R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 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, May 2008

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 G. Arnoux 1, S. Brezinsek 2, M. Beurskens 1, T. Eich 3, H. G. Esser 2, W. Fundamenski 1, A. Huber 2, B. Gulejova 4, S. Jachmich 5, A. Kreter 2, A. Loarte 6, E. de la Luna 7, J. Marki 4, G. F. Matthews 1, V. Philipps 2, E. Solano 7, M. F. Stamp 1 and JET EFDA Contributors * 1 Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 2 Institut für Energieforschung-Plasmaphysik, Forschungszentrum Jülich, Association EURATOM-FZJ, Germany 3 Max-Planck-Institut für Plasmaphysik, IPP-EURATOM Association, D Garching, Germany 4 CRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederation 5 LPP, ERM/KMS, Association Euratom-Belgian State, B-1000, Brussels, Belgium 6 ITER Organization, Cadarache, France 7 Associacion 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) with thanks to many co-authors

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Outline Introduction Experiment Impact on the divetor –Radiation –Surface temperatures ELM-wall interactions –Energy deposition –Comparison with theory Conclusions

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Introduction - ELM size limit 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) This implies an ELM energy loss,  W ELM ~ 1 MJ  ~0.3% of stored energy in an ITER Q DT = 10 burning plasma! This is lower than any Type I ELM energy so far achieved  mitigation strategies required. BUT … JET Type I ELMs can approach 1 MJ  study the effects on first wall surfaces and edge plasma Important also in preparation for JET ITER-like wall and improved understanding of ELM SOL physics

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Experiment Vertical targets, MarkIIHD div. I p = 3.0MA, B  = 3.0T q 95 = 3.1,  ~ 0.25,  ~ 1.72 Fuelling scan producing ELMs with range of frequencies and amplitudes

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Large ELMs at zero fueling Input energy ~195 MJ Energy Tile 3,7: 24.6, 70.1 MJ Radiated energy: ~82 MJ n/n Greenwald ~ 0.4 D  (inner) P TOT (MW) W DIA (MJ) T e,ped (keV) n e,ped (10 19 m -3 ) H 98Y Z eff (Brems) Time (s) #70226 – no gas fuelling R. A. Pitts et al., ITPA, Garching, 2007 Mostly NBI

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Lowest fuelling cases at ITER relevant * ped  W ELM /W ped ~ 0.2 for largest ELMs R. A. Pitts et al., ITPA, Garching, 2007 D  (inner) P TOT (MW) W DIA (MJ) T e,ped (keV) n e,ped (10 19 m -3 ) H 98Y Z eff (Brems) Time (s) #70226 – no gas fuelling ITER Mostly NBI Large ELMs at zero fueling

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Radiation during large ELMs Time (s) #70225, low fuelling D   (inner) W DIA (MJ) P RAD (MW)  E rad (MJ) 0.58 MJ 1.08 MJ 0.85 MJ 1.29 MJ Strong in-out asymmetry in ELM induced radiation for high  W ELM  probably due to layers on inner targets and preferential inboard deposition of ELM energy More details: A. Huber et al, P2-24

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 In-out ELM radiation asymmetry W ELM = 0.85 MJ W ELM = 0.61 MJ First ELM spike only For  W ELM 0.6 MJ radiation “spills over” separatrix – in-out radiation asymmetry reduced > ~  E RAD /  W ELM ~ 0.5 if  W ELM 0.6 MJ Evidence for a break at larger  W ELM < ~ Up to 70%  W ELM radiated More details: A. Huber et al, P2-24

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Target IR data not of high enough quality in this more recent experiment to quantify tile surface temperatures Return to shots from 2003 Slightly raised strike pts. compared with recent pulses Slightly lower  W ELM Vertical targets, MarkIISRP divertor I p = 1 – 3 MA, B  = 1 - 3T q 95 ~ 3.1,  ~ MA discharge #62218 similar in parameters to more recent shot #70226 Target surface temperatures?

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Target surface temperatures? Vertical targets, MarkSRP div. I p = 1 – 3 MA, B  = 1 - 3T q 95 ~ 3.1,  ~ MA discharge #62218 similar in parameters to more recent shot #70226 D  (inner) P TOT (MW) W DIA (MJ) T e,ped (keV) n e,ped (10 19 m -3 ) H 98Y Z eff (Brems) Time (s) #62218 ELM averaging period

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Target surface temperatures? Max. T surf far from sublimation Suggest thermal decomposition and ablation of inner target layers accounts for strong radiation asymmetry 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 T surf (outer) ~ 1150ºC Max T surf (inner) ~ 875ºC See also talks by A. Kreter, I-3 and T. Eich, O-17 T. Eich

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 How much ELM energy to walls? Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally 68193, 57 s Make energy balance for a single outboard poloidal limiter during H-mode phase, assume: Only ELM filaments can deposit energy on limiters No energy to upper dump plates No energy deposited in compound phases Same energy on 16 limiters For more on ELM filament wall interactions see posters by: A. Alonso, P1-39 M. Jakubowski, P1-24 D. Moulton, P2-35

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 How much ELM energy to walls? Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally 68193, 57 s s s Make energy balance for a single outboard poloidal limiter during H-mode phase, assume: Only ELMs can deposit energy on limiters No energy to upper dump plates No energy deposited in compound phases Same energy on 16 limiters ∑E tile (15 tiles)

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Wall loading and ELM size 68193, 57 s Pulse No.  gas (10 22 e - /s) No. ELMs(MJ) (kJ) I p = 3.0 MA, B  = 3.0 T, gas scan. Separatrix-midplane outer wall gap fixed at ~5.0 cm.  W ELM estimated for first ELM peak only For fixed wall gap, on average, larger ELMs deposit more energy on limiters. Pulse No.  gas (10 22 e - /s) No. ELMs(MJ) (kJ) How does wall energy fraction compare with theory?

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Pedestal profiles (#70224) 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 Pedestal width ~4 cm Reasonable assumption: filament is born in the mid- pedestal region

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Compare with filament model Mid-pedestal: T e,0 = T i,0 ~ 800 eV n e,0 ~ 3.0  m -3  ped ~ 4 cm v ELM = 600 ms -1  from previous JET studies W’ = (model) W’ = (experiment) Very good agreement given the approximations!

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Compare with filament model Pedestal top: T e,0 = T i,0 ~ 1500 eV n e,0 ~ 5.0  m -3 Separatrix: T e,0 = T i,0 ~ 200 eV n e,0 ~ 1.0  m -3 v ELM = 600 ms

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Compare with filament model Mid-pedestal Pedestal top Separatrix v ELM = 600 ms -1 v ELM = 1200 ms -1 A filament starting at the pedestal top with 2x higher v ELM deposits the same energy at the limiter Filaments starting at the separatrix must travel much more slowly (~180 ms -1 )

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 How to extrapolate to ITER? Results indicate that larger ELMs travel faster  consistent with interchange drive and sheath dissipation as mechanism for filament motion 1 1 MJ (mitigated) ELMs on ITER deposit negligible energy fraction at wall 1 O. E. Garcia et al., Phys. Plasmas 13 (2006), 2 W. Fundamenski, JNM (2007) Use previous scaling from JET H- modes 2 (v ELM = 600 ms -1 for  W ELM /W ped ~ 0.12): Natural ELM (~20 MJ) v ELM = 1000 ms -1 Desired ELM (~1 MJ) v ELM ~220 ms -1 20% 2.5%

R. A. Pitts et al., O-8 18 th PSI, Toledo, Spain 27 May 2008 Conclusions JET can access ELMs with  W ELM approaching 1 MJ at ITER relevant pedestal collisionality (but low n/n GW ) 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 W wall / = 9-12% for ~ 0.5 MJ 1 MJ ELMs on ITER will deposit very small energy fraction at first wall if interchange driven filament velocity scaling applies