Extrap and relaxed discharges in the Extrap T1 device

Experimental results from Extrap Z-pinch and relaxed discharges with an extra toroidal octupole field applied are compared. It was observed that an Extrap Z-pinch discharge that produced no global toroidal field could be maintained for typical discharge times of 0.3 ms when a sufficiently strong octupole field was applied. However for discharge currents above a certain limit, the discharge produced its own toroidal field presumably through the growth of a kink instability. If a bias toroidal field was present when the discharge was initiated, an additional self-generated toroidal field was produced. We call these relaxed discharges. It was observed that the average self generated toroidal field was a constant fraction of the edge poloidal field. The application of the octupole field with various strengths reduced the plasma current but did not change this field ratio. 7 refs, 6 figs