DYNAMICS OF PLASMA DISCHARGES USED FOR SPACE PROPULSION

Hall thrusters are a mature form of electric propulsion for spacecraft well suited for north-south station keeping, orbit transfer maneuvers, and eventually deep space travel. Despite decades of development, producing propulsion systems with high operating efficiency and specific impulse, key aspects of the physics governing these complex devices remain poorly understood and fundamental experimental and computational studies persist. The collection of plasma oscillations, instabilities, and turbulent fluctuations present in Hall thrusters make for rich electron and ion dynamic behavior that is the subject of this investigation. Laser diagnostics of plasmas avoid perturbing the discharge (unlike physical probes), and recent advances in diode laser technology allow for very narrow spectral lineshapes to be easily interrogated with continuous-wave diode lasers. This work concerns the development and implementation of laser-induced fluorescence diagnostics for probing time-resolved dynamics in systems with a quasi- periodic forced or naturally occurring oscillation. Relaxing the requirement of a fixed frequency spectrum over time distinguishes the sample-hold and fast switching LIF methods studied here from others currently in use. The methods are validated against a collisional-radiative model of a 60 Hz xenon test discharge, successfully predicting the time evolution of excited xenon state populations. A statistical consideration of the influence of a drifting frequency and amplitude spectrum in the time-resolved methods is presented. The sample-hold method is then applied to studying xenon neutral and ion dynamics and trans- port in two Hall thruster discharges: a laboratory model in the Stanford Plasma Physics Laboratory similar to the SPT-100 class of devices, and a commercial Busek Co. BHT-600 Hall thruster at the Air Force Research Laboratory at Edwards Air Force Base, California. Parallelizing the data acquisition greatly increases data collection efficiency, allowing for full time series of LIF traces with 28 points of temporal resolution to be obtained at a single spatial location in three 8–10 minute laser scans. Time-resolved ion velocity measurements in the channels and near-field plumes of these thrusters reveal propagating propellant ionization and acceleration fronts undergoing periodic mo- tion that are fundamentally linked to the breathing mode ionization instability common to such devices. The presence of low ion velocity populations exhibiting different dynamic behavior than the main accelerated population is likely due to residual ionization of neutral propellant in the near- field plume of the thrusters. A study of the poorly understood central jet in the BHT-600 also reveals new details about the asymmetric nature of this plasma feature in both space and time. Finally, the measured ion dynamics in the Stanford Z-70 thruster are compared with those predicted by a two-dimensional hybrid particle-in-cell simulation of the device with a dynamically updating electron mobility modeled after the turbulent dissipation of energy in the electron fluid. Somewhat similar modulations in ion velocity are observed in the simulation results, but the limits of these types of simulations in tracking accurate time-resolved behavior are revealed. Comprehensive time-resolved studies like these presented here will both increase our theoretical understanding of the complex dynamics of these thrusters and improve our ability to accurately model and eventually predict correct discharge physics in simulations, reducing costly test and development time