Experimental observations of spherical plasma oscillations in a positively pulsed inertial electrostatic confinement device

Abstract

A pulsed, positive polarity gridded inertial electrostatic confinement (IEC) device has been investigated experimentally, using a differential emissive probe, current measurements, potential traces and spectral measurements as primary diagnostics. Large amplitude oscillations in the plasma current and plasma potential were observed within a microsecond of the discharge onset, which are indicative of plasma oscillations as opposed to second order circuit (LC) resonance. The contextualisation of these oscillations with existing IEC work to achieve fusion relevant ion motion is the subject of this thesis. Parallels that can be drawn between this work and the periodically oscillating plasma sphere concept are of particular note. The plasma oscillations occur in two regimes. At lower currents associated with the use of series ballast resistance, lower frequency oscillations were induced which are a result of the ion-electron transit time instability about a virtual cathode. The reduction of the ballast resistance to below 100Ω resulted in a high current, high frequency oscillatory mode which we attribute to the saturated Buneman instability. This mode splitting was observed in the non-localisation of the discharge outside of the gridded volume, as well as the induction of high frequency oscillations at the ion plasma frequency in the decay phase of the Buneman oscillations. The complex interplay between particle kinetics and nonlinear spatio-temporal plasma structures that occurs during the transition between the ion transit and Buneman instability modes is still not well understood. However, ion velocity measurements suggest that the periodically oscillating plasma sphere concept may be tenable at the higher pressures, voltages and associated currents studied in this thesis.