Studies of Fast-ion-driven MHD Instabilities in Heliotron J Equilibrium by Particle-MHD Hybrid Simulation code MEGA

Understanding of fast-ion-driven MHD instabilities is crucial for realizing sustainable burning plasma. In this study, MEGA, a hybrid MHD simulation code is applied to Heliotron J, which is a low shear helical-axis heliotron.  MEGA utilizes nonlinear MHD equations for bulk plasma and a drift kinetic equation for fast-ions.   Previously, MEGA has been implemented only in Tokamak and LHD.  The objective of this study is to verify the fast-ion MHD wave resonance in Heliotron J equilibrium by MEGA. The utilized equilibrium has m/n=7/4 magnetic islands at the edge region.  This islands are located close to the experimentally observed m/n=2/1 global Alfvén eigenmode (GAE).  To simplify the calculation, the coupling between Alfvén eigenmode and magnetic island is avoided. Only eigenmodes in the core region are considered.  Two eigenmodes may exist at r/a=0.4: m/n=8/5 toroidal Alfvén eigenmode (TAE) and m/n=5/3 beta-induced Alfvén-acoustic eigenmode (BAAE).  We have found an instability with the dominant harmonic of m/n=5/3 at the BAAE gap location; however, the frequency of the mode is 1 kHz, which is less than predicted value from BAAE dispersion relation (27.4 kHz).  The properties of this instability will be discussed and other instabilites will be explored.