# Presentation: 473385

## Dependence of the H-mode Pedestal Structure on Aspect Ratio: DIII-D/NSTX/MAST Joint Experiment

- R. Maingi, T.Osborne@, P. Snyder@, A. Kirk* , G. Counsell*
- Oak Ridge National Laboratory
- @ General Atomics
- * UKAEA, Culham
- NSTX Research Forum
- Princeton NJ
- Sept. 22-24, 2004

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## Investigation of R/a dependence of pedestal could aid in understanding of multi-machine comparisons

- Previous studies from JT-60U(Hatae) indicate aspect ratio dependence of temperature width
- Recent JET/JT-60U similarity expt. yielded different pedestal/ELM regimes (Saibene, H-mode wkshp, 2003)
- MAST found poor correlation with empirical scaling of Te width in pedestal database (Kirk, ITPA ped. SD 9/03)
-  What is the aspect ratio dependence of pedestal width?
- NSTX and MAST have many of the same shape parameter windows as DIII-D (minor radius, 
- Major radius of both machines ~ 1/2 of DIII-D
-  ideal aspect ratio scan candidates
*Both DIII-D and MAST have allocated run time for this expt.*

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## Slide 3

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## Goals and Execution of Experiments

- Goals:
- Assess the effect of aspect ratio and wall proximity on pedestal height, widths and gradients in ELMy H-mode
- Assess the effect of aspect ratio and wall proximity on ELM radial extent, amplitude, and frequency
- Execution (1-2 days):
**We will test the dependence of pedestal parameters on R/a at same ****************and************ ****, matched at the top of the outboard midplane pedestal for two values of**********
- We will measure the radial extent of ELM flux in far SOL with midplane probe (no plunge needed)
- ELITE calculations used to guide toward best shape in which a difference in max. p’ could be expected

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## Edge Stability Calculations Show Aspect Ratio Dependence of Maximum P (P’) in Certain Shapes

- P. Snyder,
- ELITE calculation,
- ITPA pedestal
- Group Meeting
- San Diego, 10/03

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## Common souble-null shape achievable on NSTX, MAST and DIII-D

**Common souble-null shape achievable on NSTX, MAST and DIII-D**

**NSTX #111378 277msec**
**MAST #8851 200msec**
**DIII-D #109856 2100msec**

- NSTX vac. ves.

**NSTX #111378 277msec**
**MAST #8851 200msec**
**DIII-D #109856 2100msec**
**(shifted inward by 0.76m) **

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## Backup

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## Shape development of NSTX shape in DIII-D commenced during Heidbrink’s Alfven Mode Similarity Expt.

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## NSTX and DIII-D global profiles are similar early after the L-H transition and then evolve differently

- ne profiles hollow and Te profiles overlay after L-H
- NSTX ne profile flattens out with time, but DIII-D profile peaks
- Turn cryo off?
- Te widths match late in time but NSTX is narrower early in time(?)

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## Dimensionless parameters can probably be matched at top of outer pedestal in dedicated experiments

|  | NSTX DN
108812@0.243 | DIII-D DN
109856@1000 |
| --- | --- | --- |
| Ip [MA] | 0.82 | 0.59 |
| Bt0 , Btotped [T] | -0.45, 0.37 | -0.62, 0.51 |
|  | 1.85, 0.74 | 1.8, 0.41 |
| R, aa/r | 0.87, 0.65, 0.74 | 1.7, 0.60, 0.35 |
| q95 | 7.7 | 3.0 |
| PNBI [MW] | 5.4 | 1.5 |
| ne [1019 m-3] | 3.0 | 3 |
| neped [1019 m-3] | 3.0 | 3 |
| Teped [keV] | 0.29 | 0.26 |
| Tiped [keV] | ? | ? |
| ped= 40nT/Btot2 | 0.053 | .024 |
| *,eped, *,iped | 0.44, 0.31 | 1.35, 0.95 |
| *ped | 0.01 | 0.0075 |
| t  | 0.12 | .033 |
| p | 0.8 | 0.65 |
| WMHD [MJ] | 0.159 | 0.134 |
| E [sec], H89P | .038, 1.8 | .094, 1.6 |

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## New Data Needed from Each Machine

- DIII-D:
- Drop Bt ~ 0.5 T (e.g. 109861)
- Use NBI source with least fast ion loss - need 1-2 src
- NSTX:
- Use medium  DN shape ~ 0.4-0.5
- Drop NBI power to bring down ped
- Adjust fueling to get matched *ped
- MAST:
- Develop nearly ohmic only startup with DN and large minor radius (need to adjust Thomson channels?)

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## Comparison of Discharge Characteristics

**B****t**** [T]**

******t**

- NSTX: #108812

****

- D3D: #109856

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