High performance low and high q discharges in DIII-D

Abstract

The high performance H-mode regime on DIII-D has been extended to both low q and high q (high ) operation. In high current operation, VH-mode discharges were obtained for the first time with . These discharges had , H = 2.9, , and %-sec. was improved by approximately 50% over previous results. These discharges were obtained with neutral beam injection during the plasma current ramp up which maintained the axial q above 1. In low current operation, neutral beam heated discharges with 100% of the plasma current from non-inductive sources were obtained at high , with , H = 3.1 and . These discharges represent an extension of the high performance regime to , which was made possible by reduction in the locked mode low density limit, as the result of improvements in the magnetic field error correcting coils. These low current discharges do not exhibit some of the standard signatures of VH-mode, but appear to represent a new regime of improved H-mode confinement. Similar, non-VH-mode, high energy confinement discharges were obtained at low density and moderate q.