MHD characteristics in the high beta regime of the Large Helical Device

Abstract

The highest volume averaged beta values beta (t) of 2.2% at B-t = 0.75 T (gas puff) and 2.4% at D-t = 1.3 T (pellet) in helical devices have been achieved in LHD. The beta (t) dependence of MHD activities has been investigated in NBI plasmas. The n/m = 1/2 mode, excited in the core region, and the t = 1 resonant modes, in the peripheral region, have been observed. Both of the fluctuation amplitudes increase with beta (t) and pressure gradient. The strong n/m = 1/2 mode, which affects the plasma profile., has been observed in high beta (t) discharges, and the abrupt disappearance of the mode leads to restoration of the T-e profile. Violent instabilities which terminate the plasma and degradation of global energy confinement have not been observed so far.