolved electrode tip temperature Ttip for YAG-lamps with NTD1 (10%Dy) and NTD2 (5% Dy) salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 500− 800 mA switched-dc, high frequency operation at f = 500 Hz−1 kHz

146 5. Measurements and results

are higher than comparable results for pure Dy-lamps (cf. figure 5.40) but significantly lowerin comparison to Ttip results determined from pure Hg lamps like Ttip,Hg = 3207 K (foriRMS = 500 mA, f = 1 kHz switched-dc) and Ttip,Hg = 3539 K (for iRMS = 800 mA, f = 1 kHzswitched-dc). However, the deviations of the Ttip results for NTD1 and NTD2-lamps in figure5.45(a) are again caused by an unstable, constricted arc attachment during the measurement.The differences between the electrode tip temperatures for NTD1 and NTD2-lamps presentedin figure 5.45(b) at higher currents and with a stable arc attachment are much lower andtherefore more reliable for interpretations.While comparing the reliable Ttip results for different operating frequencies in figure 5.44(b)and 5.45(b), the general trend of a decreasing electrode temperature for an increasing op-erating frequency is again observed. Thus, the assumption of an active emitter-effect of Dyduring the anodic phase which was observed from pure Dy-lamp measurements is obviouslyalso approved for NTD YAG-lamps.

For completeness, a result of the electrode input power Pin = Ploss, which is determinedby a simulation of the electrode heat balance (cf. chapter 4.1.4) and which can be consideredas a power loss with regard to the HID-lamp efficacy, is exemplarily given for NTD-lamps andfor a 1 mg pure Dy YAG-lamp in figure 5.46. The electrode input power results in this figure

(a) i = 800 mA, low frequency (b) i = 800 mA, high frequency

Figure 5.46: Phase resolved electrode input power Pin for YAG-lamps with 1mg pure Dy, NTD1(10% Dy) and NTD2 (5% Dy) salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 800 mA switched-dc, low and high frequency operation

reflect the same correlations which were already obtained by means of the Ttip measurements:The electrode is heated during the anodic phase and cools down during the cathodic phase.Moreover, the input power of the pure Dy-lamp is significantly lower than Pin for NTD-lampsdue to the active emitter-effect of Dy, which is hampered by sodium and thallium withinthe NTD-lamps as clearly visible in figure 5.46. However, the Pin values for the NTD1 andNTD2 at low and high operating frequencies are almost similar and do not show a significantcorrelation to the different amounts of Dy filling.It can be concluded from the electrode input power results in figure 5.46 that the investigationof Pin does not provide additional information concerning the Dy emitter-effect with respectto the electrode tip temperatures. This observation was assumed before, since Pin = Ploss

5.3. Results at the YAG-lamps 147

is directly linked to the electrode tip temperature Ttip by means of the evaluated electrodeheat balance. However, the determination of the electrode tip temperature leads to a higheraccuracy (cf. chapter 4.1.4), therefore an evaluation of Pin values will be omitted within thefollowing investigations of the emitter-effect.

As it is known now from the presented Ttip measuring results that the Dy emitter-effectis reduced within NTD YAG-lamps, an interesting question occurs whether the sodium ad-dition or the thallium addition of the NTD salt mixture is mainly responsible for this effect.With an ionisation energy of Ei,Na = 5.14 eV, sodium has the lowest ionisation energy ofall three lamp salt ingredients. The ionisation energies of Ei,Dy = 5.94 eV for dysprosiumand Ei,Tl = 6.11 eV for thallium are significantly higher than the Na values. Accordingly, thesodium addition in the NTD-lamps increases to electron density, reduces the plasma temper-ature and reduces thereby the ion density of dysprosium in the gas-phase. A reduced Dy iondensity results finally in a reduced ion flux transporting Dy emitter material by cataphoresistowards the electrode surface. This assumed interrelationship might explain the observedreduction of the Dy emitter-effect in NTD-lamps and should be proved or disproved withinfurther investigations.To check the reduction of the Dy ion flux towards the electrode surface by the ionised Naparticles, the correlated Dy atom density in front of the NTD electrode is measured by thebroadband absorption spectroscopy method. The results of this nDy measurement for a low

(a) i = 500 mA

2

4

6

8

10

12

14

16x 10

14

phase

Dy

grou

ndst

ate

dens

ity /

cm−

3

0 π 2π

NTD1, 1 Hz

NTD1, 10 Hz

NTD1, 100 Hz

NTD2, 1 Hz

NTD2, 10 Hz

NTD2, 100Hz

(b) i = 800 mA

Figure 5.47: Phase resolved Dy atom groundstate density 125µm in front of the electrode forYAG-lamps with NTD1 (10% Dy) and NTD2 (5% Dy) salt fillings. Parameters: dE = 360µm,electrode length lE = 5 mm, iRMS = 500 − 800 mA switched-dc, low frequency operation atf = 1− 100 Hz

frequency operation of f = 1− 100 Hz in NTD-lamps at iRMS = 500 mA and iRMS = 800 mAis given in figure 5.47. The Dy atom density values in figure 5.47 show the typical decreasewithin the anodic phase and increase during the cathodic phase, but it is less pronouncedthan for pure Dy YAG-lamps. The time constants for the delay of the Dy atom modulationare comparable to the ones obtained for pure Dy YAG-lamps in figure 5.36. At least the ab-solute Dy atom densities for NTD-lamps in figure 5.47 show differences between the NTD1and the NTD2-lamp: The Dy atom density in the NTD1-lamp is two times higher than the

148 5. Measurements and results

density in the NTD2-lamp since the NTD1-lamp has the double amount of Dy within thesalt pool. However, this difference of the Dy atom density in front of the electrode does notshow significant influences on the Dy emitter-effect represented by the Ttip results in figure5.44 and 5.45.The Dy atom density within the cathodic phases in the NTD1-lamp and especially in theNTD2-lamp is roughly 4 − 6 times lower for iRMS = 500 mA in figure 5.47(a) than thecomparable nDY values in pure Dy YAG-lamps presented in figure 5.36(a). Also for aniRMS = 800 mA operating current, the Dy atom density in the NTD-lamps (cf. figure 5.47(b))is 3−4 times lower within the cathodic phase than the Dy density results for pure Dy YAG-lamps in figure 5.36(b). Thus, the previously introduced assumption of a reduced Dy supplyby a reduced Dy ion flux towards the electrode, represented by a lower resulting Dy atomdensity in front of the NTD electrode, is confirmed by means of this measurement.

Figure 5.48 shows the corresponding Dy atom densities in NTD YAG-lamps for a highfrequency operation of f = 500 Hz - 1 kHz. The general trends, discussed for the Dy density

(a) i = 500 mA (b) i = 800 mA

Figure 5.48: Phase resolved Dy atom groundstate density 125µm in front of the electrode forYAG-lamps with NTD1 (10% Dy) and NTD2 (5% Dy) salt fillings. Parameters: dE = 360µm,electrode length lE = 5 mm, iRMS = 500 − 800 mA switched-dc, high frequency operation atf = 500 Hz−1 kHz

in NTD-lamps for low operating frequencies, can also be found within the results for highoperating frequencies presented in figure 5.48. The pronounced modulation of the Dy atomdensity during the anodic and cathodic phase is vanished also within NTD-lamps at highoperating frequencies due to the inertia of the Dy atoms and ions. An unexpected low ab-solute nDy value is observed for the Dy atom density of the NTD1-lamp at iRMS = 500 mAand f = 1 kHz in figure 5.48(a). However, the presented relatively low Dy density results areonly slightly above the detection limit of the absorption spectroscopy measurement whichmight cause the accuracy problems in this case.Hence, the Dy atom density results in NTD-lamps for high operating frequencies, presentedin figure 5.48, show comparable absolute values to the results obtained for a high frequencyoperation of pure Dy YAG-lamps in figure 5.37. Obviously, only the supporting Dy ion fluxduring the cathodic phase is hampered within the NTD-lamps whereas the Dy density adja-

5.3. Results at the YAG-lamps 149

cent to the electrode surface for high operating frequencies is almost equal to the findings inpure Dy lamps. This result is another indication for the assumed suppression of Dy ionisationin the NTD-lamps.

A reference measurement of the Dy atom density in the middle of the NTD YAG-lampsshould explain whether the Dy density is reduced adjacent to the electrode surface or is inagreement with the Dy density within the whole gas-phase of the HID-lamp. The results of

(a) i = 500 mA (b) i = 800 mA

Figure 5.49: Phase resolved Dy atom groundstate density in the discharge middle between theelectrodes for YAG-lamps with NTD1 and NTD2 salt fillings. Parameters: dE = 360µm, electrodelength lE = 5 mm, iRMS = 500 − 800 mA switched-dc, low and high frequency operation atf = 100 Hz−1 kHz

this Dy atom density measurement in the lamp middle for an NTD1-lamp and an NTD2-lampat iRMS = 500 mA and iRMS = 800 mA is given for low (f = 100 Hz) and high (f = 1 kHz)operating frequency in figure 5.49. Within the error margins, the Dy atom densities are inagreement with those in front of the electrode. Again, a difference between the Dy atomdensity in the NTD1-lamp and the densities in the NTD2-lamp can be found in accordanceto their amount of Dy in the salt filling. Hence, in contrast to the results for pure Dy-lamps,the Dy partial pressure is obviously not saturated within the NTD2-lamp and maybe alsonot in the NTD1-lamp. But the difference between the Dy atom density in the differentNTD YAG-lamps has only a minor influence on the Dy emitter-effect efficiency as alreadydiscussed.However, the presented Dy atom densities in the middle of the NTD-lamps are only slightlylower (factor of 1.5− 2) than the Dy density values observed in the discharge middle of pureDy lamps which are presented in figure 5.38. Accordingly, the main differences between theDy atom densities in NTD and pure Dy YAG-lamps can be found only close to the electrodesurface where the ionisation of Dy and the cathodic Dy ion flux have a significant impact.This observation shows once more that the ionisation of Dy within the NTD-lamps mightbe disturbed by the addition of sodium or even thallium to the salt pool.

A final measurement of the Dy ion density adjacent to the electrode in the NTD1 andNTD2-lamps should give further insight into the assumed suppression of the Dy ionisation

150 5. Measurements and results

by Na and Tl. As already explained before, the Dy ion density measurement is also performed

(a) i = 500 mA (b) i = 800 mA

Figure 5.50: Phase resolved Dy ion density 125µm in front of the electrode for YAG-lampswith NTD1 and NTD2 salt fillings. Parameters: dE = 360µm, electrode length lE = 5 mm,iRMS = 500− 800 mA switched-dc, low and high frequency operation at f = 10 Hz−1 kHz

by means of emission spectroscopy within the NTD-lamps. The results of the Dy ion densityin the NTD1 and the NTD2-lamp are given for iRMS = 500 mA in figure 5.50(a) and foriRMS = 800 mA in figure 5.50(b). A comparison of the Dy ion density in NTD lamps, shownin figure 5.50(a), to the Dy ion densities obtained for pure Dy YAG-lamps in figure 5.41indicates clearly that the Dy ion density is one order of magnitude lower in the NTD-lampsthan in the pure Dy lamps. This result of a significantly differing Dy ion density in NTD andpure Dy YAG-lamps proves clearly the assumption of a hampered Dy ionisation by sodiumand thallium within NTD-lamps.

It can be concluded from the investigations of the NTD1 and NTD2 YAG-lamps that theemitter-effect of Dy is significantly reduced in comparison to pure Dy lamps. This Dy emitter-effect reduction is obviously caused by a reduced ionisation of Dy within the NTD-lampsdue to the additional Na and Tl salt ingredients. Because of its low ionisation energy ofEi,Na = 5.14 eV, sodium might be the main reason for the reduced Dy ionisation rate inNTD-lamps. However, investigations and simulations by Philips Lighting showed that alsothe presence of thallium in the NTD salt pool might induce a significant reduction of the Dyemitter-effect. To clarify the influence of Na and Tl in NTD-lamps, their impact on the Dyemitter-effect of YAG-lamps is investigated separately within the following chapter.

5.3. Results at the YAG-lamps 151

5.3.3 The poisoning effect of thallium on the Dy emitter-effect

To distinguish whether the reduction of the Dy emitter-effect, observed by NTD YAG-lampinvestigations, is caused by the addition of Na or by the addition of Tl to the pure Dy saltfilling, a separated investigation of both elements is performed within this chapter. Therefore,the electrode tip temperatures and Dy densities are measured in a pure Dy YAG-lamp aswell as in NTD, ND, TD and pure Hg, Na or Tl-lamps. Unfortunately, at the present time,these different YAG-lamps are only available in a set with a thicker electrode diameter ofdE = 450µm. In the consequence, the obtained absolute result values are not directly com-parable to the previously determined results for NTD1 and NTD2 YAG-lamps. Furthermore,the thicker electrodes induce YAG-lamp operation instabilities due to spot arc attachmentsespecially in the case of iRMS = 500 mA low current operation. Thus, the following results areobtained and shown exclusively for a lamp operation at its nominal current of iRMS = 800 mAdue to a higher accuracy and reproducibility of the absolute values.

For a first classifying overview, the electrode tip temperatures Ttip for different YAG-lampswith and without an NaI or TlI additive compound are measured at a f = 100 Hz switched-dc, i = 800 mA operating current and shown in figure 5.51. It is visible in figure 5.51 that

Figure 5.51: Phase resolved electrode tip temperature for YAG-lamps with different Tl-containingand non Tl-containing salt-fillings (pure Hg, Dy, Na, Tl, DyTl, NaDy) at low frequency operation.Parameters: dE = 450µm, electrode length lE = 5 mm, iRMS = 800 mA switched-dc, f = 100 Hz

the electrode tip temperature increases during the anodic phase and decreases during thecathodic phase as it was observed previously for an iRMS = 800 mA. However, the phase mod-ulation is not very pronounced due to the operating frequency of f = 100 Hz. It is significantthat the Ttip-results for pure Hg YAG-lamps and for YAG-lamps seeded only with NaI orTlI in figure 5.51 are comparable in the range of 3150− 3200 K. Obviously, an emitter-effectreducing the electrode tip temperature at least during the cathodic phase is not active forpure Na or Tl salt fillings. Also Ttip measurements within NT YAG-lamps, containing sodiumand thallium but no dysprosium, do not show a reduction by an emitter-effect, but the NT-

152 5. Measurements and results

lamp operation is so instable concerning the arc attachment that resulting Ttip values are notincluded in figure 5.51. The conclusion that Hg + NT fillings do not show any emitter-effectcompared to pure Hg fillings was also reached in earlier works, cf. [78] figure 3. However,further investigations of pure Na or Tl lamps and combinations of both are not intended asthey do not provide more information concerning the Dy emitter-effect.The lowest electrode tip temperature of Ttip = 2780 − 2840 K can be observed from figure5.51 for a pure Dy lamp filling as it was expected due to a non disturbed emitter-effect ofdysprosium in this case. However, the Ttip values for the ND-lamp, containing an additionof sodium to the Dy emitter material, are only ∆Ttip = 20 − 50 K higher than the pure Dyvalues. This result shows that the Dy emitter-effect is obviously not significantly disturbedby sodium as it was predicted from the NTD-lamp results within the previous chapter 5.3.2.In contrast, thallium reduces the emitter-effect of Dy much more than previously expected.As it can be seen in figure 5.51, the electrode tip temperature Ttip of the TD-lamp, con-taining thallium and dysprosium in the salt mixture, shows values of 2950− 3050 K roughlyin the middle between the pure Hg YAG-lamp and the pure Dy lamp results. Accordingly,the Dy emitter-effect is still active in the TD-lamp but it is reduced to roughly 50% of itsoriginal efficiency. Comparable Ttip results for NTD-lamps with the same electrode geometryare located in almost equal ranges as the TD results but they are not included in figure 5.51due to clarity reasons.

Obviously, the Dy emitter-effect in YAG-lamps is mostly hampered by an addition of thal-lium to the salt mixture. This phenomenon cannot be explained by means of the ionisationenergy differences of Dy and Tl as they are almost equal and even slightly higher for Tl atoms(cf. chapter 5.3.2). The high vapour pressure of thallium within the YAG-lamp gas-phase,determined by thermodynamic considerations, is presumably responsible for the observedpoisoning effect of Tl on the Dy emitter-effect. Additionally, Tl atoms have comparablemasses mTl = 204.4 amu to the Dy atom mass mDy = 162.5 amu whereas the light Na atom(mNa = 23 amu) can obviously not hamper the heavy Dy emitter atoms and ions significantly.This interpretation can explain the pronounced poisoning effect on the Dy emitter-effect rep-resented by the Ttip results given in figure 5.51. However, a determination of the frequencydependence of Ttip for different Tl containing and Tl free YAG-lamp salts might providemore information of its poisoning impact on Dy.

Figure 5.52 shows the averaged electrode tip temperature Ttip during the anodic and dur-ing the cathodic phase of the YAG-lamp electrode for an increasing operating frequency(f = 1 Hz - 1 kHz) of a pure Hg, a pure Dy, an ND, TD and NTD lamp. It is visible in thisfigure that the averaged anodic tip temperature is higher than the cathodic Ttip up to anoperating frequency of f = 100 Hz. For higher operating frequencies f > 100 Hz, the anodictip temperature decreases to the cathodic tip temperature level and is almost equal. Hence,as Reinelt already found in [31], the overall electrode behaviour of an AC operated HID-lampis governed by the cathode whereas the anode only reacts on the cathodic phase conditions.For a high lamp operating frequency where the time constants of the electrode heat capacityare much longer than the phase cycle time, the electrode temperature behaviour is exclu-sively defined by the thermionic electron emission demand during the cathodic phase.Figure 5.52 also shows that the addition of thallium reduces the efficiency of the emitter-effect in pure Dy YAG-lamps as the Ttip values of the TD-lamp are significantly higher thanthe electrode tip temperatures in the pure Dy lamp and in the ND-lamp. The TD-lamp tem-peratures are comparable to the values of the NTD1-lamp in figure 5.52 indicating that the

5.3. Results at the YAG-lamps 153

100

101

102

103

2700

2800

2900

3000

3100

3200

3300

operating frequency f / Hz

elec

trode

tip t

emper

ature

Tti

p/

K

anodecathodeHg

TD

NTD1

ND

Dy

Figure 5.52: Electrode tip temperature Ttip for YAG-lamps with different Tl-containing and nonTl-containing salt-fillings (pure Hg, Dy, NTD1, DyTl, NaDy) in dependence on frequency. Pa-rameters: dE = 450µm, electrode length lE = 5 mm, iRMS = 800 mA switched-dc

reduced Dy emitter-effect in NTD-lamps, observed within the previous chapter, is exclusivelycaused by the additional Tl lamp filling. On the other hand, an addition of sodium to thelamp salts does not influence the Dy emitter-effect significantly since the Ttip values of theND-lamp in figure 5.52 are almost similar to the values of a pure Dy YAG-lamp. However,an emitter-effect of Dy is still visible in the TD and NTD-lamps in figure 5.52 because theirTtip results are ∆Ttip = 160 K below the electrode tip temperature of a pure Hg YAG-lamp.It has to be mentioned that the electrode tip temperature within the TD-lamp in figure5.52 is significantly reduced for an increase of the operating frequency from f = 500 Hz tof = 1 kHz in contrast to the Ttip tendencies of all other YAG-lamps. This observation mightbe an indication that the poisoning influence of thallium on the Dy emitter-effect vanishesfor higher operating frequencies but it should not be overinterpreted. However, this effectmight also be caused by an instability of the discharge arc constriction on the lamp electrode.

A further proof for the hampering influence of Tl on Dy should be found by evaluatingthe measured Dy density results in the YAG-lamp. Therefore, the Dy atom groundstate den-sity is measured by means of the absorption spectroscopy method in front of the electrodeand in the discharge middle. The phase resolved results are plotted in figure 5.53. Herein, (a)shows the Dy groundstate density in front of the electrode in a pure Dy, ND and TD-lamp forlow (f = 10 Hz) and high (f = 1 kHz) operating frequencies whereas (b) shows comparableresults measured in the lamp middle.As already known, the Dy atom density in front of the electrode in figure 5.53(a) shows adecrease during the anodic phase and an increase during the cathodic phase for low frequencyoperation but it is almost constant for a high frequency lamp operation. In comparison, theDy density in the lamp middle, plotted in figure 5.53(b), does not show a pronounced phasemodulation even in the case of low frequency operation. However, the absolute Dy atom den-sity values in the lamp middle are generally lower than the correlated values in front of the

154 5. Measurements and results

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10x 10

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m-3

0 π 2π

pure DyNa DyDy Tl

10 Hz 1 kHz

(a) in front of electrode

2

2.5

3

3.5

4

4.5x 10

15

phase

Dy

gro

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/ c

m-3

0 π 2π

pure Dy Na Dy Dy Tl

10 Hz 1 kHz

(b) in discharge middle

Figure 5.53: Phase resolved Dy groundstate atom density nDy for YAG-lamps with different Tl-containing and non Tl-containing salt-fillings (Dy, DyTl, NaDy) in front of the electrode (a) andin the discharge middle (b) for low and high frequency operation. Parameters: dE = 450µm,electrode length lE = 5 mm, iRMS = 800 mA switched-dc

electrode within the cathodic phase, moreover they are almost equal to the anodic densitylevels in front of the electrode. Obviously, the high Dy atom density is only generated by aDy ion flux adjacent to the electrode during the cathodic phase as it was already observedfrom figure 5.47, 5.48 and 5.49.Concerning the poisoning effect of thallium, it is clearly visible in figure 5.53 that the TDYAG-lamp filled with a Tl amount contains the lowest density of Dy atoms in the gas-phaseat the electrode (a) but also in the middle of the discharge (b). This observation confirmsthe assumption of a hampered Dy emitter-effect on the electrode surface which is mainlycaused by an addition of Tl. The Dy atom density within the ND-lamp in figure 5.53 is highercompared to the density values of the pure Dy lamp corresponding to an additive of 1.57 mgDyI3 within the ND lamp compared to 1 mg DyI3 within the D lamp. Concerning the Dyemitter-effect, the different Dy atom densities in the pure Dy and the ND-lamp determinedfrom figure 5.53 do not show a significant influence since their Ttip values in figure 5.52 arealmost comparable.

In the middle of the DT lamp, a Dy density is measured which does not correspond tothe seeding of this lamp with 1.24 mg DyI3 compared to the seeding of the Dy lamp with1 mg DyI3. To characterise the poisoning effect of thallium, a measurement of the Dy iondensity in front of the electrode is performed additionally by means of emission spectroscopy.The results of the Dy ion density are shown phase resolved for the three YAG-lamp types(Dy, ND, TD) in figure 5.54. The Dy ion densities in figure 5.54 show a pronounced phasemodulation for the pure Dy and the ND YAG-lamp with a decrease during the anodic and anincrease during the cathodic phase. The anodic decrease and especially the cathodic increaseof the Dy ion density are again steeper than the Dy atom density modulation in figure 5.53since their movement is directly coupled to the alternating electric field.The absolute Dy ion density values are almost comparable for the pure Dy lamp (nDyI,Dy =1 ·1014−12 ·1014 cm−3) and the ND-lamp (nDyI,ND = 1 ·1014−16 ·1014 cm−3) whereas the Dy

5.3. Results at the YAG-lamps 155

pure DyNa DyDy Tl

10 Hz 1 kHz

2

4

6

8

10

12

14

16

18x 10

14

phase

Dy i

on d

ensi

ty /

cm

-3

0 π 2π

Figure 5.54: Phase resolved Dy ion density for YAG-lamps with different Tl-containing and nonTl-containing salt-fillings (Dy, DyTl, NaDy) in front of the electrode for low and high frequencyoperation. Parameters: dE = 450µm, electrode length lE = 5 mm, iRMS = 800 mA switched-dc

ion density in the TD lamp is one order of magnitude lower (nDyI,TD = 5 ·1013−1 ·1014 cm−3).Hence, the poisoning effect of Tl in the YAG-lamp is proven by a significant reduction ofthe Dy ion density in front of the electrode. In the consequence, the Dy ion flux towardsthe cathodic electrode is reduced in the thallium containing lamp, which hampers the Dyemitter-effect and explains the Ttip results of TD and NTD-lamps in figure 5.52. However,the almost equal Dy ion densities for the pure Dy and the ND-lamp in figure 5.54 show thatthe addition of sodium does not influence the ionisation of Dy significantly. Therefore, theefficiency of the emitter-effect within the Dy- and the ND-lamp are comparable.

An interesting investigation for a more detailed understanding of the Tl poisoning effectwould be a comparison of the absolute Na atom density and of the absolute Tl atom densityto the Dy atom densities presented in figure 5.53. However, as it was discussed in chapter5.3.2, Na possesses only a few spectral lines which are disturbed by self absorption and notsuitable for absolute emission- and absorption measurements. At least a ∆λ-measurement ispossible for Na as it was performed for Dy in figure 5.43 but it would not provide absolute Nadensities for a comparison. A comparison of absolute Na densities to Dy densities is anywaynot very important in this case since the sodium does not influence the Dy emitter-effectsignificantly.Much more interesting concerning its poisoning influence is the measurement of absolutethallium densities for a comparison to the Dy atom density results in figure 5.53. The onlysuitable spectral line for an absorption measurement of absolute Tl atom densities is lo-cated at λTl,abs = 535.05 nm. This characteristic, dominant Tl spectral line emits most ofthe visible thallium radiation in the green wavelength range and shows a very pronouncedinfluence by self-absorption. Thus, this line is only suitable for Tl density measurements bythe absorption spectroscopy method. It cannot be investigated in absolute values by meansof emission spectroscopy (cf. chapter 4.2.2). An example for a relatively calibrated spectro-scopic measurement of the spectral radiation of Tl around λTl,abs = 535.05 nm is given in

156 5. Measurements and results

533 535 537

0

0.2

0.4

0.6

0.8

1

wavelength λ / nm

spec

tral

rad

iati

on

I λ/

a.u.

YAG-lamp emissionUHP transm.+YAG em.UHP-lamp transmissionUHP-lamp emission

Figure 5.55: Example for the spectral absorption conditions of the Tl-line at λTl = 535.05 nm.Parameters: Tl YAG-lamp, dE = 450µm, electrode length lE = 5 mm, iRMS = 800 mA switched-dc

figure 5.55.Figure 5.55 shows the emission spectrum Iem,Y AGν of the Tl containing YAG-lamp itself, thecombined radiation Iresν of the YAG-lamp with the UHP-backlight source as well as the trans-mitted UHP-light, reduced by absorption Iabs,UHPν = Iresν −Iem,Y AGν . To evaluate the line pro-file of the absorption peak correctly, the normalised UHP-backlight emission, predeterminedby means of the narrow interference filter (cf. optical setup in figure 3.8), is also included infigure 5.55. It is clearly visible within this figure that the spectral intensity radiated fromthe Tl-containing YAG-lamp is comparable or even more powerful than the UHP-backlightsource. From the considerations concerning the absorption spectroscopy method in chapter4.2.3 it is known, that the UHP-backlight intensity should have a distinctly higher luminouspower output in the measured wavelength range than the investigated HID-lamp itself togain a high accuracy of the measured density result. However, this precondition is obviouslynot met in the case of Tl absorption spectroscopy as shown by the radiation profiles of theYAG-lamp and of the YAG+UHP-lamp in figure 5.55.In spite of theses radiation power problems of the UHP-lamp, it is anyway possible to deducethe absorption profile from the UHP-backlight Iabs,UHPν which is also given in figure 5.55 butit is much weaker than the YAG-lamp emission. However, if the Iabs,UHPν absorption peakhad an almost symmetric profile, it would be possible to deduce absolute Tl density resultsfrom the spectroscopic measurement presented in figure 5.55. But it is clearly visible in thisfigure that the self-absorbing Tl spectral line is extremely unsymmetric since its right wingis modified by other spectral lines. Accordingly, the corresponding absorption line profileIabs,UHPν has only a characteristic shape on its left wing whereas the right side is completelyabsorbed by the YAG-lamp. From spectral considerations and spectral distance calculationsit can be found that the additional dip around λ = 536.8 nm in the YAG-lamp spectrumplotted in figure 5.55 is caused by Tl2 molecules. Thus, the right, unsymmetric wing ofthe λTl,abs = 535.05 nm thallium spectral line represents a Tl2 molecule vibration structure.Therefore, it cannot be analysed in absolute values by means of the standard absorptionspectroscopy method developed within this work.In general, the undisturbed left wing of the Tl absorption dip from Iabs,UHPν in figure 5.55 can

5.3. Results at the YAG-lamps 157

be mirrored to the right side in order to gain a symmetric absorption structure to determinethe Tl density by the absorption method. However, due to the mirroring procedure, the ob-tained thallium density results are doubtful by the assumption of a symmetric profile. Onlyrelative, reliable Tl densities in different YAG-lamps or operating points can be deduced frommeasurements as in figure 5.55. Concerning a characterisation of the Tl poisoning effect onDy, a relative Tl density information is useless while comparing it to absolute Dy densitiesand will therefore not be evaluated further.

In conclusion, this chapter about the poisoning effect of thallium proves the assumptionthat the main disturbing influences on the Dy emitter-effect in YAG-lamps are caused by anaddition of Tl to the salt filling. It is clearly visible that the Tl addition reduces mainly theDy ion density in front of the YAG-lamp electrode and also the resulting Dy atom densitywhereas Na does not show a significant influence on the dysprosium filling. A reduced Dyemitter-effect by thallium, comparable to its reduced efficiency in NTD-lamps, is observedfrom Ttip measurements in pure Dy, ND and TD YAG-lamps. However, a spectroscopic de-termination of absolute Tl atom densities adjacent to the electrode for a comparison withthe reduced Dy atom density is not possible up to now, even by means of the absorptionspectroscopy method.

158 5. Measurements and results

5.3.4 Comparison of YAG- and PCA-lamps

To show some further possibilities and challenges of the broadband absorption spectroscopyby means of UHP-backlight, the density measurement is applied to commercial, non-transparentPCA (poly crystalline alumina) HID-lamps and compared to YAG-lamp measurements withtransparent discharge burners. The PCA-lamps are designed almost similar to the researchYAG-lamps introduced in chapter 3.1.3 but their burner material is made of ceramic, non-transparent and much cheaper PCA material. For the investigations within this work, twodifferent PCA-lamps with an NTCC (sodium, thallium, calcium, cerium) salt mixture con-taining either 2.6% Ce or 9.5% Ce are available as well as two comparable YAG-lamps withthe same salt mixture. The PCA-lamps are installed at the same position within the spec-troscopic setup as the YAG-lamps and measured by means of the UHP-backlight source.Due to the unknown transparency and light scattering factor of the PCA material, an abso-lute Ce density measurement by emission spectroscopy is in principle impossible. However,as discussed previously, the absorption spectroscopy measurement only evaluates a relativeabsorption peak ratio to determine absolute density values, which is in general also possi-ble within non-transparent PCA-lamps. Of course, the position information is partly lostwhile investigating PCA-lamps with the absorption spectroscopy due to the light scatteringcharacteristic of the PCA material. But the general 1D position information perpendicularto the arc within the PCA-lamp is only blurred by a cosine-type scattering distribution ofPCA. It can be measured and processed by the inverse Abel transformation to gain a globalradial density distribution. However, a measurement exactly 125µm in front of the electrodetip, as it is performed in YAG-lamps, is impossible in PCA-lamps since the electrode tip isnot clearly visible. Therefore, the presented Ce density measurements performed by means ofUHP absorption measurements at the atomic Ce line at λCe = 577.36 nm are always recordedat the discharge middle of PCA and YAG-lamps to compare their absolute density resultswithin the gas-phase.

A measured absolute Ce density result for a YAG-lamp containing 2.6% Ce in the saltmixture and the correlated PCA-lamp is given phase resolved for different arc currentsiRMS = 500 − 800 mA and two different operating frequencies f = 10 Hz - f = 1 kHz infigure 5.56. It is visible in figure 5.56(a) that the Ce atom groundstate density in the dis-charge middle of the YAG-lamp shows a slight phase modulation from nCe = 6 ·1014 cm−3 tonCe = 8·1014 cm−3 for iRMS = 800 mA. This observation is caused by a superposition of a cat-aphoretic and a convective movement of the Ce atoms in the vertically operating YAG-lamp.The phase modulation of the Ce density is more pronounced than that of Dy in the dischargemiddle (cf. figure 5.49) presumably since Ce atoms have lower masses (mCe = 140.1 amu)than Dy atoms (mCe = 162.5 amu). This phase modulation of the Ce atom density in YAG-lamps, presented in figure 5.56(a), is less pronounced for lower arc currents. Moreover, theaveraged absolute Ce density decreases with decreasing iRMS as it is expected due to a lowerelectrical power input into the HID-lamp an a lower cold spot temperature.In comparison, the observed phase modulation of the Ce atom density is less pronouncedin PCA-lamps as shown in figure 5.56(b). This difference might already be caused by theinsufficient position resolution of the absorption spectroscopy measurement due to the non-transparent PCA burner and should not be overinterpreted. It is much more important thatthe absolute Ce atom groundstate results in figure 5.56(b), measured by absorption spec-troscopy through the translucent PCA material, are almost similar to the Ce density values

5.3. Results at the YAG-lamps 159

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500 mA600 mA700 mA800 mA

10 Hz1 kHz

(a) YAG-lamp

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x 1014

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Ce

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m-3

0 π 2π

500 mA600 mA700 mA800 mA

10 Hz1 kHz

(b) PCA-lamp

Figure 5.56: Phase resolved cerium atom density nCe measured at the cerium atom line λCe =577.36 nm for NTCC YAG-lamp and PCA-lamp with 2.6% Ce amount in the salt filling at low(f = 10 Hz) and high (f = 1 kHz) frequency operation and varying arc current in the dischargemiddle. Parameters: YAG-lamp: dE = 360µm, PCA-lamp: dE = 500µm, electrode length lE =5 mm, iRMS = 500− 800 mA switched-dc

observed from YAG-lamp measurements in figure 5.56(a). Also the decreasing Ce densityfor a decreasing arc current iRMS is well reproduced by the UHP-backlight absorption mea-surement in PCA-lamps. Considering that the phase modulation of the Ce atom densitycannot be resolved sufficiently in PCA-lamps, the difference between comparable measure-ments in YAG-lamps and PCA-lamps by means of absorption spectroscopy is less than 20%.It has to be mentioned that YAG-lamps and PCA-lamps do not even have a completely sim-ilar geometry (e.g. burner shape, electrode diameter YAG-lamp: dE = 360µm, PCA-lamp:dE = 500µm) and it is therefore not sure, whether the real Ce density within these HID-lamps is exactly equal. Keeping in mind the different lamp properties, the correlation of theCe atom density measurements in YAG-lamps and PCA-lamps presented in figure 5.56 isalmost excellent.

To have a further possibility of comparison, the UHP absorption spectroscopy measure-ment is also applied to a YAG-lamp and a correlated PCA-lamp containing both 9.5% Ceamount within their salt filling. The phase resolved results of this Ce atom groundstate den-sity measurement for different arc currents and frequencies if given in figure 5.57. The generalCe atom density trends like a phase modulation and a decreasing averaged Ce density for adecreasing iRMS which are observed in the 2.6% Ce lamps can also be found for the 9.5% Celamps in figure 5.57. Furthermore, the global absolute Ce atom density values in this figureare 2− 5 times higher for the YAG-lamp and at least 1.8− 2 times higher in the PCA-lampas it was expected due to the higher Ce amount in the salt filling.However, for the lamps with 9.5% Ce salt filling, the agreement of the Ce density measure-ment results in the YAG-lamp in figure 5.57(a) with the ones in the PCA-lamp in figure5.57(b) is not as excellent as in figure 5.56. Moreover, the absolute Ce density values mea-sured by absorption in a YAG-lamp with 9.5% Ce content and in a corresponding PCA-lampdiffer more than a factor of 2, especially for iRMS = 800 mA (cf. figure 5.57). At least the

160 5. Measurements and results

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500 mA600 mA700 mA800 mA

10 Hz1 kHz

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23456789

101112

x 1014

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m-3

0 π 2π

500 mA600 mA700 mA800 mA

10 Hz1 kHz

(b) PCA-lamp

Figure 5.57: Phase resolved cerium atom density nCe for NTCC YAG-lamp and PCA-lamp with9.5% Ce amount in the salt filling at low (f = 10 Hz) and high (f = 1 kHz) frequency operationand varying arc current in the discharge middle. Parameters: YAG-lamp: dE = 360µm, PCA-lamp:dE = 500µm, electrode length lE = 5 mm, iRMS = 500− 800 mA switched-dc

general trends of the Ce density in YAG- and PCA-lamps for a 9.5% Ce filling are similar andthe absolute results are in the same order of magnitude. However, the observed prospects arealready a big advantage of the developed UHP absorption spectroscopy measuring methodwith respect to the PCA burner material since it strongly scatters the UHP background lightbeam.

In summary it can be proved by the presented Ce density results that the broadband absorp-tion spectroscopy measurement is even applicable to commercial HID-lamps consisting of anon-transparent PCA burner material. In spite of absolute emission spectroscopy measure-ments, which are impossible in PCA-lamps, the approximate light beam correlation preservedin an absorption spectroscopy measurement (cf. chapter 4.2.3) facilitates the determinationof absolute densities with an acceptable accuracy even within translucent HID-lamps. Theobtained absolute density values are at least within the same order of magnitude as compara-ble measurements in transparent YAG-lamps. Furthermore, the higher measured Ce densitiesfor YAG burners are in agreement with the fact that the heat conductivity of the YAG ma-terial is higher than that of the PCA material, leading to a higher cold-spot temperature andthus Ce vapour pressure in the YAG-lamps. Especially in the case of relative particle densityinvestigations, the UHP absorption spectroscopy measurement provides reliable prospectseven for commercial, non-transparent ceramic HID-lamps.

5.3. Results at the YAG-lamps 161

5.3.5 Plasma temperature measurements by emission and absorption

Finally within this last subchapter, the developed and approved UHP absorption spec-troscopy measuring method will be combined with results of the emission spectroscopymeasurement to determine not only particle densities but also the plasma temperature Tpl

independently of a Hg component within the investigated HID-lamp gas-phase. The plasmatemperature Tpl is a key plasma parameter within HID-lamp research since it is for examplenecessary to calculate absolute groundstate densities from calibrated emission spectroscopymeasurements. Furthermore, as a consequence the LTE assumption in HID-lamps (cf. chap-ter 2.2.3), the plasma temperature Tpl represents the temperature of all species within thegas-phase and therewith especially the gas temperature which is e.g. interesting for the HID-lamp designers.

The plasma temperature Tpl in HID-lamps can be determined from absolute emission spec-troscopy measurements of Hg emission coefficients according to the explanations in chapter4.2.2. However, to become independent of the Hg component in HID-lamps and to measureTpl at an arbitrary position in the HID-lamp without Hg line emission, results of the discussedUHP absorption spectroscopy measurement can be combined with corresponding emissionspectroscopy results to determine Tpl. Therefore, the line emission coefficient εL of one lampsalt ingredient, in this case Dy, is measured in absolute units and calculated according tochapter 4.2.2. The resulting emission coefficient can be written as

εL =1

4πAulh

c0

λul

nu (5.5)

leading to the population density nu of the specific excited state of this optical transition.The constants within this equation are already given for equation 2.18.By means of an additional UHP broadband absorption measurement of a spectral resonanceline, as it was discussed extensively within this PhD work, a corresponding line absorptioncoefficient αL of the same lamp salt ingredient, e.g. Dy, can be determined. It is related tothe ground state density according to

αL =e2

4πε0

· π

mec0

· flu · n0 (5.6)

wherein the comprised constants are already given for equation 2.19. The absolute ground-state density n0 of the investigated particle, herein Dy, can be derived from this line absorp-tion coefficient αL with a high accuracy as it was discussed previously.If now a local thermodynamic equilibrium (LTE, cf. chapter 2.2.3) can be assumed for theHID-lamp plasma and the energy distance of the groundstate E0 = 0 and the investigatedupper excited state Eu is sufficiently high, the ratio nu/n0 is given by the Boltzmannrelation according to

nu

n0

=gu

g0

· exp

[− Eu

kBTpl

]. (5.7)

Hence, the demanded plasma temperature Tpl in equation 5.7 can be obtained by calculatingthe ratio of nu determined from emission spectroscopy measurements and n0 determined fromcorresponding absorption spectroscopy measurements. In general, this alternative plasmatemperature measurement can be performed with an arbitrary set of density measurements

162 5. Measurements and results

from emission and absorption lines. But the combination of an excited state density mea-surement by emission spectroscopy with the groundstate density measurement by absorptionspectroscopy, performed within this PhD work, leads to a high accuracy due to the signifi-cantly high energy difference ∆E = Eu − E0 = Eu − 0 = Eu.

A first example for a plasma temperature measurement by combining a Dy emission spec-troscopy result obtained at λDy,em = 695.81 nm (cf. chapter 4.2.2) with a Dy absorptionspectroscopy measurement obtained at λDy,abs = 625.91 nm (cf. chapter 4.2.3) in a pure 1mgDy YAG-lamp is given in figure 5.58. Herein, the YAG-lamp with thin 360µm electrodes isoperated exemplarily at a iRMS = 800 mA, f = 100 Hz switched-dc current. The results in

phase

pla

sma

tem

per

ature

/ K

TP

l

0 π 2π

T from Hg

at electrodepl

T from em/abs

at electrodepl

6500

6600

6700

6800

6900

Figure 5.58: Phase resolved plasma temperature Tpl determined from a combination of absorption-and emission-spectroscopy in a 1mg Dy-lamp in front of the electrode. Parameters: dE = 360µm,electrode length lE = 5 mm, iRMS = 800 mA, f = 100 Hz, switched-dc

figure 5.58 represent the phase resolved absolute Tpl values 125µm in front of the electrodetip, measured on the one hand by the described combination of emission- and absorptionspectroscopy and on the other hand by the conventional Hg emission lines for comparison.The absolute results of Tpl determined by emission and absorption spectroscopy in figure5.58 are plotted as red dots and a general course (red dotted line) is fitted to the resultsfor the anodic and cathodic phase independently by means of a least-square spline function.As usual, the spectroscopic measurement is performed radially resolved but the results arepresented averaged over the electrode region within this figure to investigate the much moreinteresting phase modulation of Tpl.It is obvious from the results in figure 5.58 that the correlation of the plasma temperature de-termined by the combination of Dy emission-/ absorption spectroscopy and by the absoluteHg line emission spectroscopy is almost excellent in the pure Dy YAG-lamp. The absolutevalues of Ttip in front of the electrode decrease during the anodic phase from 6900 K to 6500 Kand increase during the cathodic phase due to the electric fields within the plasma boundarylayers. The plasma heating during the cathodic phase is more pronounced than the plasmacooling during the anodic phase indicated by a steeper increase and a presumable saturationof Tpl in the cathodic phase in figure 5.58 and an almost exponential relaxation during theanodic phase. This difference is caused by the significantly thinner cathodic boundary sheath

5.3. Results at the YAG-lamps 163

combined with a high voltage drop uc leading to higher electric fields heating the plasma infront of the cathode (cf. chapter 2). The observed qualitative tendencies and absolute valuesof the plasma temperature in pure Dy-lamps plotted in figure 5.58 are in good agreementwith results determined by Reinelt and Langenscheidt in [22, 26, 31].

The same general phase modulation of the plasma temperature is observed by measure-ments in pure Hg YAG-lamps. An example for phase resolved Ttip results measured in a pureHg YAG-lamp with the same geometric parameters as presented in figure 5.58 is given infigure 5.59. The results in figure 5.59 show the phase resolved, radially averaged absolute

6600

6650

6700

6750

6800

6850

6900

phase

pla

sma

tem

per

atu

reT

pl/

K

0 π 2π

50 Hz

100 Hz

250 Hz

500 Hz

1 kHz

Figure 5.59: Phase resolved plasma temperature Tpl for different frequencies (f = 50 Hz - 1 kHz)in a pure Hg YAG-lamp determined from Hg emission in front of the electrode. Parameters:dE = 360µm, electrode length lE = 5 mm, iRMS = 800 mA, switched-dc

plasma temperature values at iRMS = 800 mA switched-dc current for different frequenciesf = 50 Hz - 1 kHz. Of course, as no additional salt like Dy is introduced into this pure HgYAG-lamp, the Tpl measurement in figure 5.59 is performed by means of optical spectroscopyof the Hg emission lines.The qualitative behaviour of the plasma temperature in front of the electrode in a pureHg-lamp shown in figure 5.59 is equal to the one obtained for the 1mg Dy-lamp in figure5.58: The plasma temperature decreases from 6900 K to 6720 K during the anodic phase andincreases a bit faster up to a saturation level during the cathodic phase. In contrast to theabsolute values of the Dy-lamp, the plasma temperature in the pure Hg-lamp (figure 5.59)decreases only down to 6720 K at the end of the anodic phase, which is roughly 200 K higherthan in figure 5.58. Obviously a higher electron temperature is needed to produce a sufficientnumber of electrons within a pure mercury plasma by reason of the higher ionisation energyof the Hg than in a plasma seeded with DyI3. However, the Tpl values for a f = 1 kHz lampoperating frequency in figure 5.59 are artificially low, presumably caused by disturbing effectsduring the current-zero-crossing (CZC) and a low accuracy due to short camera exposuretimes. Hence, the obtained differences in the case of f = 1 kHz lamp operation should notbe overinterpreted.

164 5. Measurements and results

Furthermore it can be seen in figure 5.59 that the course of the phase resolved plasma tem-perature does not change significantly with increasing frequency. This observation, which iscontrary to the frequency dependency of particle densities like Dy in figures 5.36 and 5.37,shows that the plasma heating processes are mainly caused by electrons in the electric fieldand do not depend significantly on heavy particles with a high inertia.

A last plasma temperature measurement in NTD YAG-lamps should prove that the con-cept of combining Dy emission- and absorption spectroscopy for a determination of Tpl isalso applicable in HID-lamps with more complex salt fillings. Therefore, the plasma tem-perature Tpl is measured in front of the electrode in an NTD1 and an NTD2 YAG-lampby means of emission-/ absorption spectroscopy and by Hg emission spectroscopy for com-parison reasons as presented by the results in figure 5.60. The phase resolved Tpl results in

phase

pla

sma

tem

per

ature

/ K

TP

l

0 π 2π

5600

5800

6000

6200

6400

6600

6800T from Hg

at electrodepl

T from em/abs

at electrodepl

T from em/abs

in lamp middlepl

5400

(a) NTD1 YAG-lamp

phase

pla

sma

tem

per

ature

/ K

TP

l

0 π 2π

T from Hg

at electrodepl

T from em/abs

at electrodepl

T from em/abs

in lamp middlepl

5600

5800

6000

6200

6400

6600

6800

(b) NTD2 YAG-lamp

Figure 5.60: Phase resolved plasma temperature Tpl determined from a combination of absorption-and emission-spectroscopy in NTD-lamps in front of the electrode and in the discharge middle.Parameters: dE = 360µm, electrode length lE = 5 mm, iRMS = 800 mA, f = 100 Hz, switched-dc

figure 5.60(a) for the NTD1-lamp and in figure 5.60(b) for the NTD2 lamp are presented asa solid blue line for the Hg emission measurement and as red dots for the plasma tempera-ture determined from Dy emission-/ absorption spectroscopy. As known from figure 5.58, aninterpolating curve is fitted to the red dots for the anodic and cathodic phase independentlyby a least-square fitting. Additionally, the plasma temperature is measured in the dischargemiddle by combining Dy emission spectroscopy results with Dy absorption spectroscopy re-sults and plotted in figure 5.60 as green crosses with a straight, dashed green average line.This measurement of Tpl in the discharge middle is not possible with the conventional Hgemission method as Hg emission lines are only present adjacent to the electrode tip.In contrast to its behaviour within Dy or pure Hg YAG-lamps, the plasma temperature inNTD-lamps presented in figure 5.60 shows another characteristic phase modulation: Theplasma is heated during the anodic phase from Tpl = 5800K to Tpl = 6500K in the NTD1-lamp (cf. figure 5.60(a)) and cools down during the cathodic phase. In the NTD2-lamp witha lower Dy amount, the trends of the plasma temperature Tpl are the same as in the NTD1-lamp but it is slightly higher (Tpl = 5900− 6700K). The contrary phase behaviour of Tpl inthe NTD-lamps, compared to Dy and pure Hg-lamps, is caused by the presence of sodiumwith a very low ionisation energy in the gas-phase of the NTD YAG-lamps. The resultinghigh ionisation rate of Na during the cathodic phase hampers a further heating up of the

5.3. Results at the YAG-lamps 165

plasma. A lower plasma temperature is required in NTD lamps than in Dy lamps to producea sufficient number of electrons by the ionisation of Na. As previously expected, the Tpl

values measured exemplarily in the discharge middle of NTD-lamps (cf. figure 5.60) do notshow a significant phase modulation and are generally lower than the plasma temperaturesin front of the electrodes.The correlation of the plasma temperature results in the NTD-lamps presented in figure 5.60for the Dy emission-/ absorption measuring method and the Hg emission method is slightlyworse than in the pure Dy YAG-lamp (cf. figure 5.58) but still very good. Accordingly, themeasurement of plasma temperatures by combining emission- and absorption spectroscopyresults of a species like Dy in HID-lamps is an accurate and reliable alternative to conven-tional absolute Hg emission line measurements. Especially within the perspective of modern,Hg free HID-lamp, the presented plasma temperature measurement might become an in-teresting method for future HID-lamp research. Of course, its application has to be furtherapproved for other HID-lamp salt ingredients like Th or Ce, which will not be performedwithin this PhD work.

Concluding the presented plasma temperature measurements by Dy emission- and absorp-tion spectroscopy, some further interpretations can be withdrawn from the perfect agree-ment shown in figure 5.58 and figure 5.60: The almost equal Tpl results, determined either bymeasuring absolute Hg emission lines and by combining Dy emission- and absorption mea-surements, prove the existence of a local thermal equilibrium (LTE) within the HID-lampplasma adjacent to the electrode. The LTE assumption, presumed for HID-lamp plasmasat the beginning of this PhD work, is the most important pre-condition for evaluating theplasma temperature Tpl from emission- and absorption spectroscopy measurements accordingto the Boltzmann relation given in equation 5.7. Hence, the LTE assumption in HID-lampplasmas is confirmed since the obtained Tpl results correlate well with corresponding absoluteHg line measurements.Moreover, the excellent agreement of Tpl results in figure 5.58 and figure 5.60 shows thatthe Dy emission line at λDy,em = 695.81 nm, evaluated for Dy emission spectroscopy mea-surements, is definitely not influenced by self-absorption within the investigated YAG-lampoperating ranges. If the emission coefficient ε of Dy at λDy,em = 695.81 nm was reducedby self-absorption, the observed good agreement of plasma temperature results obtained bycombining Dy emission-/ absorption spectroscopy and by means of absolute Hg emissionmeasurements would not have been possible.

166 5. Measurements and results

167

6. Conclusions and outlook

Within this concluding chapter, the main facts of the obtained research results concerningthe emitter-effect in HID-lamps determined by means of 2D electrode temperature measure-ments and by the developed absorption spectroscopy method are summarised and evaluated.Furthermore, on the basis of the discussed and presented conclusions within this PhD work,some hints and prospectives are given for further research approaches and future applica-tions of the developed measuring methods. To facilitate a certain structure, the concludingremarks within this chapter are given for each investigated HID-lamp system separately.

6.1 Results at the model lamp

The Bochum model lamp is a very flexible, open system adopting the physical conditionsof a real, closed commercial HID-lamp. By means of exchanging pure tungsten lamp elec-trodes with thoriated electrodes in this model lamp, fundamental aspects of the emitter-effectinduced by Th on the electrode surface have been investigated. Therefore, electrode tip tem-perature values Ttip have been measured phase resolved (in the case of AC lamp operation)by means of an absolutely calibrated 2D infrared photography setup in dependence on arccurrent and operating frequency. The measuring results for Ttip within the model lamp havebeen compared to a simulation of the cathodic plasma sheath adjacent to the electrode pro-viding absolute values for the investigated work function reduction. The work function of asolid surface is a quantity describing the energy barrier for electrons leaving and entering thesolid material, thus, its reduction is a direct indication for the active emitter-effect of Th onthe electrode surface.

By means of model lamp investigations including Ttip measurements and theoretical sim-ulations, a clear reduction of the effective work function of a cathodic DC tungsten electrodefrom ϕ = 4.55 eV of the pure tungsten material to ϕ = 3.0 eV of thoriated electrodes in-duced by an emitter-effect of Th could be proved. Furthermore, by a consideration of wellknown research results concerning the surface coverage dependency of the Th emitter-effecton the surface temperature and its depletion from hot electrodes, the existence of a certainemitter material supply mechanism from the plasma arc could be predicted. Hence, a supplyof the emitter material thorium by means of a Th ion flux towards the cathodic electrodewas predicted and exemplified to be the essential emitter supporting effect in HID-lampswith regard to the emitter-effect of Th and other rare earth like Ba, Dy and Ce. However, acomparable emitter-effect of thorium on the tungsten DC anode could not be verified withinthe Bochum model lamp.

168 6. Conclusions and outlook

Further spectroscopic investigations of Th ions in front of the cathode should be performedwithin future model lamp experiments to prove the assumption of a Th supply by meansof an ion flux. Additional DC measurements within the Bochum model lamp equipped withvarying amounts of Th in the electrode material or with Th exclusively in the anodic elec-trode might provide a deeper insight into the dominating transport mechanisms of emittermaterials in HID-lamps.

However, to investigate the frequency dependency of the Th emitter-effect within the Bochummodel lamp with a special focus on the anode, some further phase resolved electrode tip tem-perature measurements with switched-dc currents of variable frequency have been performed.It was clearly identified by the Ttip measuring results that for a certain current value theanode is heated from the arc plasma more than the cathode which cools down during thecathodic phase, whereas this correlation is vice versa for a low operating current of the modellamp. In the case of pure tungsten electrodes, the characteristic turning point indicated byan equal plasma heating of the anode and the cathode is located between iarc = 4 − 5 Afor an electrode diameter of dE = 1 mm. The correlated turning point in the case of thori-ated lamp electrodes, inducing an active emitter-effect during the cathodic phase, is locateddistinctly lower around iarc ≈ 3 A. However, for higher switched-dc operating frequencies ofthe Bochum model lamp, the phase modulation of the electrode tip temperature was almostvanished due to the heat time constants of the solid electrode material. From the presentedmeasuring results it became obvious that the thermal behaviour of an HID-electrode is dom-inated by the electron emission demand of the cathode whereas the anode only reacts on thecathodic operating point especially in the case of high frequency operation.However, the existence of an anodic emitter-effect, which was predicted by a surviving Thelectrode coverage on the anode due to the inertia of the supported Th atoms during thecathodic phase, could not be validated within the Bochum model lamp. As a result of theanodic emitter-effect, a generally decreasing electrode tip temperature for an increasing lampoperating frequency was assumed, but the frequency dependence of Ttip was observed con-trary even with thoriated electrodes in the Bochum model lamp. At least in comparable,commercial YAG-lamps filled with a certain amount of ThI4, the anodic emitter-effect couldbe proved as it was already known from Dy containing YAG-lamps.Concluding these high frequency investigations of Th containing electrodes within the Bochummodel lamp, an emitter-effect on HID-lamp electrodes is not only formed by a sufficientlyhigh emitter ion flux. Moreover the emitter-effect is governed by a complicated correlationof emitter ionisation, adsorption, thermal desorption and the atom masses/ inertia of thespecific emitter material. A future model lamp investigation at distinctly higher operatingfrequencies in the range of f ≈ 100 kHz might lead further interpretation aspects and opti-misation parameters concerning the emitter-effect.

6.2 Results at the high-pressure sodium lamp

The high-pressure sodium (HPS) lamp investigated within this PhD work is a downscaledmodel of a standard 1000 W commercial lighting system equipped with a transparent researchdischarge vessel made of sapphire to facilitate optical observations. The emitter material, inthis case barium, is stored in a tungsten coil wound around the lamp electrode at a certain

6.2. Results at the high-pressure sodium lamp 169

distance from the electrode tip. In a correctly operating HPS-lamp, the Ba diffuses from thetungsten coil along the electrode rod to the electrode tip where it is consumed for the emitter-effect. However, recently developed HPS-lamps show a plasma arc attachment jumping fromthe electrode tip directly to the tungsten coil due to a lack of Ba supply, which destroys thewhole HID-lamp already after a very limited lifetime. Thus, an experimental investigationof the Ba diffusion from the tungsten coil to the electrode tip was needed to optimise theemitter supply within these HPS-lamps and to overcome the mentioned operating problem.

Within this work, a new broadband absorption spectroscopy (BBAS) setup and data eval-uation method was developed to measure and characterise Ba densities adjacent to theHPS-lamp electrode between the tungsten coil and the electrode tip. In this special case,an usual emission spectroscopy measurement of Ba densities was not applicable since theinvestigated measuring position within the HPS-lamp does not emit a Ba line of sufficientlyhigh intensity with respect to the radiating plasma arc. The broadband absorption spec-troscopy measurement of Ba densities was initially performed by means of a high powerLED as backlight source, but its spectral power output at the investigated Ba resonanceline (λBa = 553 nm) was too weak to produce a sufficiently high SNR. Hence, within thedevelopment of the measuring method, the LED was substituted by a high power UHP-lamp (ultra-high pressure) supplying a 100 times higher spectral power output which resultsin a good SNR for the Ba density determination. The broadband absorption measurementincluding an UHP backlight source was further optimised to obtain Ba density measuringresults with a high accuracy and a high reproducibility at an arbitrary position within theHPS-lamp. At the end, the developed, reliable absorption spectroscopy method providedabsolute density results of the groundstate atoms of Ba without the necessity of an absolutecalibration of the spectroscopy setup and without an additionally needed measurement of theplasma temperature. Accordingly, besides an independent choice of the measuring positionwithin an HID-lamp, the BBAS method is not influenced and deteriorated by an unknowntransmission of the burner material of the investigated HPS-lamp or other optical param-eters within the measuring path. This is the most important advantage of the BBAS withrespect to the formerly performed absolutely calibrated emission spectroscopy measurementof particle densities.By means of the developed absorption spectroscopy method with UHP backlight, it was pos-sible to measure the Ba density distribution adjacent to the electrode surface between thetungsten coil and the electrode tip in HPS-lamps. Thereby, the Ba diffusion process from itssource within the tungsten coil to the electrode tip could be characterised in dependence ondifferent parameters, e.g. on the distance between the coil and the electrode tip. The resultsof this Ba diffusion characterisation were used by the project partner Philips Lighting, NL todevelop a simulation of the Ba diffusion, which solved finally the discussed arc attachmentproblems within commercial HPS-lamps. However, it was also identified by the measured Badensity results that the transport of Ba along the electrode is governed by a surface diffusionprocess which cannot be investigated directly by means of optical spectroscopy methods.Moreover, even the absorption spectroscopy measurement can only reflect the Ba diffusionprocess indirectly from the Ba density produced by evaporation from the electrode surface,which is strongly dependent on the electrode temperature. Hence, it might be an interestingaspect for a future characterisation of the Ba diffusion process in HPS-lamps to investigatethe correlation between the real surface diffusion and its impact on the measured Ba gas-phase density close to the surface. Therefore, a theoretical calculation of the Ba evaporationfrom the electrode surface in dependence on its temperature has to be considered.

170 6. Conclusions and outlook

In a final BBAS measurement of Ba densities within the HPS-lamp, it was possible to char-acterise the influence of a hydrogen pollution on the lamp operation. Therefore, a specialHPS-lamp with an extended outer glass bulb was used containing a hydrogen dispenserand a hydrogen getter to control the H pressure within the outer lamp burner externally.Changes of the H amount in the outer HPS-lamp induced a different heat conduction fromthe inner lamp burner, which could be monitored by a change in the operating HPS-lampvoltage. By means of the measured Ba density adjacent to the HPS-lamp electrode, which isinfluenced by the partial pressure of hydrogen, it was possible to prove the assumption of aH diffusion from the outer HPS-lamp bulb into the inner lamp burner. However, it was notpossible to determine reliable time constants for this hydrogen diffusion process due to a highconsumption of the hydrogen supplied by the dispenser during HPS-lamp operation. Hence,a precise determination of the H diffusion time constants can only be performed in futuremeasurements if the general construction of the H dispenser/ getter HPS-lamp is changed.But the needed effort to construct another H dispenser/ getter concept in a HPS-lamp doesobviously not balance with the demand of precise H diffusion time constants at the moment.

6.3 Results at the YAG-lamp

The developed UHP absorption spectroscopy measurement was adopted and optimised fi-nally to determine Dy densities in so-called ”YAG-lamps” consisting of transparent yttrium-aluminium-garnet tubes for optical observations. These specially designed research HID-lamps are similar in most of their parameters to commercially used PCA-lamps (poly-crystalline-aluminia) and contain the emitter material dysprosium as a solid DyI3 salt whichis added during lamp production. The solid salt melts during YAG-lamp operation andevaporates to a certain partial pressure so that the Dy emitter is supplied to the tungstenelectrode mainly by means of an ion flux directly from the gas-phase. The BBAS measure-ment of Dy densities in front of the YAG-lamp electrodes was used within this work to showits perspectives and challenges with focus on the characterisation of the emitter-effect inalmost commercial ceramic HID-lamps.

An active emitter-effect of Dy within YAG-lamps could clearly be proved by means ofphase resolved 2D electrode temperature measurements. Moreover, the existence of an anodicemitter-effect was also observed in YAG-lamps by means of a generally decreasing electrodetemperature for an increasing operating frequency. Obviously, the small, closed volume of theYAG-lamps facilitates the Dy to establish a significant partial pressure within the gas-phase,which induces also a Dy coverage of the anode for higher operating frequency due to thediscussed inertia of Dy atoms. Furthermore, by an absorption spectroscopy investigation ofYAG-lamps with a pure Dy filling but different Dy amounts in the salt filling, it was foundthat the Dy density in the gas-phase is already saturated for a 1 mg Dy salt filling whereasa higher amount of Dy only leads to an undesired YAG material erosion.A further step towards an investigation of complex, commercial HID-lamp salt fillings wasperformed by a determination of the Dy density and the correlated Dy emitter-effect in NTDYAG-lamps filled with a salt mixture containing sodium (N), thallium (T) and dysprosium(D). The behaviour of these NTD-lamps is already comparable to commercial HID-lampssince the addition of Na and Tl induces a powerful light emission of the lamp within the

6.3. Results at the YAG-lamp 171

desired spectral range leading to a good colour-rendering index (CRI). Correlated, phase re-solved measurements of the electrode tip temperature by 2D pyrometry and of the Dy densityadjacent to the electrode by BBAS for different operating currents and frequencies in NTD-lamps results in a clear characterisation of the Dy emitter-effect: The Dy emitter-effect isreduced to roughly 50 % of its efficiency achieved in pure Dy-lamps but it still induces a pro-nounced Ttip reduction in comparison to pure Hg YAG-lamps without any salt filling. Thegenerally observed Dy density within NTD-lamps was reduced and obviously not saturatedwith respect to pure Dy lamps, but the measured Dy density results within NTD-lamps showa direct correlation to the relative fraction of Dy filling in the lamp salt for different typesof NTD-lamps. However, by means of Ttip and nDy measurements in NTD-lamps it was notpossible to distinguish whether the addition of Na or the addition of Tl is responsible for thereduction of the Dy emitter-effect.Accordingly, in a last logical step, the UHP absorption spectroscopy was applied to YAG-lamps filled either with Na or with Tl and several other ingredients like Dy separately. Bythe Ttip and nDy measuring results within theses Na or Tl containing YAG-lamps, the influ-ence of both salt additives was studied separately: Obviously, the Tl is mainly responsiblefor a reduction of the Dy emitter-effect in YAG-lamps, whereas an addition of Na does notchange the Dy emitter parameters significantly. A possible explanation of this correlationcould be found within the Dy ion density, measured by means of emission spectroscopy sincethe amount of Dy ions in front of the YAG-lamp electrode was distinctly reduced by an ad-dition of Tl to the gas-phase. Thermodynamical considerations showed that the high partialpressure of Tl and its relatively high atom mass (204.3 amu) must be the main reasons forthe poisoning of the Dy emitter-effect since Na has a distinctly lower ionisation energy thanDy and Tl but also a very low atomic mass (23 amu). However, an ion flux as an essentialemitter material transport mechanism towards the electrode of an HID-lamp, predicted atthe beginning of this PhD work, could finally also be validated for YAG-lamps, since thepoisoning of the Dy emitter-effect could be attributed to a reduction of the Dy ion densityby thallium.

At the end of the BBAS investigations on YAG-lamps within this work, it was proven thatthe developed UHP absorption spectroscopy method is also applicable to determine otherparticle densities like cerium and it can be used within certain limitations to measure par-ticle densities in translucent, commercial PCA-lamps. Therefore, measuring results of theCe density in transparent YAG-lamps were compared to measurements in non-transparent,commercial PCA-lamps filled with the same salt mixtures. It was proved that the determinedgeneral particle densities in translucent HID-lamps by means of BBAS measurements showcomparable values to results obtained within transparent YAG-lamps. Deviations betweenmeasurements at both lamps of a factor of two were observed, but the relative trends of thedetermined Ce densities were directly comparable. Thus, the UHP BBAS measurement isgenerally also applicable for measurements within commercial, non-transparent HID-lampsif the lack of an exact position information is accepted.Finally it was shown within this PhD work that the important plasma temperature parameterTpl can be measured in HID-lamps by a combination of emission spectroscopic measurementsin absolute units with the developed UHP absorption measurement. Therefore, the popu-lation density of an upper atom energy state was compared to the one of the groundstateleading to Tpl values by the Boltzmann correlation. The plasma temperature results ob-tained by this emission-/ absorption measurement of Dy atom densities were approved by acomparison to results determined from the common Tpl measuring method evaluating abso-

172 6. Conclusions and outlook

lute Hg emission lines within the HID-lamps. The presented, alternative plasma temperaturemeasurement is independent of any Hg content within the HID-lamp and might therefore bean important approach concerning the investigation of new, Hg-free HID-lamps. Moreover,the LTE assumption within HID-lamps could also be proved by the perfect correlation ofTpl results determined from Dy emission/ absorption spectroscopy and absolute Hg emissionline measurements.

For a further improvement of the UHP absorption spectroscopy measurement and its ap-plication for HID-lamp investigations in the future, it has to be optimised to determineparticle densities of several other lamp ingredients. Therefore it might be a good solutionto use a monochromator instead of the interference filter within the UHP backlight partsince this setup would allow an arbitrary choice of the backlight wavelength range. For theinvestigation of particles with a low atom absorption strength or even for the investigationof absorbing ions it might also be interesting to exchange the actual UHP backlight sourcewith a more powerful type which is nowadays easily available.On the other hand, within future research work concerning the BBAS setup, the investiga-tion of the plasma temperature by combining spectroscopic emission- and absorption resultsshould also be optimised for other HID-lamp salt additives like Ce. However, the use of Dyas an emitter material in HID-lamps is already the actually market technology and it will besubstituted by modern emitter elements like Ce or La in the near future. Additionally, theproposed application of the developed Tpl measurement for investigations of modern Hg-freeHID-lamps still has to be approved within future research projects.

Bibliography 173

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A - 1

A. Appendix

A.1 Relative efficiency curves of the used CCD cameras

Figure A.1: Relative efficiency curve of the PCO SensiCam used for the 2D electrode temperatureinvestigations within this work. The CCD-type in use is the SVGA version, further camera datacan be found in [79].

A - 2 Appendix

Figure A.2: Relative efficiency curve of the PCO SensiCam qe used adjacent to the spectrographfor the spectroscopic investigations within this work. Further camera data can be found in [79].

A.2. Relative filter characteristics of the used interference filters A - 3

A.2 Relative filter characteristics of the used interferencefilters

885 890 8950.02

0.04

0.06

0.08

0.1

0.12

wavelength λ / nm

rela

tive

filte

r tr

ansm

issi

on

Figure A.3: Relative filter transmission of the IR interference filter around λ = 890 nm

550 555 5600.0

0.1

0.2

0.3

0.4

0.5

wavelength λ / nm

rela

tiv

e fi

lter

tra

nsm

issi

on

(a) Ba

620 625 630 6350

0.1

0.2

0.3

0.4

0.5

0.6

wavelength λ / nm

rela

tive

filte

r tr

ansm

issi

on

(b) Dy

Figure A.4: Relative filter transmission of the Ba interference filter around λ = 550 nm (a) andof the Dy interference filter around λ = 626 nm (b)

A - 4 Appendix

530 535 5400

0.1

0.2

0.3

0.4

0.5

wavelength λ / nm

rela

tive

filte

r tr

ansm

issi

on

(a) Tl

570 575 580 5850

0.1

0.2

0.3

0.4

0.5

0.6

wavelength λ / nm

rela

tive

filte

r tr

ansm

issi

on

(b) Ce

Figure A.5: Relative filter transmission of the Tl interference filter around λ = 535 nm (a) and ofthe Ce interference filter around λ = 577 nm (b)

A.3. Spectral efficiency of the high power LED used as backlight source A - 5

A.3 Spectral efficiency of the high power LED used asbacklight source

rela

tive

spec

tral

radia

nce

1.0

0.8

0.6

0.4

0.2

0.0

wavelength / nmλ

400 450 500 550 600 650 700

GREEN

CYAN

BLUE

ROYALBLUE

Figure A.6: Relative spectral radiation curve of the high power LED used partly as a backlightsource within this work. The LED-type in use is the GREEN version, further LED data can befound in [80].

A - 6 Appendix

Lebenslauf / Curriculum Vitae

Name: Michael Westermeier

Adresse: Hermannshöhe 6

44789 Bochum

Geburtstag: 27. Februar 1981

Geburtsort: Bochum

Staatsangehörigkeit: deutsch

Familienstand: ledig

Berufspraxis

01/2011 – heute RWE Effizienz GmbH, Dortmund

Technisches Produktmanagement RWE SmartHome

02/2007 – 12/2010 Ruhr-Universität Bochum

Lehrstuhl für Allgemeine Elektrotechnik und Plasmatechnik

Wissenschaftlicher Mitarbeiter

Betreuer: Prof. Dr.-Ing. P.Awakowicz

Studium

02/2007 – 01/2011 Ruhr-Universität Bochum

Fakultät für Elektrotechnik und Informationstechnik Promotion zum Dr.-Ing.

10/2001 – 01/2007 Ruhr-Universität Bochum

Ingenieurstudium der Elektrotechnik und Informationstechnik

09/2000 – 07/2001 WR Berufskolleg der Ev. Stiftung Volmarstein, Wetter (Ruhr)

Zivildienstleistender

Schulausbildung

09/1991 – 06/2000 Städtisches Geschwister-Scholl-Gymnasium Wetter (Ruhr)

Abschluss: Allgemeine Hochschulreife

09/1987 – 08/1991 Gemeinschaftsgrundschule Volmarstein, Wetter (Ruhr)