# Presentation: 823976

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

- R. Maingi, T.Osborne@, A. Kirk*
- Oak Ridge National Laboratory
- @ General Atomics
- * UKAEA Fusion, Culham
- ORNL Fusion Energy Division Informal Seminar
- Oak Ridge, TN
- April 10-11, 2005

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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, PPCF 2004)
- MAST found poor correlation with empirical scaling of Te width in pedestal database (Kirk, PPCF 2004)
-  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

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

- P. Snyder,
- PPCF 2004

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## Goal and Execution of Experiments

- Goal:
- Assess the effect of aspect ratio and wall proximity on pedestal height, widths and gradients in ELMy H-mode
- Result in DIII-D:
**Outboard pedestal top ************e************ ~ 0.011, and local ********tot**** ~ 4-5% achieved at apparent edge ********limit**
- Two shapes achieved, with squareness spanning between MAST and NSTX
-  no obvious effect of squareness on pedestal here
- Pedestal appears rather wide in N space for DIII-D
- Will get dedicated data from MAST and NSTX (Summer ‘05)

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## DIII-D shape flexibility used to match both high (NSTX) and low (MAST) squareness in DN shape

**DIII-D shape flexibility used to match both high (NSTX) and low (MAST) squareness in DN shape**

**NSTX #111378 300ms**
**(0.96 scale)**
**MAST #11204 200ms**
**DIII-D #121503 1200ms**
**(shifted inward by 0.8m)**

- NSTX vac. ves.

**NSTX #111378 300ms**
**(0.96 scale)**
**MAST #11204 200ms**
**DIII-D #121516 1200ms**
**(shifted inward by 0.8m)**** **

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## NSTX shape in DIII-D at Bt0=0.5 T achieved sufficient flat-top for profile analysis

- 

- Ip [MA]

- Prad [MW]

- PNBI [MW]

- WMHD [MJ]

- D [m-2 s-1]

- ne [m-3]

- N

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## Target Dimensionless Profiles Achieved in DIII-D comparable with MAST (w/Ti=Te), NSTX Sample Profiles

**Target Dimensionless Profiles Achieved in DIII-D comparable with MAST (w/T****i****=T****e****), NSTX Sample Profiles**

- DIII-D
**MAST**
**NSTX**

- #121504@1175
**A. Kirk, PPCF 2004**
**#113460@460**

- ne [1019 m-3]

- Te [keV]

- Pe [keV]

- tot [%]

- 

- i

******N **

******N **

******N **

******N **

- Ti [keV]

- e

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## Pedestal Parameters Independent of Squareness

**Pedestal Parameters Independent of Squareness**

- For these collisionalities, squareness has little effect on Pedestal

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## Plasma Profiles Comparable in DIII-D Shapes with Different Squareness

**Plasma Profiles Comparable in DIII-D Shapes with Different Squareness**

- #121497@1150
**#121516@1150**

- ne [1019 m-3]

- Te [keV]

- Pe [keV]

- tot [%]

- 

- i

******N **

******N **

******N **

******N **

- Ti [keV]

- e

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## Ongoing Tasks and Analysis

- Obtain dedicated data from MAST and NSTX (Summer ‘05)
- Investigate impact of high radiated power and carbon fraction on DIII-D data
- Compare pedestal widths and gradients
- Separate Bt effects from a/R effects from DIII-D data
- Compare pedestal top parameters with analytic models
- Perform kinetic equilibrium analysis and stability calculations

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## Backup

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