Numerical Study On Resistive Wall Modes In HL-2A Tokamak

Long pulse and steady-state tokamak operation is of great significance for a future fusion reactor. There are many kinds of magnetohydrodynamics(MHD) unstable modes that can influent tokamak state operation. Especially, the external kink mode is one of the most dangerous unstable modes, that often limits the operational space of fusion devices. Theoretical and experimental study have showed that external kink mode can be stabilized completely when plasma in tokamak is surrounded closely by a perfect conducting wall which is not existent actually, a conducting wall has more or less resistance. Due to electrical resistance in conducting wall, magnetic field which is used to constrain plasma diffuses through the conducting wall. The relatively fast growing external kink mode is converted into the new far more slowly growing unstable mode when discharge time is up to the diffusion time of perturbation flux. The new MHD instability relates to the conducting wall of encircling plasma, so the new instability is called resistive wall mode (RWM), the conducting wall is called resistive wall. It is still an important problem for restraining RWM in advanced tokamak.Studies of analytical method and numerical method have demonstrated that strong edge plasma rotation and viscous force inside the plasma somewhere can stabilize the RWM. They made use of a simple model which outer conduct wall is similar with internal plasma edge in many theoretical researches. However, for HL-2A tokamak, he plasma shape is a circular and conducting wall shape is polygon. The effect of the polygon wall to suppress the external kink mode and resistive wall mode is not studied. In this work, we use model which is the real HL-2A tokamak to study the RWM and external kink mode.In this work, the eigenvalues method is used to study MHD instabilities, the eigenvalue and eigenfuction are solved by a numerical approaches. In chapter2, the equilibrium configurations are computed at several certain equilibrium current profiles by CHEASE code. For different equilibrium configuration, the growth rate of external kink mode is computed by MARS code, the critical position is confirmed through studying the effect of perfect conduct wall to external kink mode. In chapter3, we study the effect of plasma rotation with respect to external kink mode in equilibrium configuration. In chapter4, what is the effect of parallel viscosity with respect to external kink mode and RWM is discussed.Results showed that when the central zone of equilibrium current profiles is gentler, the growth rate of external kink mode is smaller in HL-2A tokamak. The real frequency of external kink mode varies along with plasma rotation and the value of frequency is equal to the rotation frequency approximately. The real frequency of RWM is always very small and do not vary along with different viscosity. The threshold of plasma rotation can be obtained when the stable window appears for established plasma viscosity. When the plasma viscosity is increase, the threshold of plasma rotation is decrease.