Observation and analysis of pellet material B drift on MAST

1
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. Valovic1, 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.
2
Overview

• Experimental set-up
• Macroscopic features
• Visual analysis
• Quantitative interpretive analysis
• First principle simulations
• Conclusions

3
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
• Ip0.66-0.76 MA,
• B0.47-0.50 T,
• ltnegt1.6-7.51019 m-3,
• Te00.7-1.2 keV,
• H-mode plasmas NBI heated (PNBI1.1-3.0 MW with
  neutral beams with energy 65-67 keV).

top pellet entry
outboard pellet entry (not used in this study)
4
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 ms,
• 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.

5
Deposition the inner zone

• Adiabatic deposition creates a distinct zone ?ne
  gt 0, doubled ?lnTe
• Simulation indicates favourable increase of
  transport
• Overtaking the pedestals role

6
Pellet retention time measurement

• Encapsulates complex post-pellet losses
• depends on fraction of gas/beam fuelling,
  non-exponential in time and inhomogeneous

7
Pellet retention time

• Correlates with status of edge transport barrier
• Diffusive tpel ?? ( a rpel)2
• CUTIE simulation in good agreement

8
Pellet retention time normalised to energy
confinement time

• The ratio tpel /tE decreases
• for rpel ? a
• For ITER-like pellets
• tpel /tE 0.2
• Further improvement normalise to tE,pel tE
  (rpel)
• (analogue to tE,ped)

9
Illustration for ITER

• Assume density controlled only by pellets and
  tpel /tE 0.2
• Then ?pel 70 Pa m3/s 70 of design
  steady-state value
• For 5mm pellets, fpel 4?/tpel, faster than in
  today plasmas

10
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.
  

B
11
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 ms) 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.

12
Camera images
Snapshots of the pellet cloud taken during pellet
ablation.
MAST shot 16335
t0.2226 s
t0.2236 s
t0.2244 s
13
Timing
Low resolution TS
High resolution TS
Camera frames

• Relative timing of the camera frames and the high
  space resolution Thomson scattering profiles.

14
Image composition

• Superimpose all the frames taken during the
  pellet ablation at intervals of 200 ms.
• Superimpose the image of the equilibrium map
• Superimpose grid at the toroidal location of the
  pellet injection plane to measure distances.

LFS
HFS
15
Visual analysis

• Flux surfaces spaced by intervals of DyN0.1.
• The surface highlighted in red corresponds to
  yN0.4 (innermost surface affected by the pellet
  perturbation according to Thomson scattering).
• Pellet ablates completely outside yN0.5-0.6. To
  affect the surface yN0.4 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
16
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.

45 cm above the equatorial plane
LFS
HFS
17
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 ?.
• 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.

18
Interpretive analysis (II)

• The post-pellet ablation profile (no drift) falls
  outside the experimental data.
• Drifted (?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).

19
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, todays poster
  session, P4.099 (benchmarking).
• Good agreement with the experiment.
• Pre-cooling has to be taken into account.

20
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.

21
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.