Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX
Authors: S.P. Gerhardt, R.E. Bell, A. Diallo, D. Gates, B.P. LeBlanc, J.E. Menard, D. Mueller, S.A. Sabbagh, V. Soukhanovskii, K. Tritz, and H. Yuh
Abstract:
This paper discusses disruption rates, disruption causes, and disruptivity statistics in the high- βN National
Spherical Torus Experiment (NSTX) [M. Ono, et al. Nuclear Fusion 40, 557 (2000)]. While the overall
disruption rate is rather high, configurations with high βN , 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. In quiescent conditions, qmin >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 qmin 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 fueling 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 rampdown is initiated incorrectly.
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Submitted to: Nuclear Fusion, (October 2012)
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