Characteristics of MHD instabilities limiting the beta value in LHD

Effects of low-n magnetohydrodynamic instabilities on plasma performance have been assessed in the regime where an achieved beta value is limited by instabilities. The unstable regime of an ideal interchange mode is characterized by enhanced magnetic hill and reduced magnetic shear. Experiments have clarified that (i) low-n modes are significantly destabilized in the ideal-unstable configurations and lead to degradation of central beta by at most 60%, and (ii) the degree of their damages strongly depends on the mode rotation velocity. The occurrence of the minor collapse is independent of an existence of an error field.