ULQ operation and transition to RFP in Extrap T1 with a resistive shell

The modified Extrap T1 device has been run also as an RFP and as an ultra low q (ULQ) configuration. The device now has a stainless steel bellows vacuum vessel and a resistive shell located inside the Extrap octupole field conductor system. The equilibrium field is controlled by the distribution of the currents in the ohmic heating primary winding. The main objective of the present experiments was to study the ULQ regime. These discharges, with toroidal currents in the range 20-50 kA, show the typical stepwise decay of the plasma current. The current steps corresponded to transitions of the edge q-value across rational values. By applying the octupole magnetic field, discharges in a magnetic separatrix configuration were obtained with toroidal plasma current 15-20 kA, q-value in the range 0.3-0.5, conductivity temperature 10-20 eV and duration 1 ms. With a programmed toroidal field RFP discharges with θ = 1.6 and F = -0.2 were obtained. The pulse lengths were up to 0.5 ms, limited by the termination of the reversed current in the toroidal field circuit. The duration is five times longer than the vertical field penetration time of the shell. (author). 3 refs, 2 figs