L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Observation and analysis of pellet material B drift on MAST
L. Garzotti1, K. B. Axon1, L. Baylor2, J. Dowling1, C. Gurl1, F. Köchl3,
G. P. Maddison1, H. Nehme4, A. Patel1, B. Pégourié4, M. Price1,
R. Scannell1, M. Valovič1, M. Walsh1
1Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, UK.2Association EURATOM-Österreichische Akademie der Wissenschaften, Austria.
3Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.4Association EURATOM-CEA, CEA Cadarache, Saint Paul-lez-Durance, France.
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Overview
• Experimental set-up
• Macroscopic features
• Visual analysis
• Quantitative interpretive analysis
• First principle simulations
• Conclusions
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
MAST pellet injection system
• On MAST deuterium pellets are injected
vertically from the top of the machine into
the high field side of the plasma.
– Typical pellet speeds are between 250 and
400 m/s.
– Nominal pellet masses are 0.6, 1.2 and 2.4
1020 atoms.
• Typical MAST target plasmas:
– Ip=0.66‑0.76 MA,
– B=0.47‑0.50 T,
– <ne>=1.6‑7.5·1019 m-3,
– Te0=0.7‑1.2 keV,
– H-mode plasmas NBI heated (PNBI=1.1‑3.0
MW with neutral beams with energy 65‑67
keV).
top pellet entry
outboard pellet entry (not used in this study)
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
MAST pellet diagnostics• Unfiltered visible images of the complete pellet trajectory inside
the plasma taken with a fast camera:
– frame rate 5 kfps, exposure time 7 s,
– core region of the cloud saturated,
– information limited to the edge of the cloud.
• Narrow spectrum (centre wavelength 457 nm and bandpass 2.4
nm) radiation (mainly brehmsstrahlung) emitted by the pellet
cloud recorded by a second CCD camera:
– frame rate 30 fps, exposure time 31 ms,
– limited field of view including only the final part of the pellet trajectory,
– images saturated on a smaller region of the pellet cloud,
– more detailed information about the structure of the cloud.
• Density and temperature profile measured:
– every 5 ms with a multiple-pulse, 34 radial points Thomson scattering
system,
– immediately after the end of pellet ablation with a single-pulse, 300
radial points Thomson scattering system.
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Deposition: the inner zone
• Adiabatic deposition creates a distinct zone: ne > 0, doubled lnTe
• Simulation indicates favourable increase of transport • Overtaking the pedestal’s role
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Pellet retention time: measurement
• Encapsulates complex post-pellet losses:
depends on fraction of gas/beam fuelling, non-exponential in time and inhomogeneous
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Pellet retention time
• Correlates with status of edge transport barrier
• Diffusive: pel ( a – rpel)
• CUTIE simulation in good agreement
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
• The ratio pel /E decreases
for rpel a
• For ITER-like pellets:
pel /E ~ 0.2
• Further improvement: normalise to E,pel = E (rpel)
(analogue to E,ped)
Pellet retention time normalised to energy confinement time
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Illustration for ITER
, ~ 0.8
pe
pel
e pla l
pel
sma
pel
n S ar a
r
• Assume density controlled only by pellets and pel /E ~ 0.2
• Then: pel ~ 70 Pa m3/s ~ 70% of design steady-state value
• For 5mm pellets, fpel= 4/pel, faster than in today plasmas
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
EXB drift
• Pellet material deposited in a
tokamak plasma experiences
a drift towards the low field
side of the torus induced by
the magnetic field gradient. E
R0
B
EB drift
B
B
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Characteristics of the drift
• Potentially beneficial effects on the fuelling efficiency, since increases the
deposition depth of the pellet material for pellets injected from the high field
side of the plasma.
• Very difficult to observe, because of the fast time scale on which it occurs
(~100 s) and the presence of other transport mechanisms in the plasma.
• Detected in the past on different machines (ASDEX-U, JET, DIII-D, Tore-
Supra, FTU and MAST).
• Since the fuelling of ITER plasma will rely significantly on the beneficial
effect of this B drift to increase the pellet material deposition depth, it is
crucial to analyse this phenomenon in detail:
– develop codes to predict it,
– compare the predictions with experimental results in present machines.
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Camera images
t=0.2226 s t=0.2236 s t=0.2244 s
Snapshots of the pellet cloud taken during pellet ablation.
MAST shot 16335
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Timing
• Relative timing of the
camera frames and the
high space resolution
Thomson scattering
profiles.
Camera frames
Low resolution TS
High resolution TS
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Image composition
• Superimpose all the frames
taken during the pellet ablation
at intervals of 200 s.
• Superimpose the image of the
equilibrium map
• Superimpose grid at the
toroidal location of the pellet
injection plane to measure
distances.
LFS HFS
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Visual analysis• Flux surfaces spaced by intervals of
N=0.1.
• The surface highlighted in red corresponds
to N=0.4 (innermost surface affected by the
pellet perturbation according to Thomson
scattering).
• Pellet ablates completely outside N=0.5‑0.6.
To affect the surface N=0.4 the pellet
material should drift by ~20 cm towards the
low field side (LFS) of the plasma.
• End of the pellet trajectory is 45 cm above
the equatorial plane.
• Clouds equally spaced vertically along the
pellet path and pellet path follows an almost
straight line.
LFS HFS
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Brehmsstrahlung imaging
• Asymmetric structure of the pellet
cloud extending towards the LFS
is visible on the images of the final
part of the pellet trajectory taken
with the filtered camera.
• Suggests that a drift is taking
place towards the LFS of the
plasma.
LFS HFS
45
cm
ab
ove
th
e
eq
ua
toria
l pla
ne
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Interpretive analysis (I)
• Interpretive analysis of the observations performed with
the code PELDEP2D (Pégourié & Garzotti EPS
Bertchesgaden 1997).
• Pellet advances along the trajectory in the cross section
of the plasma.
• Ablation calculated at each point (NGPS).
• Material distributed along the magnetic field gradient
with typical drift length Λ.
• Resulting 2-dimensional density distribution averaged
over the magnetic surfaces to give a poloidally
symmetric deposition profile.
• Adiabatic plasma cooling caused by pellet material
drifting in front of the pellet taken into account.
t1
t2
t3
Λ
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Interpretive analysis (II)• The post-pellet ablation profile (no drift)
falls outside the experimental data.
• Drifted (Λ~25 cm) profile fits well the experimental measurements.
• Drift along the magnetic field gradient ~35-40% of the plasma minor radius
• Displacement between ablation and deposition profile of 10-20% in terms of flux radial co-ordinate.
• Without pre-cooling pellet penetrates to 60 cm above the plasma equatorial plane (shorter than the observed penetration).
• With pre-cooling penetration reaches 50 cm above the equatorial plane (closer to the experimental observations).
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
First principle simulations (I)• Simulations performed with a first
principle code:
– NGPS-type ablation,
– four fluid Lagrangian drift model
(plasmoid expansion).
• Details of the code:
– B. Pégourié et al., Nucl. Fusion 47
44 (equations),
– F. Köchl, this conference, today’s
poster session, P4.099
(benchmarking).
• Good agreement with the
experiment.
• Pre-cooling has to be taken into
account.
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
First principle simulation (II)
• Simulations of the MAST experiments have been attempted also with
another similar first principle code described in P.B. Parks and L.R.
Baylor, Phys. Rev. Lett. 94 125002.
• The code underestimates the displacement of the deposition profile by
~50%.
• The reason for this is that the main mechanism driving the plasmoid
drift is the reheating of the pellet cloudlet.
• In this model background plasma temperatures over 1 keV are required
to build enough pressure in the cloudlet to accelerate it along the major
radius.
• Therefore this mechanisms is predicted to be weak in MAST plasma
simulations because of the relatively low background plasma
temperature.
L. Garzotti – Workshop on Fuelling in Magnetic Confinement Machines – Hersonissos, Crete 16th-17th June 2008
Conclusions
• Fast visible imaging and high space and time resolution Thomson
scattering have revealed the details of the pellet trajectory, ablation and
deposition profile on MAST.
• The presence of a B-induced drift, leading to a 10 cm displacement
between ablation and deposition profiles, has been identified.
• Interpretive analysis shows that this displacement is compatible with a 20-
25 cm drift of the pellet material in the direction of the magnetic field
gradient.
• There is evidence of the drift induced plasma pre-cooling in front of the
pellet playing a role in increasing the pellet penetration depth.
• These results are predicted by one of the first principle ablation/deposition
codes presently available, whereas a second code tends to underestimate
the drift because the driving mechanism is predicted to be weak on MAST.