Introduction to ELM power exhaust: Overview of experimental observations

1. Introduction to ELM power exhaust:Overview of experimental observations W.Fundamenski Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK This work was funded jointly by the UK Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The view and opinions expressed herein do not necessarily reflect those of the European Commission.

2. Growth-Transport-Exhaust picture of the ELM Growth stage: • Linear instability (eg. Peeling-Ballooning ideal MHD) forms ~ 10-20 flute-like ripples in pedestal quantities Transport stage: • Instability saturates due to transport (drift ordered) effects, generating ~10-20 filaments Exhaust stage: • Filaments move outward (eg. due to interchange/curvature drive) • Parallel losses to divertor targets produce radial decay of n, T, p W.Fundamenski et al., Plasma Phys. Control..Fusion, 48 (2006) 109

3. Schematic of ELM filament evolution Start of parallel losses • Parallel losses to divertor targets begin in the ETB region: • between rped (with reconnection at X-point) and rsep (without reconnection) • Electrons faster than ions • Acoustic response of plasma • Dispersive radial motion, development of leading front & trailing wake W.Fundamenski et al., Plasma Phys. Control..Fusion, 48 (2006) 109

4. Observable (i) : Ballooning character • Particles and energy ejected on the low field (outboard) side of the torus • Most direct evidence from double null ELMy H-mode discharges, where virtually all particles and energy seen on outer target Petrie, Nuclear Fusion (2002)

5. Observable (ii) : Many filaments, sub-structure • Exhaust stage of the ELM gives rise to many filaments which remain detectable long after the end of magnetic activity (2 ms vs. 0.2 ms on JET) • ELM filaments contain fine structure as observed with fast scanning LPs • ELM filaments appear to follow the pre-ELM magnetic field (recent evidence from MAST) Kirk, PPCF (2005) Eich, PPCF (2005) Silva, JNM (2004)

6. Observable (iii) : Vr ~ 1 km/s, Vr/csped ~ 0.01-0.1 • ELM filaments travel radially with SOL-average velocities of order 1 km/s, which represents ~1-10 % of pedestal plasma sound speed (~1% on JET with ~1-2 keV pedestal temperature) • Hence, ELM filament dynamics is drift-ordered • Similar velocities are measured for turbulent eddies (blobs) in L-mode SOL turbulence on most machines • Divertor target power profile lqELM / lqinter-ELM ~ 1-2 (Herrmann 2003) consistent with (vr/cs)ELM ~ (vr/cs)inter-ELM Kirk, PPCF (2005) Silva, JNM (2004)

7. Observable (iv) : prompt hot electron pulse • Onset of magnetic activity is quickly followed by observation of hot electrons (soft X-rays) at divertor targets • Delay between magnetic spike and electron pulse comparable to (but larger than) the electron thermal transit time

8. Observable (v) : acoustic ion pulse Loarte, POP (2004) • Bulk (ion) pulse delayed by ion transit time • Arrives faster at outer target • In-out delay also consistent with sonic time

9. Observable (vi) : (n,T)ELM ~ (n,T)L-mode • Radial (n,Te) profiles of ELM filaments roughly agree with L-mode profiles • This suggests that ELM filaments and turbulent blobs driven by similar processes • Since L-mode SOL turbulence is interchange driven and electro-static (drift-ordered), this suggests that ELM filaments motion is also • Type-III ELMs PDFs agree with L-mode PDFs Boedo, Rudakov (2004)

10. Observable (vii) : cold electrons, hot ions • Retarding field analyser measurements of ions energies in the far-SOL of JET • ELM filament Te ~ 25 eV vs. Tped ~ 400 eV • ELM filament Ti / Te ~ 3 W.Fundamenski and R.A.Pitts, PPCF, 48 (2006) 109

11. Observable (viii) : (Wlim / Wdiv)ELM Loarte, POP (2004) • Total energy expelled during the ELM found mostly in the divertor • The missing energy, identified with energy deposited on limiter tiles, increases with ELM size and wall gap

12. Open questions of relevance to ITER Growth stage: • ELM size (Dn, DT, DW) for given plasma conditions (Bt, q95, nped, Tped) Transport stage: • Time scales (duration, deposition) Exhaust stage: • Power profile on divertor target • In/out asymmetry between targets • For a given sized ELM (Dn, DT, DW), given plasma conditions (Bt, q95, nped, Tped), and given wall gap, what fraction of ELM energy deposited on the wall?, i.e. what is the radial profile of ELM energy density? W.Fundamenski et al., Plasma Phys. Control..Fusion, 48 (2006) 109

13. Discussion: ELM filament propagation 1) Are the results from various machines consistent?, eg. in terms of the observables mentioned in introduction • radial velocity, radial Mach number • convective vs. conductive ELMs • power deposition on targets and wall, etc. 2) What picture of ELM (filament) dynamics emerges? • MHD vs. drift ordered ? • how to include into 2-D transport codes ? • are there existing models sufficient? How to progress? • can we predict ELM-wall interaction on ITER? (Loarte, IAEA 2006) also IEA workshop on Edge Transport in Fusion Plasmas, Krakow, 09/2006 all talks may be found inwww.etfp.ipplm.pl

14. Discussion: ELM size control by RMP 1) How does RMP (edge ergodisation) affect ELM size & SOL transport? • ELM size/amplitude • ELM transport • Inter-ELM transport • Strike point shape, conditions (n,T,q) and detachment 2) External vs internal coils? Can we reconcile the DIII-D and JET results? • JET results promising but need to be further validated • Are the existing EFCC coils on ITER enough for ELM control? 3) Can we explain the change of Tped, nped during RMP ? • Decrease of nped : magnetic pump out effect • Increase of Tped : reduced neo-classical cond., increased R-R transport • Need to compensate reduced nped by increased fuelling. Would RMP control of ELMs be effective at higher density?

15. Acceptable (small) ELMs for ITER Can we suggest “acceptable ELM” criteria for ITER? Suggestion: 1) ELM energy, DW < 2-3 MJ; with W ~ 350 MJ this gives DW/W ~ 0.6 – 1 % • This value suggested by heat load testing of ITER components (Linke 2006) • Roughly the same limit for W as for C, although different mechanisms 2) Maximise plasma current for given toroidal field, hence q95 ~ 3 (or less). • This follows from the strong scaling of confinement with current, tE ~ q95-3 3) Outer target must remain partially detached between ELMs, Te < 2 eV • Eg. If magnetic pump out reduced edge ne, this must be increased by fuelling 4) Others ?!