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Abstract
A pure electron plasma is confined in a Malmberg-Penning trap and its confinement and stability properties studied. Of particular interest are the effect collisions between plasma electrons and background neutral gas atoms have on the plasma confinement and the m = 1 diocotron mode evolution. An understanding of the electron-neutral interactions and its effects on the plasma may allow the development of a pressure sensor in the ultra-high vacuum regime if the collision cross-sections are known, or alternatively, of measuring collision cross-sections if the background gas density is known.
The Electron Diffusion Gauge (EDG) was developed to study the effect of electron-neutral interactions on the plasma dynamics. First, a reproducible and well diagnosed pure electron plasma is obtained. Techniques for forming a reproducible electron plasma from a thoriated tungsten filament are demonstrated. Diagnostics for measuring the axially integrated plasma density and the diocotron mode are explained, with careful attention paid toward the method and accuracy of calibrations. Lastly, the density distribution is numerically reconstructed using the experimental measurements, and its accuracy is verified.
The evolving radial density profiles have been measured to determine the plasma expansion rate and changes in the electrostatic field energy. Profiles are compared to numerically calculated thermal equilibrium density profiles and good fits are often found, depending on the initial plasma formation. The fit of the equilibrium profiles provide estimates of the plasma temperature. The plasma expansion rates are measured as a function of the magnetic field strength and the background gas pressure.
The m = 1 diocotron mode is measured in the EDG device for the first time. The frequency is found to be in good agreement with theoretical predictions which consider a finite-length plasma. The resistive-wall instability has been observed and its growth rates are in good agreement with predictions over five orders of magnitude in resistance and 2.5 orders of magnitude in growth rate. The mode is studied at large, nonlinear amplitudes and a frequency shift is observed, also in good agreement with predictions. Finally, the mode amplitude evolution is found to be sensitive to the background gas pressure down to the base pressures of 5 × 10-10 Torr.
