Experimental Studies on AN Annular Nonneutral Electron Plasma

The properties of an annular pure electron plasma are examined experimentally. The electron plasma is confined radially by a uniform axial magnetic field and axially by electrostatic potentials. A conductor is placed on the axis of the machine to provide for plasma stability and increased plasma density. The low frequency electrostatic modes (plasma waves and the diocotron instability) are explored. In general, the plasma is unstable to the diocotron instability. However, this instability can be stabilized by placing the correct bias on the central conductor (relative to the outer wall), imposing axial variations in the boundary conditions, or creating shear in the magnetic field. The process of diocotron mode saturation is studied with indirect and in situ diagnostics. The nonlinear vortex states formed by the diocotron instability are observed with real-time direct measurement probes and computer simulations. The stability and lifetimes of these vortex states are determined. It is found that low azimuthal mode number states collapse rapidly onto the central conductor, via wave induced transport. High mode number states evolve slowly, dominated by diffusion and mild wave transport. Two types of vortices are observed; the expected incompressible vortices and under some conditions, highly compressible vortices. The conditions required to form compressible vortices are studied in detail. The plasma confinement time and transport processes are examined under a wide variety of conditions. The plasma confinement time is dominated by neutral collisional diffusion and can be explained by classical transport theory. The maximum density that can be contained in the device is calculated and verified experimentally. Densities well in excess of what can be contained in a typical Penning trap can be achieved in this configuration. The process by which an over-dense plasma relaxes to a stable plasma below the Brillouin limit is explored with computer simulations and experimental measurements.