Characteristics of Ultra-Low q Plasmas in the OHTE Device

The characteristics of plasmas with the safety factor q between that of tokamaks and that of reversed field pinches were studied on the OHTE device, with toroidal fields of up to 3.9 kG and plasma currents of up to 140 kA. Plasmas in this regime (0 < q < 1) are called ultra-low q plasmas. The discharges are globally stable and last much longer than the resistive decay time of the current. A unique feature of most discharges is a series of plateaus in the current (or equivalently q) as the current decreases in a 'staircase' fashion. The plateau q is related to the major rational ratios (1/4, 1/3, 1/2, 1, etc.), suggesting plasma self-organization. Profile measurements show that the q-profile is somewhat different from those of the Taylor relaxed states. The soft X-ray temperature measurement indicates that the plasma has burned through the radiation barrier.