Tokamak disruptions in LT-4 without precursor oscillations

Results are presented of observations on events leading to major disruptions in the LT-4 tokamak for q(a) approx.= 3. They show that, although steady oscillatory signals are obtained under normal conditions, all oscillatory activity effectively disappears during the predisruptive phase (i.e. when the rapid loss of plasma energy occurs). This behaviour, which exhibits no evidence of any large growing mode, contrasts sharply with the experience in other tokamaks. Although there is evidence that intermittent mode-locking to a stationary frame can occur at other times, it has not been possible to verify the existence of sufficiently large stationary islands, whether growing or not, to explain the process. (author)