Disruptions, disruptivity and safer operating windows in the high-β spherical torus NSTX

This paper discusses disruption rates, disruption causes and disruptivity statistics in the high-βN National Spherical Torus Experiment (NSTX) (Ono et al 2000 Nucl. Fusion 40 557). While the overall disruption rate is rather high, configurations with high β_N, moderate q^*, strong boundary shaping, sufficient rotation and broad pressure and current profiles are found to have the lowest disruptivity; active n = 1 control further reduces the disruptivity. The disruptivity increases rapidly for q ^* < 2.7, which is substantially above the ideal MHD current limit. Under quiescent conditions, q_min > 1.25 is generally acceptable for avoiding the onset of core rotating n = 1 kink/tearing modes; when EPM and ELM disturbances are present, the required q_min for avoiding those modes is raised to ∼1.5. The current ramp and early flat-top phase of the discharges are prone to n = 1 core rotating modes locking to the wall, leading to a disruption. Small changes to the discharge fuelling during this phase can often mitigate the rotation damping associated with these modes and eliminate the disruption. The largest stored-energy disruptions are those that occur at high current when a plasma current ramp-down is initiated incorrectly.