Vertical stability, high elongation, and the consequences of loss of vertical control on DIII-D

摘要

Recent modifications to the vertical control system for DIII-D has enabled operation of discharges with vertical elongation (kappa), up to 2.5. When vertical stability is lost, a disruption follows and a large vertical force on the vacuum vessel is observed. The loss of plasma energy begins when the edge safety factor q is 2 but the current decay does not begin until q (approximately)1.3. Current flow on the open field lines in the plasma scrapeoff layer has been measured and the magnitude and distribution of these currents can explain the observed force on the vessel. Equilibrium calculations and simulation of this vertical displacement episode are presented. 7 refs., 4 figs.