# Presentation: 837619

## 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

- Phys of ST

## 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, * as key parameters.

- Phys of ST

## Small, irregular ELMs appear in C-MOD on top of EDA H-mode at higher edge  and lower * 

*Mossessian, PoP 2003*

- Phys of ST

## Small ELMs appear in wide  and * space in NSTX

- Mixed between larger ELMs at highest 

*Maingi, JNM 2007*

*Maingi, PoP 2006*

- Phys of ST

## 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

- Phys of ST

## Difficult to reproduce small ELM regime in C-MOD

- 4.7 MW RF
-  N~1.3
- Pe,ped~19 kPa
- Good H-mode
- No evident ELMs,why?

- Phys of ST

## 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.

- Phys of ST

## MAST: No small ELMs at low *  and high ped 

**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

- Phys of ST

## 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.

- Phys of ST

## 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

- Phys of ST

## NSTX: Small ELMs observed close to DN

- Observed between 2 MW < PNBI < 3 MW.
- Probably not type-V ELMs, may be similar to MAST small ELMs in DN.

- rsep ~ -0.3 cm				 rsep ~ -0.6 cm

- Phys of ST

## NSTX: Power scan with small ELMs in LSN with lower X-point and increased q95

- Phys of ST

## 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

- Phys of ST

## 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 N
- Report results of ELM structure comparison and edge stability analysis at IAEA 2008

- Phys of ST

## Backup

- Phys of ST

## Background (PEP-12):  H-mode operating spaces in q95 vs * on C-Mod (R/a~3) and JFT-2M (R/a~5) proved very similar

- Steady EDA or HRS regime at high * (> 1.5).
- ELMy regime with low D at low * (<0.4).
- Some overlap in ELM, H-mode types at intermediate *:‘Mixed’ or EDA+ELM regime at higher , ELMy at lower on C-Mod.

- C-Mod

*Hubbard, Kamiya et al, H-mode workshop 2005, Russia*

- Phys of ST

## 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

- R. Maingi, Phys. Plasmas 13, 092510 (2006)
- R. Maqueda, PSI-17, to appear in J. Nucl. Mater. (2007)

- Small ELM DN

- Type-V ELM

- R. Maqueda

- Phys of ST

## MAST: Small ELMs appear as co-Ip rotating high-n mode

- Phys of ST