Assessment of Drift Loss in ITER with Pellet Fuelling and ELM Pace Making A' Polevoi, A' Loarte, A'

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Assessment of Drift Loss in ITER with Pellet
Fuelling and ELM Pace MakingA. Polevoi, A.
Loarte, A. Kukushkin, W. Houlberg, S. Maruyama,
D. Campbell, V. ChuyanovITER Organization,
Cadarache, France

• Pellet injection in ITER
• Model description
• Model validation
• Additional loss due to ELM pace making
• Possible RD

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Pellet fuelling is required in ITER
fuelling from the edge saturates at low level
16 Pa m3/s
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ELM mitigation is required in ITER

• Predicted natural ELM energy loss
• in ITER DWELM20 MJ (DWELM/Wped 20)

• Permissible ELM energy loss
• DWELM 1 MJ (DWELM/Wped 1 )
• Background
• - Erosion is negligible for QELM lt 0.5 MJ/m2
• Zhitlukhin et al, JNM 2007
• - Strong in/out asymmetry Pout/Pin 1 2
• Eich et al, JNM 2007
• - ELM affected area Sin 1.3 m2
• DWELM QELM x Sin x (1 Pout/Pin)
• 0.5 MJ/m2 x 1.3 m2 x 1.5 ? 1 MJ

Permissible ELM energy loss DWELM
Maximal Load gt x
ELM affected surface x Peaking factor
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ELM mitigation by pellet injection is possible

• Background
• reduction of ELM loss DWELM for high ELM
  frequency fELM (JET, ASDEX-U)
• DWELM (0.20.4) x Psep/ fELM gt
• Hermann , 2002 Urano et al, 2004
• full ELM control by pellet pace making with
  reduced DWELM is demonstrated
• (ASDEX-U)
• fELM fpel 20- 40 Hz gt
• Lang et al, 2005
• gt fpel fELM 20- 40 Hz (ITER)
• RD
• - Confinement degradation with high-frequency
  pace making?

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Independent HFS pellet fuelling and LFS ELM pace
making in ITER is possible Polevoi et al, 34th
EPS, 2007
ITER HFS and LFS Injection Flight Tubes Layout
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Requirements for ELM pace making pellets (l
Dped ?)

• Background
• For ELM pace making pellet size, dpel and speed,
  vpel must provide penetration to the top of
  pedestal
• l Dped ASDEX-U, Lang et al, 2005
• (for penetration to the half of pedestal l 0.5
  Dped )
• DIII-D, Baylor et al,35th EPS, 2008
• Ablation depth, l depends on the pellet size,
  dpel, speed vpel and plasma parameters, n, T
  along the path. It is higher for higher dpel,
  vpel and for lower n, T along the pellet paths.

ASDEX-U, Lang et al, 2005
RD Real requirements for pellet for ELM
triggering?
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Requirements for ELM pace making pellets
(pedestal?)

• ITER pedestal
• Reference ITER parameters
• n, T along the pellet paths. Top of pedestal
  is assumed to be located at 95 of poloidal
  magnetic flux
• NB predicted pedestal parameters
• height (n,T) and width (Dped )
• are rather uncertain.
• RD
• Pedestal parameters?

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Requirements for pellets for ELM pace making
maximal speed

• RD
• Dependence of intact pellet speed on size of
  the bore of a guide tube
• Dependence of intact pellet speed on
  accumulation of residual gas in the tube for
  multiple frequent injection

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Requirements for pellets for ELM pace making
minimal size

• RD
• - Validation of ablation model for 2 4 mm
  pellets for vp, 300-1000 m/s and n, T similar to
  ITER pedestal
• - Accumulation of residual gas in the tube for
  multiple frequent injection (10 loss per pellet)

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Compatibility of Pumping/Fuelling/Pace making
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Pellet model description

• Simplified Mass Ablation and Relocation
  Treatment
• Polevoi, Shimada, PPCF, 43 (2001) p.15
• - Ablation model by Kuteev
• Kuteev B, 1995, Nucl. Fusion 35 431
• Cloud size by Parks, et al
• Parks P B, Sessions W D and Baylor L R 2000
  Phys. Plasmas 7 1968
• - Mass relocation by Strauss, Parks
• Strauss H R and Park W, 1998, Phys. Plasmas 5
  2676
• RD
• Model validation for ablation, cloud size and
  relocation
• No time evolution target state gt final state

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No time evolution target state gt final state

• Input/Output parameters
• Electron and electron temperatures, Te, Ti, keV
• Ion species density, nH, (H pellet) nD,
  nT,(D,T,DT pellet) 1019 m-3
• Electron density, ne, 1019 m-3
• Input solid lines gt
• Output dashed lines

HFS pellet injection in ITER-like plasma
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Model validation (examples)

• ASDEX-U HFS injection DIII- D LFS
  injection

Satisfactory agreement
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Model validation (examples)

• JT-60U HFS (Vert) injection DIII- D
  HFS injection
• Takenaga et al, 2006, 33rd EPS
  Polevoi, Shimada, 2001

Satisfactory agreement
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Additional loss due to ELM pace making(particle
loss in ELM)
RD Minimal pellet size for positive fuelling
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Additional loss due to ELM pace making(energy
loss due to drift of LFS pellet)

• Background
• Cooling starts before ELM triggering
• ASDEX-U, G. Kocsis, 2008, 35th EPS
• Energy drop after pellet is
• higher for LHS than for HFS
• DWHFS/W 1 gt
• DWLFS/W 8
• DIII-D, L. Baylor et al, 2000, PoP
• RD
• Comparison of loss energy
• for HFS/LFS injection to
• extract drift loss

Energy loss triggered by pellet DIII-D, L.
Baylor et al, 2000, PoP
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Additional loss due to ELM pace making(energy
loss due to drift of LFS pellet)

• ITER
• Assumption
• LFS pellet is marginal for ELM triggering gt
• ELM starts after ablation and cooling
• Calculation with SMART
• - Energy drop after pellet is higher for LHS
  than for HFS
• DWHFS/Wped 1 (ELM)
• DWLFS/ Wpedlt 2.3 (ELMdrift)
• - Extra loss due to drift
• Pdrift fLFS DWdrift
• RD Comparison of loss energy for HFS/LFS
  injection to extract drift loss

Energy loss due to drift of LFS pellet
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Possible RD

• Injection system
• Dependence of intact pellet speed on size of the
  bore of a guide tube and on accumulation of
  residual gas in the tube for multiple frequent
  injection
• Accumulation of residual gas in the tube for
  multiple frequent injection
• (10 loss per pellet)

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Possible RD

• Plasma transport
• Pedestal parameters prediction
• Confinement degradation with high-frequency pace
  making
• Real requirements for pellet for ELM triggering
• Influence of pellet induced modes
• - Scaling DW (0.2 -0.4??) P/f

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Possible RD

• Pellet physics
• Validation of ablation, cloud size and relocation
  models for 2 5 mm pellets with vp, 300 1000
  m/s for n, T similar to ITER pedestal
• Minimal pellet size for positive fuelling
• Comparison of lost energy for HFS/LFS injection
  to extract drift loss

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Summary

• Pellet injection system is considered as the most
  prospective technique for fuelling and ELM
  mitigation in ITER
• Present estimates of pellet injection for plasma
  fuelling and ELM pace making suggest that for
  safe operation without confinement degradation
  and negligible additional load on the divertor
  target the speed of LFS pellet have to be
  increased to 1000 m/s.
• RD is required to validate extrapolation of
  present day experiments assumed for the ITER
  pellet injection system design.