Characteristics of low-q disruptions in PBX

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Abstract
At low qψ (2.3 ≤ qψ ≤ 4.5), in the Princeton Beta Experiment, the discharges are limited by a hard disruption following the growth and sawtooth-like 'crash' of a ≤25 kHz precursor oscillation. The disruption, which occurs even in discharges with βt well below the first stability regime boundary (2.5 μ0Ip/aBt), follows the crash of this precursor mode either immediately or with a delay of several milliseconds, with the immediate disruptions primarily occurring in the discharges with βt close to the first regime limit. The highest βt discharges also exhibit the fastest growth times and the highest level of edge MHD activity. Associated with the precursor mode crash is a loss of up to 30% of the plasma energy; thus, for non-zero delay shots, it is the crash and not the actual disruption that is the βt limiting process. The delay period is interpreted as a period during which a locked mode, consisting of several toroidal components of comparable amplitude, grows. Because of the energy loss associated with the crash, the plasma goes vertically unstable during the delay period. The results of this study indicate that even within the relatively narrow low-qψ operating space, there is a continuum in the characteristics of the low-q^ disruptions with a primary dependence on the value of βt. While the ideal external kink instability may give rise to the growing oscillations that lead up to the ultimate disruption, the instabilities are weighted towards the edge only at the lowest qψ (≤3) and highest βt. The results of this study indicate that effects outside the scope of ideal MHD theory may play a significant role in low-qψ disruptions.