Gyrokinetic Simulation Of Tearing Mode Instability

Tearing mode instability is one of the most dangerous instabilities caused by magnetic field reconnection in tokamaks.It can change the topology of the magnetic field and disrupt the magnetic freezing effect,which in turn affects particle and energy confinement and ultimately disrupts tokamak steady-state operation.During the disconnection and reconnection of the magnetic field near the tearing layer,magnetic energy can be converted into kinetic,thermal and radiative energy.Due to the multi scale nature of the tearing mode,the kinetic effect of the ions near the rational surface becomes extremely important,such as the finite orbit width effect and the finite Larmor radius effect.The magnetohydrodynamic simulation is difficult to be used for the study on the effect of plasma kinetic effect on the tearing mode,so the gyrokinetic code becomes one of the powerful tools for our numerical simulation study of the kinetic effect of the tearing mode.At the same time,the tearing mode instability is affected by a series of physical mechanisms such as microscopic turbulence,neoclassical transport,macroscopic magnetohydrodynamics instability and fast ion transport.Based on the gyrokinetic ion and fluid electron hybrid model,we have developed a three-dimensional electromagnetic perturbation field solver for low-n modes.Taking into account the effects of mode coupling and magnetic drift in toroidal geometry,the GEM code was extended so that it can be applied to the numerical simulation of low-n modes.On this basis,we deeply investigated the effect of kinetic ions and the toroidal effect on the single tearing mode and the double tearing modes.For the single tearing mode,the effect of the toroidal effect on the tearing mode is investigated by comparing the simulation results of the cylindrical geometry with the toroidal geometry for different inverse aspect ratios.The radial mode width of the tearing mode becomes narrower with the increase of the toroidal effect.At the same time,the toroidal pressure effect caused by the toroidal effect can further reduce the linear growth rate of the single tearing mode,increase the radial mode width,and change the scaling between the mode width and the plasma resistivity.The effect of kinetic ions on the tearing mode was investigated by increasing the temperature of kinetic ions.The results show that kinetic ions can effectively reduce the growth rate of the tearing mode,but do not change the scaling between the tearing mode growth rate and resistivity.It is further shown that the finite Larmor radius effect of ions is not important for the tearing mode when the resistivity is large,and the effect of ions on single tearing mode is mainly through the magnetic drift effect in the motion of ion guiding center motion.The energy analysis of each component of the ions revealed that the energy exchange between the tearing mode and the passing ions is greater than that between the trapping ions,and the energy exchange between the ions and the tearing mode is greater in the parallel direction than that in the perpendicular direction.For the double tearing mode,since the biggest difference between the double tearing mode and the single tearing mode is the coupling effect of the two rational surfaces,this study focuses on the study of its coupling effect.It is found that the separation of rational surfaces effectively affects the structure of the double tearing mode,especially the smaller the separation of the rational surfaces,the stronger the coupling effect and the higher the magnitude of the mode between the two rational surfaces.In addition,the growth rate of the double tearing mode is not monotonically varying with the separation of the rational surfaces.The toroidal pressure effect can reduce the growth rate of the double tearing mode and change the growth rate scaling when the resistivity is small.Kinetic ions can reduce the growth rate of the double tearing mode,especially when the separation of the two rational surfaces is small,i.e.,the coupling effect is strong.At the same time,kinetic ions can also enhance the coupling effect between the two rational surfaces.As same as that in the single tearing mode,the magnetic drift effect plays a dominant role in the ions affecting the double tearing mode.From the energy analysis of each component of the ions,it can be inferred that kinetic ions can effectively affect the double tearing mode through the energy exchange around the outer rational surface and in the coupling region.On the one hand,the kinetic ions can enhance the coupling effect of the double tearing mode;on the other hand,the enhanced coupling effect can induce stronger energy exchange through the ion kinetic effect in the coupling region in turn.