ELM transport in the JET scrape-off layer

1. 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

2. 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

3. Brief diagnostic overview Wide angle main chamber IR Fast reciprocating probes: TTP, RFA Diagnostic Optimised Configuration (DOC) Divertor IR and tile thermocouples

4. Divertor target ELM energy asymmetry • ELM resolved target heat flux (IR) • Type I ELM energy deposition strongly favours INNERtarget for FWD-Bj • For REV-B, some evidence for more balanced deposition, • Consistent with similar analysis from AUG (WELM < 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) T. Eich et al., PSI 2006

5. ELM filaments – main chamber IR #67384, 26.225s #66560, 5.548s • Filamentary power deposition detected with new wide angle IR • 100 Hz frame-rate, but 300 ms snapshot  catches an occasional ELM • Seen by substracting pre-ELM and ELM frames P. Andrew, G. Arnoux Ip = 2 MA, Bj = 3TWELM ~ 150 kJ Two discharges with different contact point of first limiting flux surface Coord. Transformation (x,y)  (q,j)

6. ELM filaments in the far SOL TTP r - rsep ~ 80 mm at the probe • Clear filamentary structure in the particle flux, Te and radial velocity • WELM ~ 100 kJ • Te (pedestal) ~ 500 eV • TeELM(limiter) ~ 30 eV • vrELM ~ 500  1000 ms-1 • Electrons cool rapidly in the filament as it crosses the SOL • ELM duration at the probe ~10x higher than tELM seen on MHD activity etc. C. Silva et al., J. Nucl. Mater. 337-339 (2005) 722

7. Modelling the ELM transient Losses along B WALL Present understanding: MHD perturbs pedestal  radial expulsion of plasma  parallel loss along field lines to divertor until filament hits wall Two separate approaches being followed at JET to modelling the 1D SOL parallel transport. Particle-in-Cell (PIC) simulations CPU intensiveInject ELM energy kinetically via particle source at Tped, nped for time tELM and follow particles to targets including full target sheath dynamics Transient modelFluid and kinetic versions.Simpler to solve, captures many effects of PIC simulationsIntroduces 2D nature of filament propagation by relating loss times to radial velocities

8. PIC simulations of parallel losses • More realistic description of the ELMy JET SOL using improved PIC simulations (BIT1 code) • Scan in Tped, nped to vary WELM • Most of the heat flux arrives with ions on the acoustic timescale • BUT, only ~30% of ELM energy deposited when qtarget 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 g underestimates qtarget at high WELM Example: Tped = 1.5 keV, nped = 1.5x1019m-3WELM ~ 120 kJ, tELM = 200 ms D. Tskhakaya

9. Transient model of ELM parallel losses • Key elements of model • Temporal evolution of n, Te and Ti 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,Ti > Te in the filament at wall impact Example with Ti,ped = Te,ped = 400 eV nped = 1.5x1019m-3, H+ ions W. Fundamenski, Plasma Phys. Control. Fusion 48 (2006) 109

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

11. ELM-wall power loads • Fraction of ELM energy in the divertor decreases with increasing ELM size • Up to 60% “missing” from divertor at high WELM • Dedicated plasma-wall gap expts. give far SOL power widths of lW,ELM ~ 35 mm for WELM/Wped ~ 12% • Agrees well with transient model prediction • Use this lW,ELM as reference for empirical scaling:lW,ELM 35(WELM/0.12Wped)1/2 • Factor 1/2 consistent with recent ELM amplitude scaling due to interchange motion WELM,wallWELMexp(-D/lW,ELM)f = 1 - WELM,wall/WELM T. Eich et al., subm. to Plasma Phys. Control. FusionW. Fundamenski et al. PSI 2006O. E. Garcia et al., Phys. Plasmas 13 (2006) 082309

12. 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