1. Pellet injection in ITER Model description Model validation
Assessment of Drift Loss in ITER with Pellet Fuelling and ELM Pace Making A. Polevoi, A. Loarte, A. Kukushkin, W. Houlberg, S. Maruyama, D. Campbell, V. Chuyanov ITER Organization, Cadarache, France
Pellet injection in ITER
Model description
Model validation
Additional loss due to ELM pace making
Possible R&D

2. Pellet fuelling is required in ITER
fuelling from the edge saturates at low level ~ 16 Pa m3/s

3. ELM mitigation is required in ITER
Predicted natural ELM energy loss in ITER DWELM~20 MJ (DWELM/Wped~ 20%)
Permissible ELM energy loss
DWELM= 1 MJ (DWELM/Wped~ 1% )
Background:
- Erosion is negligible for: QELM < 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
ELM losses DWELM = 1 MJ
exist in JET experiments!!!
Permissible ELM energy loss:
DWELM= Maximal Load =>
x ELM affected surface x Peaking factor

4. ELM mitigation by pellet injection is possible
Background
reduction of ELM loss DWELM for high ELM frequency fELM (JET, ASDEX-U):
DWELM= (0.2—0.4) x Psep/ fELM =>
[Hermann , 2002; Urano et al, 2004]
full ELM control by pellet pace making with reduced DWELM is demonstrated
(ASDEX-U)
fELM= fpel= Hz =>
[Lang et al, 2005]
=> fpel= fELM= Hz (ITER)
R&D:
- Confinement degradation with high-frequency pace making?

5. ITER HFS and LFS Injection Flight Tubes Layout
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

6. 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]
R&D: Real requirements for pellet for ELM triggering?

7. 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.
R&D:
Pedestal parameters?

8. Requirements for pellets for ELM pace making: maximal speed
R&D:
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;

9. Requirements for pellets for ELM pace making: minimal size
R&D: - Validation of ablation model for 2 – 4 mm pellets for vp,= m/s and n, T similar to ITER pedestal; - Accumulation of residual gas in the tube for multiple frequent injection (10% loss per pellet);

10. Compatibility of Pumping/Fuelling/Pace making

11. 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 ]
Cloud size by Parks, et al
[Parks P B, Sessions W D and Baylor L R 2000 Phys. Plasmas ]
- Mass relocation by Strauss, Parks
[Strauss H R and Park W, 1998, Phys. Plasmas ]
R&D:
Model validation for ablation, cloud size and relocation
No time evolution: target state => final state

12. No time evolution: target state => final state
Input/Output parameters:
Electron and electron temperatures, Te, Ti, keV
Ion species density, nH, (H pellet) nD, nT,(D,T,DT pellet) m-3
Electron density, ne, 1019 m-3
Input – solid lines =>
Output – dashed lines
HFS pellet injection in ITER-like plasma

13. Model validation (examples)
ASDEX-U HFS injection DIII- D LFS injection
Satisfactory agreement

14. Model validation (examples)
JT-60U HFS (Vert) injection DIII- D HFS injection
[Takenaga et al, 2006, 33rd EPS] [Polevoi, Shimada, 2001]
Satisfactory agreement

15. Additional loss due to ELM pace making (particle loss in ELM)
R&D: Minimal pellet size for positive fuelling

16. Energy loss triggered by pellet [DIII-D, L. Baylor et al, 2000, PoP]
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% =>
DWLFS/W = 8%
[DIII-D, L. Baylor et al, 2000, PoP]
R&D:
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]

17. Energy loss due to drift of LFS pellet
Additional loss due to ELM pace making (energy loss due to drift of LFS pellet)
ITER:
Assumption
LFS pellet is marginal for ELM triggering =>
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/ Wped< 2.3% (ELM+drift)
- Extra loss due to drift
Pdrift = fLFS DWdrift
R&D: Comparison of loss energy for HFS/LFS injection to extract drift loss
Energy loss due to drift of LFS pellet

18. Possible R&D
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);

19. Possible R&D
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 = ( ??) P/f

20. Possible R&D
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

21. 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.
R&D is required to validate extrapolation of present day experiments assumed for the ITER pellet injection system design.