Numerical Simulation Of Drift-tearing Mode In Tokamak

Controlled nuclear fusion is an important research direction to solve energy and environmental problems in the future.Tokamak,as one of the most feasible magnetic confinement fusion devices,is the most promising way to achieve this goal.Tearing modes are among the most dangerous instabilities related to magnetohydrodynamics(MHD)in tokamak discharges.They are driven by magnetic free energy,which is determined by the form of the current profile.They can change the topology of the magnetic field via magnetic reconnection and lead to the formation of magnetic islands,thus enhancing cross-field transport and degrading the particle and energy confinement,and eventually lead to the failure of plasma confinement.Therefore,it is of great sig-nificance to study the physical mechanism of tearing mode for exploring the method of Tokamak steady-state operation.It has been shown that in a hot magnetically confined plasma,the influence of diamagnetic drift motions on the tearing modes is significant.When the diamagnetic drift frequency caused by the equilibrium pressure gradient,which is perpendicular to the magnetic field direction,is comparable to the growth rate of the resistive tearing mode,The coupling of drift modes to tearing modes leads to a substantial reduction in the growth rate and the production of a real frequency which is approximately equal to the diamagnetic drift frequency.The coupled modes are called the drift tearing mode.The classical(m=2,n=1)resistive tearing mode is verified in a cylindrical geometry with a fluid model successfully using the Gyrokinetic Toroidal Code(GTC),where m and n are the poloidal and toroidal wave numbers,respectively.The dependence of the growth rate of the resistive tearing mode on the beta value of the plasma is obtained and is found to qualitatively agree with the theoretical prediction.Then,the fluid model of the drift tearing mode is obtained by introducing the equilibrium pressure gradient term into the electronic force balance equation.The mode structure and complex frequency of the drift tearing mode are obtained through numerical simulation.The broadening of the radial mode structure and the decrease of the growth rate due to the diamagnetic drift effect have been seen.The scaling relation between the diamagnetic frequency and the growth rate of the drift-tearing mode has been calculated.This relation shows good agreement with the theoretical prediction for a relatively small resistivity;however,an obvious deviation arises when the resistivity is large.Since a narrow current layer is the main restriction of the "constant-ψ assumption",this assumption obviously introduces larger error for a larger resistivity and a wider layer.After that,the influence of temperature gradient and toroidal effects on drift tearing modes are also studied.By introducing thermal force term into the electron force bal-ance equation,it is found that the equilibrium temperature gradient will cause a signifi-cant increase in the growth rate of the drift tearing mode and a broadening of the mode structure;toroidal geometry is mainly reflected in the modification of the equilibrium magnetic field,simulation results show that the toroidal effects will increase the growth rate of the drift tearing mode,and the contours of perturbation field will "squeeze" to the strong field side in poloidal section.Finally,the hybrid model for fluid electron and kinetic ion is studied briefly,the dispersion relation of the drift tearing mode under the influence of the ion FLR effects is obtained,and the numerical solution of dispersion equation is derived by using Matlab.Comparing with the dispersion relation under the fluid model,we can see that the FLR effects of ions have a stabilizing effect on drift tearing mode.