Imaging of Instabilities in a Pure Electron Plasma

Experiments on three different instabilities in a nonneutral plasma are presented. The investigation of these instabilities was made possible by a novel, microchannel plate-based imaging system designed for the pure electron plasma trap at U.C. Berkeley. This system yields two -dimensional images with a high resolution and a large dynamic range. The expected performance and actual operational characteristics of the system are detailed, with special attention given to the properties of a microchannel plate used in a pulsed imaging mode. The diocotron instability affects hollow, cylindrically shaped pure electron plasmas. Since these plasmas behave like two-dimensional fluids with low viscosity, their evolution parallels the evolution of two-dimensional fluid vorticity annuli which are subject to the Kelvin-Helmholtz instability. Initially, a perturbation to the annular plasma shape grows exponentially, in agreement with theoretical models for the appropriate fluid and plasma systems. The formation and subsequent interaction of vortices caused by the instability is highly dependent on the initial plasma geometry. Small initial perturbations can dramatically increase the symmetry and repeatability of the dynamics. We report the first observation of the winding instability which affects asymmetric, annular plasmas. The asymmetric annulus undergoes a complex evolution which is quite different from that of a symmetric annulus. During the first, "active", phase, the asymmetries grow until the annulus collapses, leaving a large vortex at the device center. In the next, "passive", phase, the remainder of the annulus passively winds around this central vortex into an ever tighter spiral. Finally, slow shear instabilities destroy the structure of the highly evolved spiral. The first direct observation of the ion resonance instability in a pure electron plasma trap contaminated with a small population of ions is reported. The ion population is sustained by ionization of the background gas by energetic plasma electrons. The instability causes the plasma to move steadily off-center while undergoing l = 1 diocotron oscillations. The observed scaling of the parameter values at which maximum growth occurs and the dependence of growth rate on ion density are both discussed. Several aspects of the observed behavior are not in agreement with previous theory but are explained as the result of the transitory nature of the ion population.