Characterization of the plasma current quench during disruptions in the National Spherical Torus Experiment

A detailed analysis of the plasma current quench in the National Spherical Torus Experiment (Ono et al 2000 Nucl. Fusion 40 557) is presented. The fastest current quenches are fit better by a linear waveform than an exponential one. Area-normalized current quench times down to 0.4 ms m^-2 have been observed, compared with the minimum of the 1.7 ms m^-2 recommendation based on conventional aspect ratio tokamaks; as noted in previous ITPA studies, the difference can be explained by the reduced self-inductance at low aspect ratio and high elongation. The maximum instantaneous dI_P/dt is often many times larger than the mean quench rate, and the plasma current before the disruption is often substantially less than the flat-top value. The poloidal field time derivative during the disruption, which is directly responsible for driving eddy currents, has been recorded at various locations around the vessel. The I_P quench rate, plasma motion and magnetic geometry all play important roles in determining the rate of poloidal field change.