Simulation Of Energetic Electron Effects On Alfvén Eigenmodes In Tokamak Plasmas

In future reactors,alpha particles of 3.5 MeV,produced from the deuterium-tritium(D-T)nuclear fusion reaction,will be used to keep the plasma burning by heating the bulk plasma.The alpha particles can resonate with Alfven eigenmodes(AEs)in a collisional slowing-down process,and destabilize the AEs.The destabilized AEs can transport alphas and reduce their heating efficiency,finally leading to the degradation of the fusion reactor performance.In addi-tion,both energetic ions and electrons will be produced in abundance by the high-power heating and fusion reaction,which could further amplify the AEs.Therefore,to actively control them,it is very crucial to understand the instabilities driven by alphas and other energetic particles.In the past decades,interactions between AEs and energetic ions which are accelerated by neutral beam injection and ion cyclotron resonance frequency heating,have been extensively studied in various toroidal devices.By contrast,far less attention has been paid to energetic-electron driven instabilities.Indeed,the study of energetic electrons is important not only for understanding its own behavior in driving Alfven instabilities,but also for understanding the behavior of alpha particles in burning plasmas.Besides,electron cyclotron(EC)wave is a unique and very promising tool for AE control,because the localized EC wave can be flexi-bly and precisely targeted to the AEs that can be excited at a wide range of radial positions.On the other hand,the effects of EC wave on AE are quite complicated and the effects of EC wave generated energetic electrons on the energetic-ion driven AE is an unexplored area.In this dissertation,AEs driven by energetic electrons are investigated via hybrid simulations of an MHD fluid interacting with energetic electrons.Moreover,energetic electron effects on an energetic-ion driven AE are studied using an extended version of MEGA code,where both ki-netic energetic ions and kinetic energetic electrons are included.The content of this dissertation is summarized as follows.Firstly,the destabilizations of energetic-electron driven toroidal Alfven eigenmode(TAE)and elliptical-Alfven-eigenmode-type(EAE-type)mode are numerically investigated.These Alfven eigenmodes are destabilized via the free energy stored in the energetic electron density gradient.Both energetic electrons with centrally peaked beta profile and off-axis peaked profile are considered.For the centrally peaked energetic electron beta profile case,a TAE propagating in the electron diamagnetic drift direction is found.The mode is mainly driven by deeply trapped energetic electrons.It is also found that a few passing energetic electrons spatially localized around rational surfaces can resonate with the mode.For the off-axis peaked energetic electron beta profile case,an AE propagating in the ion diamagnetic drift direction is found when a q-profile with weak magnetic shear is adopted.The destabilized mode is an elliptical-Alfven-eigenmode-type mode which has a spatial profile peaking at the rational surface and a frequency close to the second Alfven frequency gap.It is found that passing energetic electrons and barely trapped energetic electrons are responsible for the destabilization of this EAE-type mode.Secondly,the saturation levels are compared for a TAE with the same linear growth rate among energetic-electron driven modes and energetic-ion driven modes with isotropic and anisotropic velocity space distributions.The saturation level of TAE driven by trapped ener-getic electrons is comparable to that driven by energetic electrons with isotropic velocity space distribution where the contribution of trapped particles is dominant.Saturation levels are also compared for a TAE driven by centrally peaked and off-axis peaked energetic electrons.The results show that the off-axis peaked energetic electrons with a narrower beta profile lead to a lower saturation level.Besides,it is found that the trapped-energetic-ion driven TAE has a larger saturation level than the passing-energetic-ion driven TAE,which indicates the difference in particle trapping by the TAE between trapped and passing energetic ions.Thirdly,both energetic electrons with centrally peaked beta profile and off-axis peaked profile are considered for interacting with an energetic-ion driven TAE.An obvious stabilization of TAE is found when an off-axis peaked energetic electron beta profile is employed.Further increasing the energetic electron beta can even fully suppress the energetic-ion driven TAE.It is found that the stabilizing effect comes from the energetic electron beta profile.The decrease of the linear growth rate is mainly due to the decrease of energetic ion driving rate,rather than the increase of the damping rate.The strongest stabilization occurs when the energetic electron beta profile is near the TAE center and high-n TAE is more easily to be stabilized.Besides,the energetic electron beta profile can also affect the TAE frequency.Both downshift and upshift of mode frequency are found,which is dependent on the gradient of energetic electron beta profile.Finally,in summary,the destabilization mechanism of energetic-electron driven TAE and EAE has been studied and clarified in this dissertation,which will help to understand the ex-perimentally observed energetic-electron driven AEs.Besides,energetic electron effects on an energetic-ion driven TAE are systematically investigated.It is found that off-axis peaked en-ergetic electrons can effectively stabilize the TAE,which will provide a channel for active AE control.