Gyrokinetic-magnetohydrodynamic hybrid simulation of the transition from toroidal Alfvén eigenmodes to kinetic ballooning modes in tokamaks

A gyrokinetic-magnetohydrodynamic (MHD) hybrid simulation code has been developed in order to study high-n (where n is the toroidal mode number) MHD instabilities driven by energetic particles in finite-β tokamaks. Here β is the ratio between plasma and magnetic pressures. Specifically, it is observed that as the core plasma β value increases, there is a corresponding transition from the toroidal Alfvén eigenmode (TAE) to the kinetic ballooning mode (KBM). The energetic particle mode (EPM) branches of both the toroidal Alfvén mode (TAM) and KBM are shown to be important in this transition. KBM are preferentially excited when the energetic particle velocity is small compared to the Alfvén velocity.