Prediction of wall fluxes and implications for ITER limiters

1. Max-Planck-Institut für Plasmaphysik Prediction of wall fluxes and implications for ITER limiters Arne Kallenbach, ASDEX Upgrade Team

2. Topics of this talk: guidelines on load specifications •  steady state particle main chamber fluxes from spectroscopy • estimates of connected power fluxes and decay lengths • contribution due to ELMs (enhancement factor)

3. Current ITER Guidelines (PID V3.0): Only radiation and CX load to first wall, 0.5 MW/m2 transport and drifts lead to parallel heat fluxes in far SOL diffusive transport between ELMs blobby transport between ELMs (radial outward convection) ELM SOL transport (like large blobs) - parallel drift towards high-field side - strong recycling around inner X-point additional players in particle transport:

4. Main chamber spectroscopy at ASDEX Upgrade Ralph Dux

5. Innner and outer wall plasma-surface interaction in AUG from CII spectroscopy: very sensitive on in-out alignment R lim 8 m2 0.3 m2 • inner heat shield major recycling region except plasma close to outer limiter • lower inner wall flux dominated from inner divertor • upper inner wall flux has radial e-folding length ~ 2-3 cm

6. How to estimate the stationary power flows on the limiters • estimate the total radial ion outflux* • estimate the deposited energy per electron-ion pair • estimate the effective wetted area • or peak load and decay length average value from different models, be conservative and use upper end  limiter power flux density *IO calculates in terms of parallel power fluxes and decay lengths

7. 1) Total radial ion wall flux in ITER [i] scaling like diffusive transport Radial SOL particle flux in ITER ansatz with effective D: H-mode  = D dn/dr recycling rises D = 3 m2/s this value typical for SOL wing in many devices dn/dr = 21019 m-3 / 0.05 m conservative, can be larger  = 1.21021 m-2 s-1 AUG edge density profiles from Li-beam Transport balloons around outer midplane: total main chamber ion influx: multiply  with 1/3 of plasma surface area F ITER = 680 m21/3  ~ 31023 s-1

8. Total radial ion wall flux in ITER [ii] some alternative ways of estimation • Same flux density as in AUG discharge with high similar fGreen, P/R, • and absolute density, scaled with area ITER: 100 MW/6 m, not possible in AUG, scale P0.24[NF 42 (2002) 1184] AUG 21015/17, 7.5 MW, ne=1020 m-3, =21022 1/s, drXP=3 cm 4.41023 s-1 b) Same flux density as in AUG discharge with high similar fGreen, P/R2 and absolute density, scaled with area  16 ITER: 100 MW/36 m2, 7.5 MW in AUG, AUG # as above 3.21023 s-1 c) Same flux density as in JET discharge with high similar fGreen, P/R2 and absolute density, scaled with area (best use 4 MA, 25 MW discharge) JET 70054, 3.5 MA, 24 MW, 1e20, midplane H2072 m2  main=7.21022 s-1 2.91023 s-1 Over all, 3(1-5)1023 s-1 seems reasonable estimate

9. Energy per electron ion pair Te in the SOL wing of a high density H-mode discharge is typically 5-10 eV, Ti tends to be moderately higher We assume for ITER Te= 10 eV, Ti= 20 eV standard model for sheath power deposition (negl. secondary el. emission) P= ei (2Ti + 3Te + Erec) + ee 2Te  100 e per 11023 part/s 100 eV per e-i pair  1.6 MW

10. 3) effective wetted area and resulting loads The wetted area depends on actual wall design ! wetted width depends on decay length in limiter shadow ITER quick guess: 18 protruding ribs, height 5 m, 0.05 m wetted width  4.5 m2 for HFS and LFS each (good alignment required !) 3 1023 ions/s  4.8 MW • charge exchange is expected to increase this number by 10-20 %  ELMs contribute to recycling fux by factor 1.5 • radiation is expected to contribute < 0.2 MW/m2 • some contrib. by fast ion losses on LFS overall, expected peak loads about 1 MW/m2 not problematic, but safely  1 MW/m2 would allow to avoid active cooling Of course, the upper X-point region takes more power and must be strengthened

11. ELM contributions to average particle influxes: small for D, C, dominnat for W Outer limiter ELM-cycle averaged, D and C fluxes increased by ~ 1.5 but: 70 % of the W influx due to ELMs (increased yield) R. Dux

12. Decay length depends on connection length to limiters Increasing the number of limiters can reduce the power load. However: the decay length shortens with reduced connection length and more precise alignment will be required measurements in AUG limiter shadow by H.W. Müller

13. ITER expects negligible loads on inner wall - does the existence of a 2nd sep. shield the inner wall ? No ! further investigations needed on inner wall load close DN dRXP= 3 mm

14. Comparison to previous estimate based on JET-AUG Recycling scaling (Tarragona meeting, July 2005) new insight: predominantly HFS recycling  multiply with S/4 only: Rtot= 1024 s-1 (ne,sol= 4.7 1019) Strong dependence of total recycling on ne,line-av (power 4)  If pellets are needed to reach 1020 m-3 in ITER, this number comes down: If ITER produces ne= 7.5 1019 by recycling only, Rtot= 3 1023, ne,sol= 2.6 1019

15. Conclusions •  Main chamber recycling occurs predominantly on the high field side • and on wall structures touching the innermost flux surfaces •  Effect supposed to be connected to strong drifts towards HFS • Strong plasma wall interaction with the inner wall close to DN operation is not understood: fluxes close to the separatrix or ExB drifts around upper X-point ?  Expected total particle fluxes 3 2 1023 part/s, power fluxes ~ 5 MW • How will the ITER FW will look like ?

16. ELMs: Simple size scaling and effect to wall materials • Size scaling based on empirical findings: • natural type-I ELM size ~ 10 % of pedestal energy, 3.5 % of plasma energy • ELMs carry 30 % of the power flux • simple algebra: PELM= 0.3 Ploss = 0.3 Wtot/tE = 0.035 fELM Wtot = fELMWELM • fELM = 8.6/tE • ITER Sc. 1: Wtot = 353 MJ, tE=3.4 sAUG typ.: 0.8 MJ, 0.1 s •  fELM = 2.5 Hz, WELM= 12 MJ80 Hz, 28 kJ • controlled ELMs: if fELM is changed, WELM scales ~ 1/fELM ITER PID: uncontrolled ELMs fELM = 1 Hz, WELM= 15-20 MJ controlled ELMs fELM = 5 Hz, WELM= 3-4 MJ o.k.

17. ELMs: Simple size scaling and effect to wall materials Material properties: 1556 K 3683 K 3640 K (subl.) melting/ablation limits: Be 20, W 60, CFC 60-70 [MJ m-2s-0.5] example: ELM 1 MJ/m2, 0.5 ms duration  45 MJ m-2s-0.5 recent lab exps. (Russian-EU collab.) suggest limit below 0.7 MJ/m2 both for W and CFC (fatigue, crack formation)  reduce peak load by factor 0.5 Divertor peak power load: ITER PID assumed effective wetted divertor area of 7.5 m2 (w= 0.1 m along targets) resulting maximum loads were 2.7 MJ/m2 (uncontr.), 0.5 MJ/m2 (contr.) the maximum allowed ELM was controlled to 4 MJ latest changes: no ELM power broadening lp  5 mm (fact 2/3) in-out asymmetry 2:1 – (fact ¾), recover factor 2 safety margin 0.5  0.25 MJ/m2 ?  maximum “ELM” ~ 1 MJ too pessimistic – ignores large lp inner div

18. Open points = possible AUG contributions 1) midplane inter-ELM power width midplane Te decay length scales ~ machine size A. Kallenbach et al., ITPA SOL&Div Topical Group, PSI 2004 expected power width 2/7  Te AUG: 1.3 mm omp considerably broader widths observed in divertor (mapped to omp) good topic for future AUG / inter-machine exps (L. Horton)