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The Impact of large ELMs on JET

The Impact of large ELMs on JET. Presented by R. A. Pitts CRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederation on behalf of JET Task Force E and JET EFDA Contributors 18 th International Conference on Plasma-Surface Interactions, Toledo, Spain, 23-26 May 2008.

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The Impact of large ELMs on JET

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  1. The Impact of large ELMs on JET Presented byR. A. PittsCRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederationon behalf of JET Task Force E and JET EFDA Contributors18th International Conference on Plasma-Surface Interactions, Toledo, Spain, 23-26 May 2008

  2. with thanks to many co-authors G. Arnoux1, S. Brezinsek2, M. Beurskens1, T. Eich3, H. G. Esser2, W. Fundamenski1, A. Huber2, B. Gulejova4, S. Jachmich5, A. Kreter2, A. Loarte6, E. de la Luna7, J. Marki4, G. F. Matthews1, V. Philipps2, E. Solano7, M. F. Stamp1 and JET EFDA Contributors* 1Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 2Institut für Energieforschung-Plasmaphysik, Forschungszentrum Jülich, Association EURATOM-FZJ, Germany 3Max-Planck-Institut für Plasmaphysik, IPP-EURATOM Association, D-85748 Garching, Germany 4CRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederation 5LPP, ERM/KMS, Association Euratom-Belgian State, B-1000, Brussels, Belgium 6ITER Organization, Cadarache, France 7Associacion Euratom/CIEMAT para Fusion, Madrid, Spain *See appendix of M. Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA Vienna (2006)

  3. Outline • Introduction • Experiment • Impact on the divetor • Radiation • Surface temperatures • ELM-wall interactions • Energy deposition • Comparison with theory • Conclusions

  4. Introduction - ELM size limit Important also in preparation for JET ITER-like wall and improved understanding of ELM SOL physics Material damage poses a limit on the maximum ELM size tolerable on ITER Current estimates indicate that ELM power fluxes must remain below ~0.5 MJm-2 at the ITER divertor targets (see J. Roth, paper R-1) JET Type I ELMs can approach 1 MJ  study the effects on first wall surfaces and edge plasma This implies an ELM energy loss, DWELM~ 1 MJ ~0.3% of stored energy in an ITER QDT = 10 burning plasma! This is lower than any Type I ELM energy so far achieved  mitigation strategies required. BUT …

  5. Experiment Fuelling scan producing ELMs with range of frequencies and amplitudes Vertical targets, MarkIIHD div.Ip = 3.0MA, Bj = 3.0Tq95 = 3.1, d ~ 0.25, k ~ 1.72

  6. Large ELMs at zero fueling #70226 – no gas fuelling Da (inner) PTOT(MW) WDIA(MJ) Te,ped (keV) ne,ped(1019m-3) H98Y Zeff(Brems) Time (s) Mostly NBI Input energy ~195 MJEnergy Tile 3,7: 24.6, 70.1 MJRadiated energy: ~82 MJn/nGreenwald ~ 0.4 R. A. Pitts et al., ITPA, Garching, 2007

  7. Large ELMs at zero fueling #70226 – no gas fuelling Da (inner) PTOT(MW) WDIA(MJ) Te,ped (keV) ne,ped(1019m-3) H98Y Zeff(Brems) Time (s) Mostly NBI ITER Lowest fuelling cases at ITER relevant n*ped DWELM/Wped ~ 0.2 for largest ELMs R. A. Pitts et al., ITPA, Garching, 2007

  8. Radiation during large ELMs 0.85 MJ 1.29 MJ 1.08 MJ 0.58 MJ #70225, low fuelling More details: A. Huber et al, P2-24 Da(inner) WDIA (MJ) PRAD (MW) Erad (MJ) Strong in-out asymmetry in ELM induced radiation for high DWELM  probably due to layers on inner targets and preferential inboard deposition of ELM energy Time (s)

  9. In-out ELM radiation asymmetry DERAD/DWELM ~ 0.5 if DWELM 0.6 MJ Evidence for a break at largerDWELM > ~ For DWELM 0.6 MJ radiation “spills over” separatrix – in-out radiation asymmetry reduced < ~ First ELM spikeonly Up to 70% DWELM radiated WELM = 0.61 MJ WELM = 0.85 MJ More details: A. Huber et al, P2-24

  10. Target surface temperatures? Target IR data not of high enough quality in this more recent experiment to quantify tile surface temperatures Vertical targets, MarkIISRP divertorIp = 1 – 3 MA, Bj = 1 - 3Tq95 ~ 3.1, d ~ 0.253.0 MA discharge #62218 similar in parameters to more recent shot #70226 Return to shots from 2003 Slightly raised strike pts. compared with recent pulses Slightly lower DWELM

  11. Target surface temperatures? Da (inner) PTOT(MW) WDIA(MJ) Te,ped (keV) ELM averaging period ne,ped(1019m-3) H98Y Zeff(Brems) #62218 Vertical targets, MarkSRP div.Ip = 1 – 3 MA, Bj = 1 - 3Tq95 ~ 3.1, d ~ 0.253 MA discharge #62218 similar in parameters to more recent shot #70226 Time (s)

  12. Target surface temperatures? Inter-ELM power loads higher at outer than inner as usual Clear affect of surface layers on inner target For largest ELM (~0.7 MJ):Max Tsurf (outer) ~ 1150ºCMax Tsurf (inner) ~ 875ºC T. Eich Max. Tsurf far from sublimationSuggest thermal decomposition and ablation of inner target layers accounts for strong radiation asymmetry See also talks by A. Kreter, I-3 and T. Eich, O-17

  13. How much ELM energy to walls? Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally For more on ELM filament wall interactions see posters by: A. Alonso, P1-39M. Jakubowski, P1-24D. Moulton, P2-35 Make energy balance for a single outboard poloidal limiter during H-mode phase, assume:Only ELM filaments can deposit energy on limitersNo energy to upper dump platesNo energy deposited in compound phasesSame energy on 16 limiters 68193, 57 s

  14. How much ELM energy to walls? 17.405 s 20.016 s 13 12 11 ∑Etile (15 tiles) Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally Make energy balance for a single outboard poloidal limiter during H-mode phase, assume:Only ELMs can deposit energy on limitersNo energy to upper dump platesNo energy deposited in compound phasesSame energy on 16 limiters 68193, 57 s

  15. Wall loading and ELM size Ip = 3.0 MA, Bj = 3.0 T, gas scan. Separatrix-midplane outer wall gap fixed at ~5.0 cm. DWELM estimated for first ELM peak only 68193, 57 s For fixed wall gap, on average, larger ELMs deposit more energy on limiters. How does wall energy fraction compare with theory?

  16. Pedestal profiles (#70224) Pedestal width ~4 cm Filament parallel energy loss model(W. Fundamenski, R. A. Pitts, PPCF 48 (2006) 109) Assume ELM filament released in pedestal region with constant radial speed. Propagate to walls and track power exhaust due to parallel energy loss Reasonable assumption: filament is born in the mid-pedestal region

  17. Compare with filament model Mid-pedestal:Te,0 = Ti,0 ~ 800 eVne,0 ~ 3.01019 m-3Dped ~ 4 cmvELM = 600 ms-1 from previous JET studies W’ = 0.094(model) W’ = 0.088(experiment) Very good agreement given the approximations!

  18. Compare with filament model Pedestal top:Te,0 = Ti,0 ~ 1500 eVne,0 ~ 5.01019 m-3 Separatrix:Te,0 = Ti,0 ~ 200 eVne,0 ~ 1.01019 m-3 vELM = 600 ms-1 0.394 0.094 0.037

  19. Compare with filament model Mid-pedestalPedestal topSeparatrix vELM = 600 ms-1vELM = 1200 ms-1 A filament starting at the pedestal top with 2x higher vELM deposits the same energy at the limiter Filaments starting at the separatrix must travel much more slowly (~180 ms-1)

  20. How to extrapolate to ITER? Natural ELM (~20 MJ) vELM = 1000 ms-1 Use previous scaling from JET H-modes2 (vELM = 600 ms-1 for DWELM/Wped ~ 0.12): Desired ELM (~1 MJ) vELM ~220 ms-1 20% 2.5% Results indicate that larger ELMs travel faster  consistent with interchange drive and sheath dissipation as mechanism for filament motion1 1 MJ (mitigated) ELMs on ITER deposit negligible energy fraction at wall 1O. E. Garcia et al., Phys. Plasmas 13 (2006), 2W. Fundamenski, JNM 363-365 (2007)

  21. Conclusions • JET can access ELMs with DWELM approaching 1 MJ at ITER relevant pedestal collisionality (but low n/nGW) • Strong in-out divertor radiation asymmetry – up to 70% of the ELM energy drop can be radiated, mostly in the inner divertor volume • Divertor surface temperatures too low for sublimation  thermal decomposition and ablation of inner target co-deposited layers • ELM filaments seen clearly at main chamber limiters • Deposited energy fraction increases with ELM size • Wwall/<DWELM> = 9-12% for <DWELM> ~ 0.5 MJ • 1 MJ ELMs on ITER will deposit very small energy fraction at first wall if interchange driven filament velocity scaling applies

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