Perpendicular Fast Ion Generation in Toroidal Plasmas
Abstract
This thesis research is concerned with the generation of fast ions in a wide variety of toroidal plasmas for which direct resonant ion heating (e.g. ICRH) is not expected to occur. Among the various types of plasmas in which this phenomenon is observed are: (1) RF-produced plasmas in the frequency range omega_{ rm LH} < omega < omega_{ rm ce}, omega_{rm pe }; (2) ECR-produced discharges; (3) low density ohmic discharges; and (4) RF-heated tokamak plasmas in which Omega_{rm ci} <<omega < omega_{rm LH}.. A scannable fast neutral spectrometer was developed to study the spatial and pitch-angular dependence of these ions. It is found that fast ions ({bf E}_{rm i}/{bf T }_{rm i} ~ 100) with large ratios of perpendicular-to-parallel energy ({bf E}_|/{ bf E}_parallel ~ 100) are present in the RF-, ECR-produced and low density ohmic plasmas. Similarities are noted between these results and observations of ion conics in the Earth's magnetosphere. Measurements of the spatial dependence of the neutral flux indicate that the source of fast ions is localized about the upper-hybrid layer in the ECR-produced plasmas, and is distributed throughout the plasma volume in the RF-produced discharges. Using this information, a model of the ion distribution function is developed, for which the computed angular dependence of the neutral flux is in good agreement with the data. Superthermal ions ({bf E} _{rm i}/{bf T}_ {rm i} ~ 20) are also observed in the RF-heated tokamak discharges, but the ion distribution is nearly isotropic with respect to pitch angle. It is argued that, due to the rapid rate of collisional relaxation expected in a tokamak plasma, the observation of near-isotropy of the neutral flux does not exclude the possibility that perpendicular ion acceleration also occurs in this case. A series of exploratory studies have been performed to elucidate the mechanism of perpendicular fast ion acceleration. The results of these studies suggest that a universal mechanism exists that allows for preferential acceleration of ions perpendicular to the magnetic field direction in plasmas in which waves and/or instabilities are present, but its exact nature has not yet been determined.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1992
- Bibcode:
- 1992PhDT........28E
- Keywords:
-
- FAST IONS;
- Physics: Fluid and Plasma; Physics: General; Physics: Atmospheric Science