ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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.

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Growth-Transport-Exhaust picture of the ELM Growth stage: Linear instability (eg. Peeling- Ballooning ideal MHD) forms ~ 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

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Schematic of ELM filament evolution W.Fundamenski et al., Plasma Phys. Control..Fusion, 48 (2006) 109 Start of parallel losses Parallel losses to divertor targets begin in the ETB region:  between r ped (with reconnection at X-point) and r sep (without reconnection) Electrons faster than ions Acoustic response of plasma Dispersive radial motion, development of leading front & trailing wake

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Observable (iii) : V r ~ 1 km/s, V r /c s ped ~ 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 q ELM / q inter-ELM ~ 1-2 (Herrmann 2003) consistent with (v r /c s ) ELM ~ (v r /c s ) inter-ELM Kirk, PPCF (2005) Silva, JNM (2004)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Observable (v) : acoustic ion pulse Bulk (ion) pulse delayed by ion transit time Arrives faster at outer target In-out delay also consistent with sonic time Loarte, POP (2004)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Observable (vi) : (n,T) ELM ~ (n,T) L-mode Radial (n,T e ) 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)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Observable (vii) : cold electrons, hot ions Retarding field analyser measurements of ions energies in the far-SOL of JET  ELM filament T e ~ 25 eV vs. T ped ~ 400 eV  ELM filament T i / T e ~ 3 W.Fundamenski and R.A.Pitts, PPCF, 48 (2006) 109

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Observable (viii) : (W lim / W div ) ELM 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 Loarte, POP (2004)

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Open questions of relevance to ITER Growth stage: ELM size (  n,  T,  W) for given plasma conditions (B t, q 95, n ped, T ped ) Transport stage: Time scales (duration, deposition) Exhaust stage: Power profile on divertor target In/out asymmetry between targets For a given sized ELM (  n,  T,  W), given plasma conditions (B t, q 95, n ped, T ped ), 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

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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 in

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E 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 T ped, n ped during RMP ? Decrease of n ped : magnetic pump out effect Increase of T ped : reduced neo-classical cond., increased R-R transport Need to compensate reduced n ped by increased fuelling. Would RMP control of ELMs be effective at higher density?

ITPA DSOL meeting, Toronto W. Fundamenski9/11/2006 TF-E Acceptable ( small) ELMs for ITER Can we suggest “acceptable ELM” criteria for ITER? Suggestion: 1) ELM energy,  W < 2-3 MJ; with W ~ 350 MJ this gives  W/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 q 95 ~ 3 (or less).  This follows from the strong scaling of confinement with current,  E ~ q ) Outer target must remain partially detached between ELMs, T e < 2 eV  Eg. If magnetic pump out reduced edge n e, this must be increased by fuelling 4) Others ?!