Remarkes on Relaxation Phenomena in Toroidal Discharge

Abstract

MHD relaxation phenomena have been studied for a toroidal plasma in a safety factor q<1 regime. Ultra low-q(ULQ) equilibria with (n+1)⁻¹<q<n⁻¹ (n≥1) are set-up through MHD relaxation, and are sustained quasistatically. A ULQ equilibrium has a q profile with dq⁄dr<0 (r: minor radius) for a major region in the plasma, so that it is stable against global MHD modes. In the time-scale of the classical field-diffusion, the ULQ equilibrium deforms to take a peaked-current profile and destabilizes a global mode. Then a catastrophic transition to a higher q(lower n) state is observed.