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Divertor plasma-surface interaction and induced radiation under large ELM impact

Divertor plasma-surface interaction and induced radiation under large ELM impact. 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.

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Divertor plasma-surface interaction and induced radiation under large ELM impact

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  1. Divertor plasma-surface interaction and induced radiationunder large ELMimpact 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

  2. 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

  3. 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

  4. 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

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

  6. Example: 3.0 MA, no fuelling (#70226) ne Da inner PNBI, PRF 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

  7. 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)

  8. 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)

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

  10. Location in parameter space: ne*(neo) A. Loarte et al., PPCF 45 (2003) 1549 ITER New data • 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

  11. Location in parameter space: Te,ped New dataTe,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

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

  13. Surface ion fluxes (example 70224) NB: Both LP and IR see the strike points lower than found with EFIT Da inner 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

  14. ELM radiation distribution: example Da inner Wdia (MJ) Prad (MW) Erad (MJ) 70225: Ein = 194 MJ, DWELM = ~0.9 MJETile3 = 23.3 MJ, Etile7 = 73.9 MJ

  15. Total radiated energy vs. DWELM Entire ELM (incl.compound phase) First ELM spike • 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

  16. 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

  17. 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 MJErad,ELM = 0.220 MJ DWELM = 0.846 MJErad,ELM = 0.579 MJ • Radiation “spills over” into outboard X-point region for large ELMs

  18. 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)

  19. Reserve slides

  20. 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

  21. 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).

  22. 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

  23. 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

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