Characterization of disruption halo currents in the National Spherical Torus Experiment

This paper describes the general characteristics of disruption halo currents in the National Spherical Torus Experiment (Ono et al 2000 Nucl. Fusion 40 557). The commonly observed types of vertical motion and resulting halo current patterns are described, and it is shown that plasma discharges developing between components can facilitate halo current flow. The halo current fractions and toroidal peaking factors at various locations in the device are presented. The maximum product of these two metrics for localized halo current measurements is always significantly less than the worst-case expectations from conventional aspect ratio tokamaks (which are typically written in terms of the total halo current). The halo current fraction and impulse is often largest in cases with the fastest plasma current quenches and highest quench rates. The effective duration of the halo current pulse is comparable to or shorter than the plasma current quench time. The largest halo currents have tended to occur in lower beta and lower elongation plasmas. The sign of the poloidal halo current is reversed when the toroidal field direction is reversed.