Radial ion transport in a nonaxisymmetric magnetic mirror

An experimental study of radial ion transport in a nonaxisymmetric magnetic mirror is presented. It is found that the ion confinement time perpendicular to the magnetic field is an order of magnitude shorter than predicted by classical and neoclassical theories, and that radial transport can be the dominant ion loss mechanism. Transport experiments are performed in hydrogen, helium and argon plasmas by measuring equilibrium radial profiles of plasma density, ionization source, end loss current, electric field, electron temperature and ion temperature. The radial profiles of the perpendicular diffusion coefficient (D±) are presented, and range from a radial average of ~ 5 x 10' cmn/sec for hydrogen to ~ 2 x 10' cm 2 /sec for argon. These coefficients are a factor of ten larger than the maximum possible classical and neoclassical diffusion coefficients in all three gasses. The effect of low frequency RF power in the ion cyclotron frequency range on the radial ion transport rate is also investigated. RF power increases the ion perpendicular transport, which then becomes the dominant loss mechanism. With sufficient RF power, the ion perpendicular loss rate exceeds the ionization source, with a resultant loss of plasma equilibrium. Application of RF power increases the radial transport rate of plasmas with resonant ions, which are also heated by the RF waves, as well as plasmas whose ion cyclotron resonance is not inside the confinement region. The increased transport rate during application of RF power shows up as an increased D±. This indicates that the radial ion transport is due to a direct interaction between the ions and the RF field, rather than to radial profile changes or the enhanced ambipolar potential which are other RF effects. The effect of RF power on plasma potential is also studied, and is shown to be an electron effect, whose power scaling is consistent with theory. Thesis Supervisor: Richard S. Post Title: Senior Research Scientist, Plasma Fusion Center