Divertor plasma-surface interaction and induced radiation under large ELM impact Appreciate to have the opportunity In-vessel view into JET … no more time for promoting fusion… R. A. Pitts, A. Huber, A. Loarte, M. Stamp, S. Brezinsek, S. Jachmich, J. Marki, M. Maslov, E. de la Luna, H. Leggate, G. F. Matthews E. Solano and JET EFDA Contributors 7 May 2007

Motivation JET experiments in 2003 at 3.0 MA (2 discharges only) identified an apparent threshold for the evolution of plasma radiation above a certain ELM energy drop (WELM) for large ELMs Suspected link to divertor target surface ablation (bulk or layers?) Experiment repeated in recent campaign under controlled conditions and with improved diagnostics A. Loarte et al., Phys. Scripta T128 (2007) 222

Configuration (I) Seek highest possible divertor tile surface temperature Work at 3.0 MA and max. power, low fuelling  largest ELMs possible, low power folding width Strike points on Tiles 3 and 7 (CFC fibre plain in the toroidal direction  poloidal spreading minimised) Improved diagnostics D1Z_VSFE_LT: Ip = 3.0 MA, Bj = 3.0 T, q95 = 3.2, outer-wall gap: ~ 8 cm, du ~ 0.22, dl ~ 0.28, k = 1.73

Extract from session pulse list Experiment performed on March 8 2007 Did a little better than previous 2003 discharges in terms of injected energy and power loading Shot EIN (MJ) ENBI,ERF (MJ) Etile3,7 (MJ) Gas 1022 el/s Ttile3,7 @t=0 (ºC) Ttile3,7 peak avge Comments 70220 195 174, -- 18.4, 66.8 2.1 140, 150 217, 430 First @3.0MA, no KL7 70221 183 158, 7.1 18.1, 63.7 1.5 153, 182 230, 437 ½ gas, lost PINIs, KL7 back (wide angle) 70222 191 163, 10.2 19.5, 66.3 1.2 193, 164 244, 492 Less gas (but not much less), KL7 wide angle 70223 178 149, 11.0 19.7, 63.7 0.9 175, 211 257, 486 Still less gas, lost PINIs, KL7 (wide angle) 70224 159 128, 12.2 19.0, 58.8 179, 212 261, 430 Less gas, lost PINIs, KL7 (wide angle). Outer strike up ~2 cm 70225 194 164, 12.5 23.3, 73.9 158, 167 264, 387 No gas, good, got full NBI. KL7 subarray, upper dump, lower div. 70226 163, 12.8 24.6, 70.1 174, 200 286, 408 Repeat, good – full beams. KL7 subarray as in 70226 70228 172 163, 5.0 23.4, 66.8 171, 191 290, 380 Repeat, lost one PINI, KL7 subarray to outer limiter 62220 166 152, -- 19.1, 52.7 0.55 130, 145 203, 341 Ref. DOC-L, March 2003

Example: 3.0 MA, fuelled (#70220) Da inner PNBI, PRF ne Wdia Te,ped Ggas 174 MJ injected, 88.2 MJ radiated, Erad/Etot = 0.51, Tile 7: 67 MJ, Tile 3: 18.4 MJ

Example: 3.0 MA, no fuelling (#70226) Da inner PNBI, PRF ne Wdia Te,ped Ggas 177 MJ injected, 82.4 MJ radiated, Erad/Etot = 0.47, Tile 7: 70.9 MJ, Tile 3: 24.6 MJ

Large ELMs are (very) often compound #70226 Da inner Da inner Wdia (MJ) Wdia (MJ) Prad (MW) Prad (MW) Erad (MJ) Zeff (brems) Zeff (brems)

Extracting ELM induced radiation (I) Da inner #70222 Wdia (J) Da inner Initial drop in Wdia an artefact of measurement – take value ~4 ms after fast drop Erad (J)

Extracting ELM induced radiation (II) Da inner #70226 Da inner Wdia (J) Erad (J)

Location in parameter space: ne*(neo) A. Loarte et al., PPCF 45 (2003) 1549 New data ITER Lowest fuelling cases working at ITER relevant pedestal n* Consistent with earlier findings – increased database population A. Loarte et al., Phys Plas 11 (2004) 2668

Location in parameter space: Te,ped New data Te,ped from ECE A. Loarte et al., Phys. Scripta T128 (2007) 222 Da inner Large ELMs asscoiated with large drop in Teped New 3.0 MA data populates the scaling beyond DTeELM/Te,ped > 0.4

Max. divertor target temperatures 70220, high fuelling, Ein = 195 MJ 70228, no fuelling, Ein = 172 MJ Da inner Da inner TINNER, ºC TOUTER, ºC TINNER, ºC TOUTER, ºC NOTE: Preliminary IR data – calibration issues still to be resolved Clear effect of inner target surface layers Bulk ablation temperatures not achieved in any of the discharges Large compound ELMs produce high inner surface (layer) temp.

Surface ion fluxes (example 70224) Da inner NB: Both LP and IR see the strike points lower than found with EFIT 14 jsat INNER (106 Am-2) 29 jsat OUTER (106 Am-2) 8 jsat LIM (105 Am-2) Fast Langmuir probe acquisition for a few ELMs only (250 kHz) Extremely rich structure after first “peak”of compound ELM – not seen on volume averaged Da Strong far-SOL limiter interaction Activity dies out faster at outer than inner (in this case) jsat LIM (105 Am-2) x10

ELM radiation distribution: example Da inner Wdia (MJ) Prad (MW) Erad (MJ) 70225: Ein = 194 MJ, DWELM = ~0.9 MJ ETile3 = 23.3 MJ, Etile7 = 73.9 MJ

Total radiated energy vs. DWELM First ELM spike Entire ELM (incl. compound phase) Clean break in radiation above given DWELM not (clearly) seen in new data Approx. 50% of the ELM energy radiated at lower DWELM due to “first” spike Similar trend when including whole compound phase

In-out ELM radiation asymmetry First ELM spike, unfuelled shots only In-out divertor volume, in-out radiation asymmetry during the ELM linearly dependent on DWELM up to ~0.6 MJ Erad,ELM(IN)/Erad,ELM(OUT) as high as 6 Break in asymmetry for DWELM > 0.6 MJ – not understood ELM exacerbates the inter-ELM in-out radiation asymmetry Higher inboard ELM radiation consistent with higher ELM power there

Radiation distribn.: large vs. small DWELM First ELM spike only – reconstruction for 4 ms averaging period #70225: 19.42 s #70225: 16.46 s DWELM = 0.447 MJ Erad,ELM = 0.220 MJ DWELM = 0.846 MJ Erad,ELM = 0.579 MJ Radiation “spills over” into outboard X-point region for large ELMs

Conclusions Operation at high Ip and low fuelling has produced a few large ELMs (DWELM <0.9 MJ) at ITER n*ped Up to ~195 MJ delivered to plasma, in-out divertor energy deposition ratio 2.5-3.0:1 Surface (layer) temperatures do not exceed ~ 2000ºC at inner target. Max. outer target temperature ~ 800 ºC (no layers) In the range 0.1 MJ < DWELM < 0.9 MJ, ~50% of DWELM is radiated, but there is considerable scatter ELM induced radiation always higher at inner than outer divertor: approx. linear increase in asymmetry up to DWELM ~ 0.6 MJ then decrease for higher DWELM Higher inner divertor induced radiation consistent (but not only due to) higher ELM energy deposition at inboard side (T. Eich et al., PSI 2006)

Reserve slides

Fast divertor visible spectroscopy KS3 div. view KE9 inner div. view Acquisition at 250 kHz BeII and CIII react at same time ~300 ms after fall in Wdia Often secondary peaks in CIII at outer divertor hardly seen at inner

70228 (new experiment) 62220 (DOC-L: old experiment) Configurations (II) 70228 (new experiment) 62220 (DOC-L: old experiment) Operational restrictions prevented strike point positions as high as had been achieved in DOC-L Less favourable for high power loading (tile geometry).

Divertor target ELM energy asymmetry ELM resolved target heat flux (IR) Type I ELM energy deposition strongly favours INNER target 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

3.0 MA, DOC-L ref. (#62220) PNBI (106 W) ne (1019 m-3) Wdia (106 J) Da out, in Wtot, Prad (107 J) Erad, Pin (108 J) Ggas (1022 els-1) 152 MJ injected, 74.0 MJ radiated, Erad/Etot = 0.49, Tile 7: 52.7 MJ, Tile 3: 19.1 MJ