Magnetohydrodynamic hybrid simulation of Alfvén eigenmodes in Heliotron J, a low shear helical axis stellarator/heliotron

Abstract

Alfvén eigenmodes destabilized by energetic particles in Heliotron J, a low magnetic shear helical axis stellarator/heliotron with four toroidal equilibrium field periods, are investigated with MEGA, a hybrid simulation code for energetic particles interacting with a magnetohydrodynamic fluid. The objectives of this study are to validate the MEGA simulation results on Alfvén eigenmodes in Heliotron J, and to clarify the properties of Alfvén eigenmodes. The experimentally observed global Alfvén eigenmode (GAE) was reproduced with MEGA simulations based on the experimental bulk plasma temperature and density profiles. Beside GAE, GAE was also observed with lower amplitude and linear growth rate. The frequency and spatial location of GAE are close to those of the experimentally observed energetic particle mode (EPM). For GAE, it was found that the three-dimensional spatial profile of the GAE was primarily composed of n = 2 harmonics while the contribution from the other toroidal mode numbers such as n = 2 ± iNfp was weak, where ‘i’ and ‘Nfp’ are arbitrary integer and toroidal field period, respectively. This indicates that the coupling of harmonics with the same toroidal mode number through toroidicity is dominant for the GAE spatial profile, and the coupling with the different toroidal mode numbers through helicity and bumpiness has minor effects. For GAE, the coupling of harmonic through toroidicity is weaker than that of GAE. The non-linear evolutions of the GAE and GAE destabilized by energetic particles with a bump-on-tail distribution and a slowing-down distribution were compared. Weak dependence of the linear growth rate for both and modes on charge-exchange time were implied.