Axial Translation of Field-Reversing Relativistic Electron Rings.

This thesis work concerns an experimental study and related theoretical analyses on the axial translation of field-reversing relativistic electron rings. The experiments represent the first serious test of the moving ring concept which has been incorporated into a number of magnetic fusion energy reactor schemes. In addition to the basic physics knowledge gained, several programmatic advances were obtained as a consequence of these experiments: (1) rings were generated for the first time in a low pressure ambient neutral gas ((TURN)10 mTorr H(,2) and D(,2)), increasing their collisionally limited field-reversal times to over 1 millisecond or more than five times over that previously observed; (2) the first experimental test of adiabatic magnetic compression resulted in greater than factor of ten increases in the ring kinetic energy densities; (3) two axially separated non-field-reversed rings, generated from a single accelerator pulse, were successfully combined or "stacked" to form one field-reversed ring. In particular, the achieved translation and subsequent ring compression constitute a major step towards future plasma heating and confinement studies with these rings. This thesis is divided into three major parts: theoretical background, experiments, and a quantitative comparison. The theoretical background consists of two calculations of the axial retarding forces that were found to greatly influence the ring motion. First, the force due to ring -induced image currents in the surrounding cylindrically symmetric, resistive wall is calculated as a function of ring geometry, current strength, axial speed, and wall resistivity. Second, calculations are performed of the force due to similar currents driven in the ring-created background plasma. To understand the magnetized plasma current response to the moving ring, tensor electrical conductivity model is developed. The experiments were performed using the Cornell RECE-Christa device. Following descriptions of the E-beam accelerator, Christa apparatus, and the associated diagnostics, two major sets of data are then presented. First, results from experiments involving ring translation by external magnetic field gradients into low pressure H(,2) and D(,2) regions are summarized; discussed here are the important programmatic advances mentioned earlier. Second, results from detailed translation studies are presented. Ring axial velocities, which were measured to be 10('6)-10('7) cm/sec, were found to be very dependent on a variety of parameters such as ring current and geometry, accelerating force, wall resistivity, gas species and pressure, and the background plasma density. In the final part, a quantitative analysis of the translation data is made using the retarding force calculations. In all cases, the rings moved axially at the terminal speed associated with a balance between the accelerating and retarding forces. It is found that conditions existed where the major contribution to the retarding force was due to either the resistive wall or plasma currents. The wall (plasma) force dominated when the rings were moved through the low (high) pressure background gas and inside of the higher (lower) conductivity wall. Good qualitative and quantitative agreement between theory and experiment is obtained for both of these limiting situations, in addition to several intermediate situations where the contributions from the two retarding force components were about equal.