Numerical Studies On Effects Of Plasma Resistivity And Plasma Flow On Internal Kink Modes In Tokamaks

In tokamak plasma,an internal kink mode is a special kink mode happening at q=1 magnetic surface.Theoretical,modeling and experimental study of the internal kink mode instabilities is an important research area in the magnetic confinement fusion plasmas.In tokamak discharges,the central safety factor q0 decreases as the toroidal plasma current diffuses into the plasma center.When q0 falls below one,the n=1 internal kink mode in the plasma becomes unstable.That is characterized by a radial monotonically ascending q profile with q0<1 in ITER's standard operation baseline scenario,edge-localized,high confinement modes.Thus,the research of internal kink mode instability plays an important role in guiding the operation of ITER in the future.As a very important magnetohydrodynamic instability,the internal kink mode is responsible for both fishbone and sawtooth activities observed in tokamak plasmas.These unstable modes have great influence on the plasma macroscopic confinement.Therefore,it is necessary and important to better understand physics of internal kink mode.This dissertation subsequently focuses on the internal kink mode instability in tokamak plasma.Details are organized as following:In Chapter 1,the background is briefly reviewed on nuclear fusion,and tokamak,the most promising fusion device for fusion energy.It is then introduced that tokamak devices with their operating parameters are currently in operation,and under construction.A brief review is given on the research status of MHD instabilities:internal kink mode,fishbone,and sawtooth.In Chapter 2,the two code,CHEASE and MARS,are briefly introduced.The CHEASE code solves the fixed boundary Grad-Shafranov equation that the tokamak plasma equilibrium satisfies,and to provide the corresponding equilibrium parameters for the MARS code and other codes.For toroidal geometric configuration of tokamaks,there are several versions of MARS codes.The full toroidal,ideal MHD code MARS-F and the nonperturbative MHD-kinetic hybrid code MARS-K solve the linearized MHD equations and numerically investigate the macroscopic magnetohydrodynamic stabilities in tokamak plasmas,such as the kink modes,the resistive wall modes and others.In Chapter 3,the influence of energetic particles and plasma resistivity on the n=1 internal kink and fishbone modes in tokamak plasmas is numerically investigated with MARS-K code.The results show that energetic particles can either stabilize or destabilize the ideal internal kink mode,depending on the radial profiles of the particles' density and pressure.Resistive fishbones with and without an ideal wall are investigated.It is found that,in the presence of energetic particles as well as plasma resistivity,two branches of unstable roots exist,for a plasma which is ideally stable to the internal kink instability.One is the resistive internal kink mode.The other is the resistive fishbone mode.These two-branch solutions show similar behaviors,independent of whether the initial ideal kink stability is due to an ideal wall stabilization for high-beta plasmas,or due to a stable equilibrium below the Bussac pressure limit.For a realistic toroidal plasma,the resistive internal kink is the dominant instability,which grows much faster than the resistive fishbone.The plasma resistivity destabilizes the resistive internal kink while stabilizes the resistive fishbone.In Chapter 4,the study focuses on two physics effects:the sheared toroidal flow of plasma and the drift kinetic effects from thermal particles on the internal kink instability with MARS-K code.Within the subsonic flow assumption,the flow and flow shear effects on the internal kink are generally weak,but the kinetic effects can be strong.A significant reduction in the mode growth rate is predicted by the nonperturbative MHD-kinetic hybrid computations,when the precessional drift resonance contributions from both particle species are included and when the mode is not too strongly unstable according to the fluid theory.The stabilization/destabilization of sheared toroidal flow depends on the radial location of the local flow shear.Strong destabilization(stabilization)occurs when a negative(positive)flow shear is located near the q=1 rational surface.In Chapter 5,effects of parallel or poloidal flows,as well as the flow shear,on the internal kink mode instability have been numerically investigated in toroidally rotating plasmas with MARS-F code.A significant difference between these flows is that the one dimensional background toroidal flow frequency is symmetric with respect to the poloidal angle,whilst both the poloidal and toroidal projections of the additional parallel flow are functions of both the plasma minor radius and poloidal angle.It is found that the stability of the internal kink mode is hardly modified by the parallel flow.However,the negative shear parallel flow has a strong destabilization with the fixed frequency of one dimensional toroidal flow component.The poloidal projection and toroidal projection provide the strong destabilization on ideal internal kink mode.Compared with the poloidal projection and toroidal projection of the uniform parallel flow,it is found that the destabilization of the two projections of shear parallel flow on the internal kink mode is slightly weakened.In Chapter 6,it is presented that the summary of this thesis as well as outlook of the future work.