Cross-field diffusion and fluctuation spectra in a levitated octupole in the presence of a toroidal field

The diffusion coefficient D⊥ for a collisionless hydrogen plasma was measured in the levitated octupole when a weak toroidal field was added. A twenty-fold decrease in the anomalous diffusion was observed with Bt/Bp≳0.1. This reduced level of diffusion was still two orders of magnitude larger than the classical value and the residual level of convective cell activity was responsible for the enhanced transport. Locally trapped particles in the poloidal field mirrors allow the convective cell activity (f<600 Hz) to persist even when the toroidal field was added. The principal effect of the added toroidal field was observed to be a localization of the convective cells in the region where particles are trapped and a shift in the k⊥ spectrum to shorter wavelengths.