Stability in high-beta plasmas of LHD

In Large Helical Device (LHD), the volume averaged beta value <βdia> of 4.5%, which is the highest value in all of heliotron/stellarators, was achieved by optimizing the magnetic configuration from a viewpoint of MHD characteristics, transport and heating efficiency efficiency of neutral beam. In this study, characteristics of MHD activities in extended <βdia> range over 4% and the configuration dependence have been investigated. The dominant MHD modes moved from inner region to outer one when <βdia> increases, and the mode excited in the outermost resonance near plasma edge are enhanced in the <βdia> range over 4%. The dependence of saturated amplitude of the mode on magnetic Reynolds number is close to that of linear growth rate of resistive interchange mode. When the magnetic shear was decreased and the plasma is close to m/n=1/1 ideal stability boundary, the m/n = 1/1 mode suddenly increased and led to a minor collapse in the core region. The results suggest the significance of magnetic shear and a validity of a linear theory on ideal interchange mode. The mode can be stabilized easily by using external resonant field. (author)