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Self-organized stationary states of inductively driven tokamaks

Author: Stephen C Jardin
Requested Type: Consider for Invited
Submitted: 2015-01-19 09:43:25

Co-authors: N. Ferraro, I. Krebs, J. Chen

Contact Info:
Princeton Plasma Physics Laboratory
P.O. Box 451
Princeton, NJ   08543
United States

Abstract Text:
We report on a mechanism for preventing the current and temperature profiles from peaking in a stationary state tokamak. For certain parameters, regardless of the initial state, the plasma profiles will evolve into a self-organized state with the safety factor q slightly above 1 and constant in a central volume. This large shear free region is unstable to interchange modes for any pressure gradient, and the instability drives a strong (1,1) helical flow. This flow has the property that V x B is nearly the gradient of a potential, so it does not strongly affect the magnetic field evolution in the stationary state. The driven flow has two effects. (1) It appears in the temperature evolution equation and dominates over the thermal conductivity in the center of the discharge, keeping the central temperature (and resistivity) profiles relatively flat; (2) It acts to broaden the current profile, countering the tendency of the current to peak. These two effects combine to preserve the self-organized state with q slightly above 1 and nearly constant in the central volume. This mechanism was discovered with the M3D-C1 toroidal 3D MHD code, and could possibly explain the mechanism at play in non-sawtoothing discharges with q0 just above 1 such as hybrid modes in DIII-D and ASDEX-U and long-lived modes in NSTX and MAST.

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