Numerical Simulation Of Tearing Mode Stabilization By Current Modulation

Tearing mode, which is one of the most dangerous magnetohydrodynamic in-stabilities in tokamak, can change the topology of the magnetic field, destroy the equilibrium of plasma, and in extreme cases the tearing mode can terminate the plasma discharge and lead to grave consequences. Therefore, to ensure the normal operation of a tokamak,the tearing mode must be stabilized effectively. Various methods have been used to stabilize the tearing mode, but these methods have some technical difficulties. An alternative way to control the tearing mode is to make the resonant surface oscillate with time by modulating the plasma current. With a modulation current which oscillates with time added on the equilibrium current, the radial position of the resonant surface can be oscillated. Tearing modes can be stabilized, if the frequency of the oscillation is higher than the clas-sical growth rate of the tearing mode, and the amplitude of the resonant surface oscillation is large enough. The power needed for the lower hybrid current drive to oscillate the resonant surface and suppress the tearing mode is comparable to the conventional method of tearing mode suppression by the electron cyclotron wave current drive. As the parameters of tokamak improved, the neoclassical effects play important roles on the stabilization of tearing modes by current modulation. Neoclassical effects enhance the resistivity and reduce the resistive diffusion time of the modulation current. Therefore, the oscillating current can penetrate deeper into the plasma. With an oscillating loop voltage, the plasma oscillates radially in the Ware-pinch velocity. These neoclassical effects improve the efficiency of tearing mode stabilization by the current modulation.