Self-organization of equilibria of toroidal plasma in ultra low-q regime

Quasistable toroidal discharges in an ultra low-q (ULQ) regime, ie., 1/2≤qa<1 have been realized and sustained longer than the resistive diffusion time. The ULQ regime lies in the intermediate range between tokamak and RFP.MHD phenomena in the ULQ discharge are extensively studied by experiments on TORIUT-6 simulated by a 3-D MHD code. Both lead to a consistent understanding of the ULQ physics. The ULQ equibrium is characterized by dq/dr<0, which is self-organized through MHD relaxation promoted by the m=1 kink instability and degraded by the resistive diffusion. ULQ dynamics is a competitive process between the resistive diffusion and MHD relaxation. The former is typical in the tokamak regime and leads to peaked current profile(dq/dr>0). The later is typical in the RFP regime and leads to a broad currnet profile (dq/dr<0). (author)