Simulation Studies Of Resistive Tearing Mode Instability In Tokamaks

The controlled nuclear fusion is the most prospect and could be finally to solve the energy dilemma and the environmental problem. The tokamak, as one of the most feasible magnetically confined fusion (MCF) devices at present, is widely studied. The tearing mode instability is an important dynamic process in both space and laboratory plasmas. In MCF devices such as the tokamak, the tearing mode instabilities can break magnetic flux surfaces, enhance unfavorable transport, degrade plasma confinement, and even lead to major disruption. The properties of the tearing modes and the methods to stabilize or control these modes are well worth studying.Based on full resistive MHD model, we develop a new initial-value 3D code (CLT) in the toroidal geometry to study the MHD instabilities in the toroidal devices. Using this code, we examine the effects of toroidal plasma rotation and the driven current on the resistive tearing mode (mainly, m/n=2/1) in tokamaks.For the influence of the toroidal equilibrium plasma rotation on the tearing modes, it is found that the toroidal rotation with or without shear can suppress the tearing instability, and the stabilizing effect increases with the rotation frequency. The Coriolis effect in the toroidal geometry is found to play a dominant role on the rotation induced stabilization. For low viscosity plasmas (τR/τV<1, where τR and τV represent resistive and viscous diffusion time, respectively), the rotation shear can reduce this stabilizing effect when the rotation is large. For high viscosity plasmas (τR/τV>>1), the effect of the rotation shear combined with the viscosity appears to be stabilizing.For the influence of the driven current on the tearing mode, we first consider a uniform driven current with a Gaussian distribution in the radial direction imposed around the unperturbed rational surface (q=2). it is found that a suitable driven current can locally modify the profiles of the current and safety factor, such that the tearing mode becomes linearly stable. The stabilizing effect increases with increase of the driven current Icd or decrease of its width δcd, unless an excessively large driven current reverses the magnetic shear near the rational surface and drives other instabilities such as double or triple tearing modes. The stabilizing effect can be negligible or becomes reversed if the maximum driven current density is not at the unperturbed rational surface.We also preliminarily study the influence of a helical driven current along the magnetic island. It can be seen that this kind of driven current can suppress the tearing mode more effectively than the former one if it matches the O-point of the magnetic island. However, when the island almost disappears, if the driven current unchanges its distribution (location and phase) with time (or island evolution), it becomes to drive the tearing mode to be unstable. The driven current varied along with the island evolution can suppress the tearing mode at a very low level for long time.