Electron plasma in a toroidal penning trap

Abstract

Summary form only given. Rugged equilibrium of electron plasmas seen in linear, cylindrical machines are unknown for toroidal traps from a theoretical standpoint, due to lack of invariants. Earlier experiments have successfully demonstrated equilibrium in toroidally continuous traps; however, breaking the toroidal symmetry to create a Penning-trap arrangement, has additional unknown consequences for the equilibrium. So a toroidal Penning trap has been created in a low aspect-ratio device to experimentally investigate the equilibrium, stability and confinement properties in such geometries. Electrons are injected along the field lines with a negatively biased thermionic source placed on a poloidal cross-section. Electrostatic confinement is achieved in the toroidal direction between the source and a negatively biased end-wall placed behind the source. The radial confinement is achieved by means of a pulsed toroidal magnetic field of few hundred Gauss. The trap has therefore an injection-hold-dump mechanism as in a linear penning trap, but the geometry retains the strong toroidal features, especially due to its unique low aspect ratio (R/a/spl sim/1.2.; R and a are major and minor radii respectively). Pressures are maintained at 10/sup -8/-10/sup -7/ torr. The parallel injection process has been studied and characterised for this geometry. Capacitive probes on inner and outer walls, as well as a charge collector on the end wall reveal a long lasting plasma, presently limited only by the duration of the B field (/spl sim/2 ms). Another interesting observation has been of very coherent, periodic oscillations on the wall probes. These are signatures of a k/sub /spl perp// mode-similar to the diocotron mode seen in linear machines-but with interesting toroidal features. The mode is non-linear and unstable, and the harmonics are very phase coherent. The frequency is typically around 100 kHz at 200 Gauss and changes with time. The growth rates of the instability have been found to be strongly dependent on neutral pressure and has a direct bearing on the confinement times too. Other things addressed in our experiments are the role of a resistive wall, role of ion-formation and the possible existence of a hollow profile - all likely candidates for driving the modes unstable. The end-wall, as a charge collector, allows us a new diagnostic to ascertain the lifetime and energy-distribution of such electron clouds in toroidal geometries, results of which will also be discussed.