Simulation Of Several Instabilities Driven By Energetic Particles In Tokamaks

So far,tokamak,one kind of magnetic confinement fusion devices,is thought to be the most promising apparatus to realize controlled nuclear fusion and solve the energy problem ultimately.Even though significant progress has been made through decades of investigation and exploration there are still many key physical and engineering problems remaining to be addressed urgently.Among them.energetic particle(EP)physics is especially crucial for the steady state operation of today's tokamaks and predicting the behavior of future burning plasmas,because energetic particles can heat the plasmas through collisions with thermal electrons and drive various instabilities.These EP induced instabilities can further cause significant loss of alpha particles,degrade confinement,and even lead to serious damage to the first wall of the device.Energetic particles are mainly generated by two different ways.Firstly,they can be generated by various auxiliary heating methods,such as Neutral Beam Injection(NBI).Electron Cyclotron Resonance Heating(ECRH),and Ion Cyclotron Resonance Heating(ICRH).Secondly,high energy alpha particles can be generated by deuterium-tritium fusion reaction.There are two kinds of energetic particle driven instabilities,one is the magnetohydrodynamic(MHD)type of instabilities,such as the fishbone mode.The other is various Alfvén Eigenmodes(AEs),such as the Toroidal Alfvén Eigenmode(TAE).In addition,various auxiliary heating methods are necessary for the tokamak devices at present,such as HL-2A,EAST,and D?-D.On the other hand,many alpha particles with high energies can be generated by deuterium-tritium fusion reaction in future tokamaks such as International Thermonuclear Experimental Reactor(ITER)and China Fusion Engineering Test Reactor(CFETR).As a result,in this dissertation we systematically carry out numerical investigation of the physical mechanism of the energetic particle mode with dominant high-order harmonics,the dynamics of fishbones and AEs in D?-D tokamak,and the instability of AEs driven by alpha particles in future CFETR device.The content of this dissertation is summarized as following.In chapter 1,the background and importance of the research in this dissertation are mainly introduced,and the background of the fishbone instabilites and AEs are briefly summarized.In chapter 2,the physical models used in the codes are described,including the kinetic-MHD hybrid code M3D-K and the gyrokinetic code GEM.In chapter 3,we investigate the excitation dynamics and characteristics of the energetic particle modes of high-order harmonics driven by trapped particles.It is found that with a weak magnetic shear in the core region,the high-order harnonics energetic particles modes can be driven unstable.The energetic particle modes become more unstable when the pressure of energetic particles becomes higher.Based on the analysis of the resonant conditions,there exist multiple resonant conditions for high toroidal mode number(n)components,while there is only one resonant condition for the m/n-1/1 harmonics.In addition:the effect of fluid nonlinearity on the mode saturation level is analysed,and the results show that the fluid nonlinearity has a relatively strong effect on the saturation level of the low n modes,while it hardly affects those of the high n components.During the nonlinear phase,the frequencies of high n modes chirp down or chirp up,and the chirping range is small,which is different with the typical fishbone instability.Finally,the changes of energetic particles'distributions due to different high-order harmonic modes are compared and analysed.In chapter 4,the fishbone instabilities and AEs driven by energetic particles in D?-D tokamak are investigated.In D?-D hybrid discharges,the intense AE activity driven by NBI that is typically observed can be suppressed and replaced by fishbone modes when Electron Cyclotron Current Drive(ECCD)is centrally applied.As a result,two typical shots are chosen to simulate and explain this phenomenon.There is only NBI applied in one of these two shots,and both NBI and ECCD are applied in the other shot.Based on the analysis of mode structure and mode frequency relative to the Alfvén continuum spectrum,it is found that the beta-induced Alfvén eigenmode(BAE)with dominant toroidal mode number n=3 is excited without ECCD.while the fishbone with dominant n=1 is destabilized with ECCD.By comparison,it is found that the main differences between these two shots are the ratio of energetic particle pressure to total pressure(Phot/Ptotal)and the safety factor va]ue at the magnetic axis(q0).As a result,a systematic scan of different n has been performed in the paraneter space of Phot/Ptotal and q0.The results show that when q0 and Phot/Ptotal are changed,the transition between AEs and fishbones can occur,and a stable region is found,inside which both AEs and fishbone modes are stable.These results provide useful guidance for tokamak experiments in future.In chapter 5,based on the parameters and profiles from the CFETR physical design,AEs driven by energetic particles are simulated by the gyrokinetic code GEM.By comparison,it is found that the contribution from beam ions on the AE instability is negligible as compared to that from alpha particles.As a result,the alpha particles generated by fusion reaction are the only energetic particle species considered in this work.Firstly,by analysing the modes with different toroidal mode numbers,it is found that the toroidal mode number of the most unstable mode is around n=10,and it is a TAE mode.As a result,detailed simulation and analysis are carried out for the n=10 instability.In general,the safety factor profile has a significant effect on the AE instability,so the effect of minimum value of safety factor(qmin)on the AE instability is analysed in detail.When only one mode is considered,it is found that the stability of AEs with a single toroidal mode number is sensitive to qmin.Moreover,the ratio of alpha particles' birth speed to Alfvén velocity(vh/vA)and the alpha particles'Larmor radius normalized by the minor radius(?h/a)are two key parameters determing alpha particle drive of AEs,so the effect of these two parameters on the mode is analysed in the simulation.It is found that the alpha particle drive is maximized for CFETR values of these two parameters.Finally,when the density and temperature of thermal plasma change,the other parameters,especially the density and other parameters related to alpha particle distribution,will change correspondingly.By considering all the parameters related to the change of thermal ions,we analyse the influence of the thermal ion density/temperature change on the growth rate.The results show that the alpha particle-driven AE's growth rate decreases as the thermal ion temperature/density decreases/increases at fixed plasma pressure,and the mode can be stable when both the density and temperature reach a threshold..Finally in chapter 6,conclusions of the dissertation are summarized,and the prospects for future work are given.