Studies On Influences Of Electron Cyclotron Current Drive On Tearing Modes

Tearing mode is one of the most important nonideal magnetohydrodynamics instabilities presented in tokamaks. Tearing modes, especially neoclassical tearing modes can degrade the confinement by inducing energy loss, temperature and velocity reduction of the core plasma, which even lead to toroidal current disturbance and tokamak disruption. A great number of experimental studies aimed on tearing modes and their control have been carried out in the main tokamak devices throughout the world. The experiments indicate that tearing modes can be avoided or suppressed by multiple methods, such as adjusting operating parameters, applying external magnetic field or current drive, etc. Among these methods, electron cyclotron current drive (ECCD) is considered to be one of the most promising ways for controlling tearing mode instability, which has also been listed as the primary tool for neoclassical tearing mode control in ITER (International Thermonuclear Experimental Reactor). ECCD forms local current at rational surface which can significantly suppress tearing modes and neoclassical tearing modes by enhancing linear stability and compensating the perturbed bootstrap current induced by-magnetic islands. However, due to influences of various parameters, negative effects and some specific characteristics of tokamak plasma, mechanisms of ECCD become complicated, and many phenomena observed in experiments remain unclear and need to be interpreted.The present thesis focuses on the studies of the influences of driven current characteristics and circumstances such as plasma rotation on suppression effect of external current drive. The thesis consists of five chapters, which are summarized in the following paragraphs.The first chapter is introduction, in which the main mechanisms of tearing mode, neoclassical tearing mode and their control strategies are presented. Then theoretical and experimental research progresses on those strategies, especially on electron cyclotron current drive are demonstrated.Chapter two firstly presents the compressible magnetohydrodynamics model used in this thesis, then the study on influences of electron cyclotron current drive characteristics and parameters on the tearing modes suppression result are presented. Through simulations on the effected island final width with different driven current parameters such as current density distribution, deposition duration and locations, the relations between driven current parameters and suppression effects are obtained; the optimal deposition duration is proposed; furthermore, the X-point shift and island deformation caused by external driven current are also investigated. The results show that the suppression effect increases along with the current magnitude, but a marginal value exists, beyond which the driven current will promote the growth of tearing mode, a transformation of O-point to X-point takes place at the driven current location and the island width grows afterwards, which is regarded as "flip" instability; the driven current deposited at X-point of magnetic island can lead to X-point shift, which is the main reason of poor suppressing effect of X-point deposition.Chapter three uses a self-consistent bootstrap current model to form a neoclassical tearing mode model, then the relationship between current drive and perturbed bootstrap current is investigated, the required conditions of driven current parameters for complete suppression are also proposed. The study indicates that the stabilizing effect depends greatly on two parameters, which are the ratio of deposition width and saturated island width, and the ratio of the peak current density of driven current and bootstrap current. A minimal requirement for complete suppression needs a condition that both ratios are greater than one; the criterion can relax a little when the bootstrap current accounts for a large portion of the total current; the suppression ability increases with the total current strength, i.e., energy input; as soon as the driven current deposition width exceeds the island width, the suppressing result won't improve because current deposited outside the island separatrix has no inhibiting effect. Furthermore, the pre-emptive strategy used in experiments is investigated, if a current drive is loaded at the predicted location in a magnetic island before the mode is excited, the mode can be controlled by approximately half magnitude of the current needed for the one which has already saturated.Chapter four mainly investigates the influences of continuous and modulated driven current on rotating island; the negative effects of phase misalignment and rational surface displacement are also studied; besides, multiple current drive strategy is proposed and explored. The study shows that when island rotation is considered, the suppressing ability not only depends on the driven current value but also the deposition width. When deposition width is smaller than the island saturation width, the suppression effect of modulated current drive is significant; when the deposition width is bigger than saturation width, the effected saturation width has a linear relation with deposition width. The optimal duty cycle is estimated to be 0.5. The radical and poloidal misalignments of driven current deposition location can greatly degrade the suppression effect, when the deposition center is located at the edges of island separatrix or the midpoint of O and X-points, the driven current has no effect on the instabilities. Multiple current drive strategy for rotating islands suppression is proposed, and it is found that multiple current drive has better suppressing effect than single current drive.In the final chapter, the main research results and conclusions obtained in this thesis are summarized, then the original contributions and prospects are outlined.