(TH/P3-13) Theoretical Studies on the Physics of Magnetic Islands

Abstract.  Three aspects of the physics associated with magnetic islands are investigated. (a) The threshold for the onset of magnetic island in tokamak is studied using the two fluids equations. For a sufficiently high beta plasma like that of ASDEX-Upgrade, the stability of a small island is found to be mainly determined by the electron diamagnetic drift frequency and the perpendicular heat diffusivity, and it can be driven unstable by the electron temperature gradient for a certain range of the diamagnetic drift frequency. With experimental data as the input, the spontaneous growing tearing mode observed on ASDEX-Upgrade is obtained from the numerical simulation. In the nonlinear stage the island decreases the local electron temperature gradient, which in turn leads to the mode saturation at a small amplitude. When including the bootstrap current perturbation, the mode can further develop into a large amplitude. The saturated island width is found to decrease for large diamagnetic drift frequency. (b) The heat diffusion across a local stochastic magnetic field is studied numerically. With the increase of the ratio between the parallel and the perpendicular heat diffusivity, the enhanced radial heat diffusivity due to the parallel transport along the field lines is found to be determined first by the additive effect of individual islands and then by the field ergodicity. A quasi-linear analytical theory are developed, which agrees with the numerical result. (c) Numerical modelling on the stabilization of NTMs by localized RF current drive are carried out. When the RF deposition width is much larger than the island width, the modulated RF current drive to deposit the RF current around the island's o-point is found to have a stronger stabilizing effect than a non-modulated one. A more effective way for stabilizing NTMs is found by using both the RF wave and a resonant helical field. The helical field decreases the island rotation frequency, leading to a longer island rotation period comparing with the slowing down time of the fast electrons and therefore a larger stabilizing effect by the RF current.

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