The highest volume averaged beta values β t of 2.2% at B t = 0.75 T (gas puff) and 2.4% at B t = 1.3 T (pellet) in helical devices have been achieved in LHD. The β t dependence of MHD activities has been investigated in NBI plasmas. The n/m = 1/2 mode, excited in the core region, and the ι - = 1 resonant modes, in the peripheral region, have been observed. Both of the fluctuation amplitudes increase with β t and pressure gradient. The strong n/m = 1/2 mode, which affects the plasma profile, has been observed in high β 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.
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Sakakibara et al. (2001) studied this question.