The highest volume averaged beta values βt of 2.2% at Bt = 0.75 T (gas puff) and 2.4% at βt, = 1.3 T (pellet) in helical devices have been achieved in LHD. The βt 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 βt and pressure gradient. The strong n/m = 1/2 mode, which affects the plasma profile, has been observed in high βt 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.
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