Observation and analysis of pellet material B drift on MAST

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.


Overview

• Experimental set-up

• Macroscopic features

• Visual analysis

• Quantitative interpretive analysis

• First principle simulations

• Conclusions


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)


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.


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


Pellet retention time: measurement

• Encapsulates complex post-pellet losses:

depends on fraction of gas/beam fuelling, non-exponential in time and inhomogeneous


Pellet retention time

• Correlates with status of edge transport barrier

• Diffusive: pel ( a – rpel)

• CUTIE simulation in good agreement


• 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


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


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


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.


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


Timing

• Relative timing of the

camera frames and the

high space resolution

Thomson scattering

profiles.

Camera frames

Low resolution TS

High resolution TS


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


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


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


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


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).


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.


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.


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.

Observation and analysis of pellet material B drift on MAST

Documents

Transcript of Observation and analysis of pellet material B drift on MAST