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Max-Planck-Institut für Plasmaphysik. Prediction of wall fluxes and implications for ITER limiters. Arne Kallenbach, ASDEX Upgrade Team. Topics of this talk: guidelines on load specifications. steady state particle main chamber fluxes from spectroscopy
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Max-Planck-Institut für Plasmaphysik Prediction of wall fluxes and implications for ITER limiters Arne Kallenbach, ASDEX Upgrade Team
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)
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:
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
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
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 = 21019 m-3 / 0.05 m conservative, can be larger = 1.21021 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 m21/3 ~ 31023 s-1
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, =21022 1/s, drXP=3 cm 4.41023 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 3.21023 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 H2072 m2 main=7.21022 s-1 2.91023 s-1 Over all, 3(1-5)1023 s-1 seems reasonable estimate
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= ei (2Ti + 3Te + Erec) + ee 2Te 100 e per 11023 part/s 100 eV per e-i pair 1.6 MW
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
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
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
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
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
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 ?
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.
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
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)