Phenomenology of internal reconnections in the National Spherical Torus Experiment

Abstract

The behavior of large scale magnetohydrodynamic modes was investigated in the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] during reconnection events using combined analysis of magnetic probe signals, and soft x-ray data. The comparison of mode dynamics during precursor and disruption stages in T-11M (V. S. Vlasenkov et al., in Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Berchtesgaden, 6–13 October 1976, p. 95) (small circular plasma), Tokamak Fusion Test Reactor [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] (large circular plasma), and NSTX (large spherical plasma) was done. The analysis shows that the sequence of events of minor and major internal reconnection events (IRE) in NSTX is essentially similar to that for disruptions in moderate aspect ratio tokamaks. The main feature of disruption dynamics apparently affected by the small aspect ratio in NSTX appears in the relatively slow thermal quench event (5–10 times longer compared to ordinary tokamaks), which precedes the major IRE. The coincidence of the electron and neutron quench times during the major IRE leads us to the conclusion that the fast ions and hot electrons leave the center of a plasma column simultaneously, i.e., convectively.