ELM transport in the JET scrape-off layer

R. A. Pitts et al. 1 (12) IAEA, Chengdu 16-21 Oct. 2006

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


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


Brief diagnostic overview

Divertor IR and tile thermocouples

Wide angle main chamber IR

Fast reciprocating probes: TTP, RFA

Diagnostic Optimised Configuration (DOC)


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 (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)


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

Ip = 2 MA, B = 3TWELM ~ 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, 26.225s


ELM filaments in the far SOLTTP 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– Te

ELM(limiter) ~ 30 eV– vr

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. 337-339 (2005) 722


Modelling the ELM transient

WA

LL

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 intensiveInject ELM energy kinetically via particle source at Tped, nped 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 simulationsIntroduces 2D nature of filament propagation by relating loss times to radial velocities


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 underestimates qtarget at high WELM

Example: Tped = 1.5 keV, nped = 1.5x1019m-3

WELM ~ 120 kJ, ELM = 200 s D. Tskhakaya


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

W. Fundamenski, Plasma Phys. Control. Fusion 48 (2006) 109

Example with Ti,ped = Te,ped = 400 eV nped = 1.5x1019m-3, H+ ions


Model consistent with RFA hot ion data

• Good agreement with transient model for i-side peak fluxes– Predicts Ti,RFA/Ti,ped = 0.30.5– Te,RFA/Te,ped = 0.130.25– ne,RFA/ne,ped = 0.30.4

• Consistent with low Te on TTP probeR. A. Pitts et al., Nucl. Fusion 46 (2006) 82W. Fundamenski, PPCF 48 (2006) 109

RFA

Current of ions with energy > 400 eV

• Filaments on plasma and hot ion fluxes– WELM ~ 50 kJTi,ped ~ 400 eV

• Lower ion energy in successive filaments

• Net “flow” to inboard side!

ELM enters SOL mainly on the outboard side

r - rsep ~ 80 mm at the probe


ELM-wall power loads

T. Eich et al., subm. to Plasma Phys. Control. FusionW. Fundamenski et al. PSI 2006

O. E. Garcia et al., Phys. Plasmas 13 (2006) 082309

• 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 W,ELM ~ 35 mm for WELM/Wped ~ 12% – Agrees well with transient

model prediction– Use this W,ELM as reference for

empirical scaling: W,ELM 35(WELM/0.12Wped)1/2

– Factor 1/2 consistent with recent ELM amplitude scaling due to interchange motion

WELM,wallWELMexp(-/W,ELM)

f = 1 - WELM,wall/WELM


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

Documents

ELM transport in the JET scrape-off layer