Numerical investigation into the peripheral energetic-particle-driven MHD modes in Heliotron J with free boundary hybrid simulation

The interaction between energetic particles (EPs) and EP-driven magnetohydrodynamic (MHD) instabilities in Heliotron J, a low-shear helical axis stellarator/heliotron, is investigated with MEGA, a hybrid MHD-EP simulation code with the free boundary condition, on the last closed flux surface. The n/m = 1/2 energetic particle mode (EPM) and the n/m = 2/4 global Alfven eigenmode (GAE) in the peripheral plasma region of Heliotron J are successfully modeled with the free boundary condition. The free boundary condition affects the EP driving rate of the n/m = 1/2 EPM and the n/m = 2/4 GAE through the changes in the mode spatial profile. Under the fixed boundary condition, the linear growth rate of the EP-driven MHD mode with low mode numbers is underestimated. The interaction between EP and these experimentally-observed modes is kinetically analyzed. It is found that the strongest EP-shear Alfven wave interactions arise from the toroidicity-induced resonances in the high-velocity region. These high-velocity EPs can efficiently interact with the peripheral EP-driven mode. The additional toroidally-asymmetric resonances are localized in the low-velocity region; therefore, their effects are weak for the bump-on-tail EP velocity distribution function.

Automated summary

The study investigates the interaction between energetic particles and EP-driven MHD instabilities in Heliotron J using the MEGA simulation code with free boundary conditions. It is found that the free boundary conditions affect the spatial profile of modes, while under fixed boundary conditions, the linear growth rate of EP-driven MHD modes with low mode numbers is underestimated. The strongest interactions between EP and experimentally-observed modes occur in the high-velocity region.