Study On Influence Of Kinetic Effects On Resistive Wall Modes And Toroidal Alfvén Eigenmodes

Macroscopic magnetohydrodynamic(MHD)instabilities are one of the main reasons resulting in plasma major disruption.For example,resistive wall mode(RWM)and toroidal Alfvén eigenmode(TAE)that are driven by plasma pressure gradient,plasma current and energetic particles.The high β value of plasma is limited due to the RWM instability,and the value of energy gain factor will decrease.The TAE instability triggered by energetic particles can cause energetic particles loss and redistribution,and then lower the energy constraint level.Therefore,a further study of the linear property of RWM and TAE is important to realize the fusion in the future.There are two methods for the RWM stabilization:active means utilizing the feedback coils,and passive approach relying on combination of the toroidal plasma flow with certain energy dissipation mechanisms.So far,research and application of the active method have been sufficiently mature.However,there are still some unsolved problems for the passive means,such as critical value of plasma toroidal rotation for stabilization of the RWM,energy dissipation mechanisms.In addition,the energetic particles can not only stabilize the RWM but also excite Fishbone-like mode(FLM)instability.On the other hand,conventionally,a fixed boundary condition at the plasma surface is assumed in the TAE study,which is neglected in the vacuum region between plasma surface and conducting wall.However,the current conclusions about low n(n is the toroidal mode number)TAE may be modified as a result of vacuum region between the plasma surface and the conducting wall.Therefore,on the basis of above reasons,this thesis will focus on dynamic behaviors of the RWM and TAE utilizing theoretical analysis and numerical computation methods.In addition,the FLM instability excited by energetic particles is briefly discussed.In Chapter Ⅰ,the background and aim of this thesis are introduced,and the equation and research methods for MHD instabilities are also presented.Furthermore,the research progress of RWM and TAE are described,and also a brief introduction of MARS-K code is given.In Chapter Ⅱ,the influence of plasma collision frequency and temperature ratio of thermal ion to electron on the RWM instability is investigated,considering the kinetic contribution from trapped thermal particles included.The MHD-kinetic hybrid theory predicts a bifurcation of the mode dynamics while varying certain physical parameters of the plasma,such as the thermal particle collisionality or the ratio of the thermal ion to electron temperatures.Qualitatively similar bifurcation features are also observed in full toroidal computations,based on non-perturbative hybrid formulation.In addition,this bifurcation does not depend on the collision model.In Chapter Ⅲ,kinetic effects of both trapped thermal and energetic particles on the RWM and FLM instability are theoretically investigated for the collision and non-collision cases.The results demonstrate that thermal particle collisions can either stabilize or destabilize the RWM,depending on the energetic particles pressure βh.Furthermore,the critical value of βh for triggering FLM is enhanced when the thermal particles contribution is taken into account.The critical value also relies on the plasma collision frequency and toroidal plasma rotation frequency.The plasma inertia has a negligible influence on the FLM.due to the low frequency of FLM.In Chapter Ⅳ,the dynamic behaviors of RWM in a resistive plasma are investigated based on both analytical model and numerical approach.The results indicate that there are two unstable branches of RWMs,if the toroidal favorable average curvature effect(GGJ effect)is taken into account in the resistive layer.The plasma resistivity can either stabilize or destabilize the RWM,depending on the plasma rotation.Similarly,the plasma rotation can stabilize or destabilize the RWM,depending on the plasma resistivity.In addition,the GGJ effect can not only directly affect the mode growth rate,but also indirectly modify the mode stability by changing the continue damping through modifying the mode real frequency in the plasma frame.There is good qualitative agreement between theoretical analysis results and numerical results.In Chapter Ⅴ,the n=1 TAE instability,excited by trapped energetic particles(EPs),is numerically investigated in a free boundary tokamak plasma,using the non-perturbative MHD-kinetic hybrid formulation based on MARS-K code.Compared with the fixed boundary condition at plasma edge,a free boundary not only enhances the critical value of the EPs kinetic contribution for driving the TAE,but also induces the finite perturbations at the plasma edge.An anisotropic distribution of EPs,in the particle pitch angle space,strongly enhances the instability and results in a more global mode structure,compared with the isotropic case.The plasma resistivity is also found to play a role in the EPs-destabilized TAE.A mode conversion,from the modified ideal kink by the EPs kinetic effect to the TAE,is observed while increasing the birth energy of EPs.Computational results suggest that the TAE mode structure can be modified by certain key plasma parameters,such as the EPs kinetic contribution,the equilibrium pressure,the plasma resistivity,the distribution of EPs,as well as EPs birth energy.More importantly,numerical results show that near the marginal point,the TAE mode structure is different from that of the conventional TAE.In Chapter Ⅵ,the summary of this thesis and outlook of future work are presented.