Observed instabilities in the trapped ion regime in the levitated octupole

Abstract

Two unstable modes were observed in the average minimum-B region of the levitated octupole operating with a weak toroidal field (safety factor of approximately 0.25) and collisionless ions. One mode propagated in the electron diamagnetic drift direction and the other mode propagated in the ion diamagnetic drift direction. The frequencies of the modes were much greater than the ion detrapping collision frequency and slightly less than the trapped ion bounce frequency. The electron collisionality was intermediate between the collisionless trapped ion mode and the dissipative trapped ion mode regimes. Density fluctuation levels from 5% to 50% and enhanced transport approaching Bohm diffusion were the manifest effects of the instabilities. When the ion collisionality was increased by cooling the ion temperature, the unstable fluctuations were not seen.