Simulations On Influence Of Fast Ions On Fishbone And Internal Kink Modes In Tokamaks

The reserch of energetic particle physics is an important topic in the field of magnetic con-finement fusion,as energetic particle can not only heat the plasma,but also excite various in-stabilities.Energetic particle can be generated by auxiliary heating methods,such as radio fre-quency waves and neutral beam injection,also can be theαparticles from fusion reactions.In the tokamak plasma,energetic orαparticles interact with magnetohydrodynamic instabilities,such as the internal kink mode and fishbone mode,while the modes will lead into redistribution and radial transport of energetic orαparticles and result in a reduced heating efficiency for the plasma and limitation on the performance of the devices.The lost energetic orαparticles will transport to the wall and even can damage the first wall of the device directly.Therefore,deep understanding of the physical mechanisms of the interaction between energetic orαparticles and the internal kink mode as well as fishbone mode,in order to suppress or control these in-stabilities,is crucial for the steady-state operation of tokamak device.Based on above research background,in this thesis,the kinetic influence of thermal particles as well as fast ions on the fishbone modes and internal kink modes are numerically studied by using MARS-K code.It should be noted that the thermal particles mentioned in this thesis refer to thermal ions and ther-mal electrons in the background plasma,while fast ions refer to energetic particles generated by neutral beam injection.The contents are organized as follows:In chapter 1,a brief introduction of research background and scientific significance of this thesis is presented.It contains fusion energy,magnetic confinement fusion device as well as the main parameters of domestic and international major tokamak devices.Previous works and research progress of the unstable modes that are related to our topic,such as internal kink modes,fishbone modes,and sawtooth oscillations are also reviewed.In chapter 2,a short description of the CHEASE code which is used to solve the Grad-Shafranov equation to give the tokamak plasma equilibrium and the MARS-K code used in the simulations.In introducing the physical model of MARS-K code,the fast ion distribution function as well as the key terms of the drift kinetic physics is emphasized.In chapter 3,based on the non-perturbative approach,the fishbone instabilities driven by trapped fast ions in a toroidal plasma with q profile nearly being flat or non-monotonic is studied,the dependence of the fishbone on the fast ion distribution,safety factor（q）profile,kinetic effects of thermal particles and plasma resistivity are explored.It is found that the mode is more easily triggered in an equilibrium with two q=1 surfaces,compared with the case with one or no q=1 surface.Kinetic contributions from transit resonance of passing fast ions and from bounce resonance of trapped fast ions strongly enhance the mode instability,while the passing thermal ions induced Landau damping has a strong stabilization effect.The plasma resistivity have a significantly stabilizes effect on fishbone mode near the marginally unstable point.The synergistic effect of plasma rotation and parallel sound wave damping weakly suppresses the fishbone mode instability,and this stabilization effect is mainly attributed to the damping caused by the parallel sound wave damping.Furthermore,the kinetic contribution from particles has a modification to the mode structure.In chapter 4,we find that the anisotropic distribution of trapped fast ions excite a sub-unstable branch of fishbone mode when we scan the parameter q_minin chapter 3.The growth rate of this sub-unstable branch is less than that oh the classical fishbone mode,while the frequency is greater.Although the sub-unstable branch has the same frequency as the classic fishbone mode at some q_minvalues,it can be distinguished by the mode structure and the growth rate.In addition,this numerical result predicted by MARS-K is well experimentally validated on the HL-2A device.The results show that the instability of the sub-unstable branch increases with increasing the plasma toroidal pressure,and kinetic contribution from the trapped fast ions toroidal precession resonance enhances the mode instability.Besides,the plasma resistivity has little influence on the sub-unstable branch,while the plasma rotation has a slightly stabilizing effect on the sub-unstable branch.Although mode structure of the sub-unstable branch is similar to that of the double fishbone mode,its excitation does not depend on whether the q profile has negative shear feature,which means that the monotonic q distribution can also drive this instability.Moreover,contrary to the double fishbone mode,with the reduction of the magnetic shear and the distance between two q=1 rational surfaces,the growth rate and real frequency of the sub-unstable branch decrease,and the radial displacement becomes sharper.In chapter 5,the kinetic effects of thermal particles and fast ions on the internal kink mode are numerically investigated.It is shown that either thermal particles or fast ions have stabilizing influence on the internal kink instability.However,the former can not fully stabilize the internal kink mode,while the latter can suppress the internal kink mode.In addition,the synergistic effect from thermal particles and fast ions induces more stronger damping on the internal kink instability.The kinetic effects from particles significantly raise the critical value of poloidal beta(β_p^crit)for driving internal kink mode in the toroidal plasma.The results shown above imply a method of controlling the internal kink mode or sawtooth mode in the high-β_pdischarge scenario of tokamak.The plasma rotation has a slightly destabilizing effect on the internal kink mode,and when the plasma rotation and collision effects work synergistically,the rotation in the same direction as the plasma current enhances the mode instability,while the rotation opposite to the plasma current makes the mode stable.It is also noted that,at the q=1 rational surface,the mode structure becomes sharper due to the self-consistent modification by particles’kinetic effect.In chapter 6,a brief summary and outlook of the future work are presented.