Diffusion and fluctuation measurements in the trapped electron and trapped ion regimes

Abstract

Experiments have been performed in the Wisconsin Levitated Octupole with a weak toroidal magnetic field added to the octupole field. When the ions were collisionless or collisional, the diffusion scaling observed in the absolute minimum-B regions indicated convective diffusion driven by mirror-trapped electrons. When the ions were collisionless, both a dissipative trapped ion mode and a collisionless trapped ion mode were observed in the average minimum-B region. These modes were distinguished by their propagation direction in the plasma frame and were not present without the weak toroidal field or when the ions were made collisional. When these modes were present, the plasma transport was increased by an order of magnitude. Experiments have been performed in the General Atomic dc Octopole in the regimes in which dissipative trapped electron and trapped ion modes are expected to occur. Dissipative trapped electron modes have been observed and studied in detail. The trapped ion regime has been attained by means of rf ion heating at the lower hybrid frequency, and fluctuations that appear to be collisionless ion drift waves driven by ion Landau resonances have been observed in the private flux regions. These ion modes may provide a confinement limit on tokamaks that are operated in parameter ranges for which the dissipative trapped particle modes are unimportant.