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

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