Sawtooth triggering of a disruptive 2/1 tearing mode in HBT-EP is controlled by wall stabilization and sawtooth period

Abstract This work characterizes the sudden appearance of a sustained, disruptive 2/1 tearing mode after a sawtooth crash, from a preexisting low-amplitude coupled 2/1–3/1 tearing-kink mode, on the HBT-EP tokamak. Using HBT-EP’s radially movable resistive wall we perform an experiment to show empirically that the onset of the disruption-causing tearing mode is delayed as the wall is moved closer, extending the shot and allowing the plasma current to grow to a higher value before the mode appears and the plasma disrupts. Concurrently, as I p grows, we observe a lengthening of the sawtooth period τ st , which has been previously theorized as a major driver for opening islands. The experimental observations are consistent with this mechanism being important in our dataset in overcoming the increased stabilization from the wall, particularly in the absence of significant changes to single-helicity Δ 2 / 1 ′ .