Influences of ballooning modes with moderate wave number on MHD equilibrium in LHD

Abstract

In order to investigate the influences of ballooning modes with moderate wave numbers on MHD equilibrium, fully three-dimensional nonlinear MHD simulations with or without a large parallel thermal conductivity κ∥ are carried out for a fairly Mercier-unstable low-β MHD equilibrium in the inward-shifted Large Helical Device (LHD) configuration, which is considered to be avoided by pressure profile control in experiments. In the case without a κ∥ corresponding to a nearly non-dissipative plasma, such ballooning modes are so dangerous that a pressure collapse occurs in the core region and the plasma expands beyond the outermost flux surface. On the other hand, pressure deformation is limited in the core region for the case with a large κ∥, because ballooning modes with moderate wave numbers are significantly suppressed by κ∥. Since the core pressure collapse in a low-β plasma will become an obstacle on the way to attaining a high-β plasma in experiments, the operation regime or pressure profile in the inward-shifted LHD configuration might be restricted, at the least low-β regime, which is consistent with the experimental observation.