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TH/4-1 · Mechanisms for ITB Formation and Control in Alcator C-Mod Identified through Gyrokinetic Simulations of TEM Turbulence
D.R. Ernst1), N. Basse, P.T. Bonoli, P.J. Catto, W. Dorland, C.L. Fiore, M. Greenwald, A.E. Hubbard, E.S. Marmar, M. Porkolab, J.E. Rice, K. Zeller, K. Zhurovich
1) Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America
2) Department of Physics University of Maryland College Park, Maryland, USA
Abstract: Mechanisms for transport barrier control are elucidated via nonlinear gyrokinetic turbulence simulations of trapped electron mode (TEM) turbulence in internal particle transport barriers in Alcator C-Mod, produced with off-axis RF heating. The simulations reveal new nonlinear physics of TEM turbulence and explain the observed transport barrier (density profile) control with on-axis RF heating. The critical density gradient for onset of TEM turbulent transport is nonlinearly up-shifted. Upon exceeding this nonlinear critical gradient, the turbulent particle diffusivity from GS2 gyrokinetic simulations matches the particle diffusivity from transport analysis, within experimental error bars. A stable equilibrium is established with the TEM turbulent diffusion balancing the Ware pinch in the ITB. This equilibrium is sensitive to temperature through gyroBohm scaling of the turbulent transport, which allows control of the density profile with on-axis RF heating. With no core particle source and ~ 1 mm resolution density diagnostics, the C-Mod experiments provide a nearly ideal test bed for particle transport studies.
Keywords: trapped electrons,
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