The reduction of neoclassical tearing modes by ECCD
is demonstrated experimentally. It is shown that with an averaged
ECCD power of only 4-8% of the total heating power injected into the discharge, the island width can be reduced by
40%, provided that the centre of deposition is very close to the resonant surface.
The reduction in mode amplitude results in a partial recovery of the loss of stored energy induced by the mode.
This experimental result is well reproduced by modelling calculations.
The theory of tearing mode stabilization in toroidal plasmas by RF driven currents that are modulated in phase with the island rotation is investigated. A timescale analysis of the phenomena involved indicates that transient effects, such as finite time response of the driven currents, island rotation during the power pulses and the inductive response of the plasma, are intrinsically important. A dynamical model of such effects is developed, based on a 3-D Fokker-Planck code coupled to both the electric field diffusion equation and the island evolution equation. Extensive applications to both ECCD and LHCD in ITER are presented.
Numerical modeling of the stabilization of neoclassical tearing modes by localized radio frequency (rf) current drive is presented. The bootstrap current is self-consistently calculated from the pressure evolution equation, the rf current source is obtained from the ray-tracing code and the Fokker–Planck code, and the change of the driven current profile due to the change of the magnetic island width has also been taken into account by modeling the two-dimensional transport of the fast electrons induced by the rf wave. It is found that both parallel and the perpendicular transport of the fast electrons play important roles in the stabilization, and modulated and nonmodulated rf current drive have approximately the same stabilizing effect. The neoclassical tearing modes is shown to be stabilized by a continuous rf current drive. The simulation results essentially agree with experimental observations.
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