Suppression of the n = 2 rotational instability in field-reversed configurations

Compact toroid plasmas formed in field-reversed theta pinches are generally destroyed after 30--50 μsec by a rotating n = 2 instability. In the reported experiment, instability is controlled, and the plasma destruction is avoided in the TRX-1 theta pinch through the application of octopole magnetic fields. The decay times for loss of poloidal flux and particles are unaffected by the octopole fields. These decay times are about 100 μsec based on inferences from interferometry and excluded flux measurements. The weak, rotating elliptical disturbance (controlled n = 2 mode) also made possible a novel determination of the density profile near the separatrix using single-chord interferometry. The local density gradient scale length in this region is found to be about one ion gyrodiameter