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Comparative Analysis of Small ELMs in Alcator C-MOD, MAST, and NSTX

Study on similarities and differences between small ELMs in different tokamaks, investigating physics governing transitions, threshold determinations, and structure comparisons using imaging diagnostics.

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Comparative Analysis of Small ELMs in Alcator C-MOD, MAST, and NSTX

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  1. Office of Science Supported by Status of Small ELM Comparison Experiments: Comparison of Small ELMs in Alcator C-MOD, MAST, and NSTX (ITPA PEP-16, XP721) R. Maingi1, A. Hubbard2, H. Meyer3, the Alcator C-MOD, MAST, and NSTX Teams 1 Oak Ridge National Laboratory 2 MIT 3 UKAEA Fusion NSTX Results Review Princeton NJ July 23-24, 2007 Page 1

  2. Goals of PEP-16 • Identify similarities and differences between the EDA-H-mode in C-MOD and the Type V ELMy H-mode in NSTX: • Does the same physics govern these regimes (and the HRS-mode on JFT-2M)? • Is the Type V ELMy H-mode specific to ST’s? • Establish dimensionless pedestal comparison with small ELMs between C-MOD, MAST, and NSTX • Match q95 and range in ped* • Perform a scan in ped (and ped*?) to find small ELM thresholds • Compare structure of small ELMs with imaging diagnostics • This XP is ITPA PEP-16 • Follows a successfully completed comparison (PEP-12) which established similarity between C-Mod EDA and JFT-2M HRS regime at higher R/a and identified q95, n* as key parameters. Page 2

  3. Small, irregular ELMs appear in C-MOD on top of EDA H-mode at higher edgeband lower n* Mossessian, PoP 2003 Page 3

  4. k=2.2 d=0.75 Small ELMs appear in wideband n* space in NSTX • Mixed between larger ELMs at highest b Maingi, JNM 2007 Maingi, PoP 2006 Page 4

  5. PEP-16 Status • C-MOD: Some data in ‘06; MP448 planned summer ‘07 • MAST: M6/059 (2 sessions in May 2007) • Mapping of existence space of small ELMs • 5 point power scan 1 MW < PNBI < 3 MW • More data on ELM structure and pedestal parameters • PhD project on inter ELM filaments (N. Ben-Ayed, EPS 2007) • Failed to access regime in L-SN with rsep < -8 mm. • NSTX: XP721 (3 sessions in 2007) •  scan in L-SN (rsep = -6 mm). • 3 point power scan 2 MW < PNBI < 4 MW. •  scan in DN (rsep = -2 mm). • 3 point scan 1 MW < PNBI < 3 MW. • Failed to access H-mode w/rsep < -8 mm (high X-point) •  scan in L-SN (rsep = -15 mm) at higher q95 Page 5

  6. Difficult to reproduce small ELM regime in C-MOD • 4.7 MW RF • bN~1.3 • Pe,ped~19 kPa • Good H-mode • No evident ELMs,why? Page 6

  7. MAST: Small ELMs vanish at high input power • Even with PNBI=1 MW small ELMs are present. • Mixed with threshold Type III ELMs • With PNBI = 3.2 MW small ELMs appear only during early H-mode. • Pressure pedestal seems to be narrow compared to ELM free phases (PEP-9). • Width ~ 1.5%-2.5% of N (half of the width seen in PEP-9) • Higher pressure gradient. Page 7

  8. MAST: No small ELMs at low*and high ped • Small ELMs exist over a wide range in * and ped. • At low power they are mixed with type-III ELMs Page 8

  9. MAST summary • Lower power threshold hard to determine • Small ELMs mixed in between type-III ELMs • Small ELMs disappear at high power and low collisionality * < 1.5 and high pedLFS • High n filaments rotate co-current, not all of them detach. • Larger ELMs: all filaments detach, medium n. • The characteristics seem different from type-V ELMs. • Failed to access H-mode with rsep < -8 mm with  < 1.7 with PNBI < 3.4 MW. Page 9

  10. #123644@0.3s #123645@0.3s NSTX: H-mode access at low  difficult in LSN • No H-mode access at <1.7 with with rsep = -15 mm. • Max PNBI= 6 MW. • No problems at  > 1.9 Page 10

  11. NSTX: Small ELMs observed close to DN drsep ~ -0.3 cm drsep ~ -0.6 cm • Observed between 2 MW < PNBI < 3 MW. • Probably not type-V ELMs, may be similar to MAST small ELMs in DN. Page 11

  12. NSTX (H-mode OK) NSTX (no H-mode) 124656 123644 @ 0.28s 124657 @ 0.22s 124657 124658 NSTX: Power scan with small ELMs in LSN with lower X-point and increased q95 Page 12

  13. NSTX summary • Small ELMs observed only close DN at  ~ 1.8 and high X-point • Small ELMs vanish at high power • These ELMs appear to drift vertically down, in ion grad-B drift direction and opposite to type-V ELMs • Failed to access H-mode with rsep < -8 mm with < 1.7 with PNBI < 6 MW and high X-point • Type V ELMs obtained with lower X-point, rsep ~ -15 mm Page 13

  14. Plans • In discussion: consider re-running XPs on NSTX and MAST with X-point shifted downward, perhaps forgoing the q95 match for a qcyl match • Re-establish small ELMs in C-MOD at higher bN • Report results of ELM structure comparison and edge stability analysis at IAEA 2008 Page 14

  15. Backup Page 15

  16. Background (PEP-12): H-mode operating spaces in q95 vsn* on C-Mod (R/a~3) and JFT-2M (R/a~5) proved very similar C-Mod Hubbard, Kamiya et al, H-mode workshop 2005, Russia • Steady EDA or HRS regime at high n* (> 1.5). • ELMy regime with low Da at low n* (<0.4). • Some overlap in ELM, H-mode types at intermediate n*:‘Mixed’ or EDA+ELM regime at higher b, ELMy at lower b on C-Mod. Page 16

  17. The structure of the small ELMs in this experiment is different from type-V ELMs • Small ELMs: • high mode number, multiple-filaments moving downwards in video • Type-V ELM: • single filament moving upwards in video. • Small ELMs are similar to the ones observed on MAST Type-V ELM Small ELM DN R. Maingi, Phys. Plasmas 13, 092510 (2006) R. Maqueda, PSI-17, to appear in J. Nucl. Mater. (2007) R. Maqueda Page 17

  18. MAST: Small ELMs appear as co-Ip rotating high-n mode Page 18

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