R. A. Pitts et al. 1 (12) IAEA, Chengdu Oct ELM transport in the JET scrape-off layer R. A. Pitts, P. Andrew, G. Arnoux, T.Eich, W. Fundamenski, E. Gauthier, A. Huber, S. Jachmich, C. Silva, D. Tskhakaya and JET EFDA Contributors 18 October 2006

R. A. Pitts et al. 2 (12) IAEA, Chengdu Oct OUTLINE ELM divertor energy asymmetries ELM filamentary structure Modelling the ELM transport –Particle-in-cell (PIC) simulations –Transient modelling of ELM filament parallel losses –Main wall particle energies –Main wall power deposition Conclusions

R. A. Pitts et al. 3 (12) IAEA, Chengdu Oct Brief diagnostic overview Divertor IR and tile thermocouples Wide angle main chamber IR Fast reciprocating probes: TTP, RFA Diagnostic Optimised Configuration (DOC)

R. A. Pitts et al. 4 (12) IAEA, Chengdu Oct Divertor target ELM energy asymmetry T. Eich et al., PSI 2006 ELM resolved target heat flux (IR) –Type I ELM energy deposition strongly favours INNER target for FWD-B  –For REV-B, some evidence for more balanced deposition, –Consistent with similar analysis from AUG (W ELM < 20 kJ) and linked to passage of net current through target plates Favourable trend for ITER target power loading (since always more energy to OUTER target inter-ELM)

R. A. Pitts et al. 5 (12) IAEA, Chengdu Oct ELM filaments – main chamber IR Filamentary power deposition detected with new wide angle IR –100 Hz frame-rate, but 300  s snapshot  catches an occasional ELM –Seen by substracting pre-ELM and ELM frames I p = 2 MA, B  = 3T W ELM ~ 150 kJ Two discharges with different contact point of first limiting flux surface #66560, 5.548s P. Andrew, G. Arnoux Coord. Transformation (x,y)  ( ,  ) #67384, s

R. A. Pitts et al. 6 (12) IAEA, Chengdu Oct ELM filaments in the far SOL TTP r - r sep ~ 80 mm at the probe Clear filamentary structure in the particle flux, T e and radial velocity –W ELM ~ 100 kJ –T e (pedestal) ~ 500 eV –T e ELM (limiter) ~ 30 eV –v r ELM ~ 500  1000 ms -1 Electrons cool rapidly in the filament as it crosses the SOL ELM duration at the probe ~10x higher than  ELM seen on MHD activity etc. C. Silva et al., J. Nucl. Mater (2005) 722

R. A. Pitts et al. 7 (12) IAEA, Chengdu Oct Modelling the ELM transient WALL Losses along B Present understanding: MHD perturbs pedestal  radial expulsion of plasma  parallel loss along field lines to divertor until filament hits wall Particle-in-Cell (PIC) simulations CPU intensive Inject ELM energy kinetically via particle source at T ped, n ped for time  ELM and follow particles to targets including full target sheath dynamics Two separate approaches being followed at JET to modelling the 1D SOL parallel transport. Transient model Fluid and kinetic versions. Simpler to solve, captures many effects of PIC simulations Introduces 2D nature of filament propagation by relating loss times to radial velocities

R. A. Pitts et al. 8 (12) IAEA, Chengdu Oct PIC simulations of parallel losses More realistic description of the ELMy JET SOL using improved PIC simulations (BIT1 code) –Scan in T ped, n ped to vary W ELM –Most of the heat flux arrives with ions on the acoustic timescale –BUT, only ~30% of ELM energy deposited when q target peaks –Electrons account for ~30% of target energy deposition –Strong transient increase over “Maxwellian” sheath transmission factors during the ELM –Fluid code assumption of fixed  underestimates q target at high W ELM Example: T ped = 1.5 keV, n ped = 1.5x10 19 m -3 W ELM ~ 120 kJ,  ELM = 200  s D. Tskhakaya

R. A. Pitts et al. 9 (12) IAEA, Chengdu Oct Transient model of ELM parallel losses Key elements of model –Temporal evolution of n, T e and T i in the filament frame of reference –Time and radius related by filament propagation velocity –Parallel loss treated as conductive and convective removal times –Radial expansion included Filament cools faster than it dilutes, electrons cooled more rapidly than ions  in the far SOL,T i > T e in the filament at wall impact W. Fundamenski, Plasma Phys. Control. Fusion 48 (2006) 109 Example with T i,ped = T e,ped = 400 eV n ped = 1.5x10 19 m -3, H + ions

R. A. Pitts et al. 10 (12) IAEA, Chengdu Oct Model consistent with RFA hot ion data Good agreement with transient model for i-side peak fluxes –Predicts T i,RFA /T i,ped = 0.3  0.5 –T e,RFA /T e,ped = 0.13  0.25 –n e,RFA /n e,ped = 0.3  0.4 Consistent with low T e on TTP probe R. A. Pitts et al., Nucl. Fusion 46 (2006) 82 W. Fundamenski, PPCF 48 (2006) 109 RFA Current of ions with energy > 400 eV Filaments on plasma and hot ion fluxes –W ELM ~ 50 kJ  T i,ped ~ 400 eV Lower ion energy in successive filaments Net “flow” to inboard side!  ELM enters SOL mainly on the outboard side r - r sep ~ 80 mm at the probe

R. A. Pitts et al. 11 (12) IAEA, Chengdu Oct ELM-wall power loads T. Eich et al., subm. to Plasma Phys. Control. Fusion W. Fundamenski et al. PSI 2006 O. E. Garcia et al., Phys. Plasmas 13 (2006) Fraction of ELM energy in the divertor decreases with increasing ELM size –Up to 60% “missing” from divertor at high W ELM Dedicated plasma-wall gap expts. give far SOL power widths of W,ELM ~ 35 mm for W ELM /W ped ~ 12% –Agrees well with transient model prediction –Use this W,ELM as reference for empirical scaling: W,ELM  35(W ELM /0.12W ped ) 1/2 –Factor 1/2 consistent with recent ELM amplitude scaling due to interchange motion W ELM,wall  W ELM exp(-  / W,ELM ) f = 1 - W ELM,wall /W ELM

R. A. Pitts et al. 12 (12) IAEA, Chengdu Oct CONCLUSIONS Significant progress at JET in the measurement and modelling of ELM SOL transport –Strong asymmetry in divertor Type I ELM energy deposition favouring inner target –ELM filaments seen on several diagnostics –Sophisticated 1D PIC modelling now providing scalings of target heat flux with ELM energy –Available data in good agreement with new transient parallel energy loss model –Implies that filaments detached from pedestal plasma –ELM ions can reach limiters with high energies See poster by A. Loarte (IT/P1-14) for more applications of the transient model to ITER wall power loads