Dependence of Pedestal Structure on Ip and Bt A. Diallo, R. Maingi, S. Zweben, B.P. LeBlanc, B. Stratton, J. Menard, S. Gerhardt, J. Canick, A. McClean, S. Kubota, Y. Ren, et al. XP Group Review NSTX Supported by

Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review 2 Pedestal height and gradient increase until an ELM is triggered. The peeling ballooning stability sets the critical pressure gradient. The kinetic ballooning mode turbulence have been proposed to set the pedestal width. Combination of the two constraints yields the pedestal height. We plan on testing the applicability of model such as EPEDxx on NSTX. This model has had some success with higher R/a tokamaks (AUG, DIIID, C-MOD, JET): Specifically, we will test the peeling ballooning pressure gradient constraint. Initial analysis will attempt to differentiate pedestal width and height scalings based on  * and  pol. We propose to vary the current and the magnetic field to generate pedestal structure data over a wide range of B t and I p. As a by-product of these scans, we will assess the impact of turbulence on the pedestal using using fluctuation diagnostics from this and other XPs. Systematic experiments using high resolution edge diagnostics are required. In the case of NSTX, we conditionally average the MPTS profiles to enhance the pedestal resolution. Pedestal Structure and Stability are Tightly Coupled

3 Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review Analysis of existing database shots shows large dataset near 0.44 T but limited systematic discharges for higher Bt 3 Shot list provided by M. Bell and S. Gerhardt Time of maximum electron energy >0.5s Outergap ~ 10 cm Ip > 0.6MA P_NBI > 3.5MW, Deuterium gas fuelling, No lithium

5 Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review Example Conditionally-Averaged Profile Reconstructions (I) 5 Conditionally averaging enables better spatial resolution of the profile gradients.

5 Example Conditionally-Averaged Profile Reconstructions (II) Preliminary analysis show modest variations of the pedestal heigh and width: need dedicated scans to insure the same conditions and better statistics, and fluctuations measurements in the pedestal region.

Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review 6 This XP targets FY 2011 Joint Research Milestone on pedestal physics: ``Conduct experiments on major fusion facilities to improve the understanding of the physics mechanisms responsible for the structure of the pedestal and compare with the predictive models described in the companion theory milestone. [..] Initial measurements of the turbulence in the pedestal region will also be performed to improve understanding of the relationship between edge turbulent transport and pedestal structure.’’ Perform systematic scan of Ip and Bt (keeping the shaping constant) to maximize the range of achievable pedestal height in the Peeling Ballooning limit (ELMy regimes). Current pedestal height on NSTX ranges between kPa with some sparse high pedestals pressure obtained last run campaign. Goal is to Obtain Complete, Broad Measurements of the Pedestal height and Width Test the KBM hypothesis: the pedestal width scaling can be assessed in Maingi’s XP as a wider range can be achieved. Assess the impact of turbulence on pedestal structure using the V-band reflectometer, which will cover deep into the pedestal.

Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review 7 Addition of the V-Band Reflectometer to Obtain Radial Profile of the Density Fluctuations in the Pedestal Region A combination of both the Q- and V- band reflectometers will enable unprecedented spatial resolution in the pedestal and SOL especially in high performance discharges. Q-band 8 Channels V-band 8 Channels Pedestal region

Keep  between 0.7 and 0.8 & drsep ~ cm - 1cm EFC coils as early as possible Outer gap should remain at 10 cm for optimum MPTS resolutions Source A (60 ms) + B (80 ms) + C (200 ms) at nominal voltage Elmy H-mode is needed 1. Setup D 2 plasma with I p = 700 kA, 1. 3 MA, 1.2MA at B T = 5.0 kG [9 shots] 2. Repeat for 5.5 kG for Ip = 700 kA, 1.2MA, and 1.0 MA [9 shots] 3. Reduce Bt to 4.5 kG, for Ip = 700kA, 1.2MA, and 900kA [9 shots] 4. Fill in shots as needed in steps of 100kA for good pedestal profile analysis Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review 8 Run Plan: Fine Ip Scan for three Bt Settings (1 run day) Run Plan: Fine Ip Scan for three Bt Settings (1 run day)

Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review 9 Requirements & Analysis Requirements & Analysis Diagnostics & Machine: Thomson scattering, MSE, ERD, CHERS, reflectometers, High-k, BES,GPI Need LITERs for divertor conditioning Source A + B + voltage Might need the V-band reflectometer Array of analysis tools: EFIT, kinetic EFIT,Osborne profile analysis tools, ELITE, PEST, M3D

4 Study of Pedestal Structure (Diallo) March 12, 2010 NSTX Boundary Physics TSG Review Characteristics of Discharge Compatible with Parameter Space Scans targeted by this XP 4

11 Effect of Lithium on Electrons Pressure Profile I Pre-Li Elmy Post-Li

12 Range of Pedestal Collisionality