(IT/P1-12) Dependence of the H-mode Pedestal Structure on Aspect Ratio

R. Maingi1), T. Osborne2), P. Snyder2), A. Kirk3), S. Sarelma3), R. Scannell3), H. Meyer3)
1) Oak Ridge National Laboratory, Oak Ridge, TN USA
2) General Atomics, San Diego, CA USA
3) UKAEA Fusion, Culham, United Kingdom

Abstract.  We report on a set of experiments between NSTX, MAST, and DIII-D to determine the aspect ratio dependence of the pedestal. The dimensionless parameters of electron collisionality ( ν*e) and normalized ion gyroradius ( ρ*i) were matched at the top of the outboard pedestal, and the widths and gradients were assessed. These experiments were motivated in part by the fact that many of the machines in the international database used for scaling the pedestal parameters for the International Thermonuclear Experimental Reactor (ITER) have different aspect ratios, and the experiments were conducted through the ITPA pedestal physics working group. A significant dependence of the pedestal on aspect ratio would not be surprising, because variation of the aspect ratio primarily affects the edge magnetic topology. A common double-null shape was developed for these experiments with triangularity 0.5 and elongation 2. The toroidal fields and plasma currents used were 0.45-0.55 T and 0.6-0.8 MA in all three machines. The dimensionless parameters ν*e∼1 and ρ*i∼0.01 were matched at the top of the outboard pedestal by variation of the target density and neutral beam heating power while maintaining ELMy H-mode. The pedestal widths and gradients were analyzed in each machine using a `standard’ modified hyperbolic tangent function; the ranges of pedestal top parameters obtained in this manner were neped: 3-5  ×1019 m-3, Teped: 100-250 eV, and Peped: 0.4-1.5 kPa. The pedestal ne, Te and Pe widths measured in DIII-D for these discharges were between 6-8% in ΨN (normalized poloidal flux), i.e. almost twice as large as the normal range of widths at the normal Bt = 2.1T. In comparison, the pedestal widths in MAST were between 1.5-4% in ΨN, and final assessment of the widths in NSTX is still in progress. Edge stability analysis has commenced and will be presented at the conference for all three machines.
* Sponsored in part by U.S. Dept. of Energy Contracts DE-AC05-00OR22725, DE-FC02-04ER54698, and DE-AC02-76CH03073, and the U.K. Engineering and Physical Sciences Research Council.

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