Though xenon is the traditional propellant used for most Hall thrusters and ion engines, its rarity and high cost create the need for alternatives. Bismuth and krypton are suggested as possible replacements for xenon to meet mission requirements.
Computer-based Hall thruster simulations are performed to analyze the performance potential of these propellants and to suggest qualitative optimizations of thruster design and operating conditions. Simulations presented indicate that a thruster optimized for bismuth propellant should have a shorter channel than one optimized for xenon; with a shortened channel, bismuth offers distinct performance advantages over xenon. Simulations of krypton propellant show that with the same mass flow, the performance penalty incurred relative to xenon propellant is likely to be rather small.
To assist in future experimental measurements of the suggested propellants, analytical background work has been done to design optical diagnostics for measuring particle velocities and densities. Atomic resonance absorption spectroscopy is suggested for measuring propellant number densities. Laser-induced fluorescence (LIF) and emission spectroscopy are suggested for measuring velocities. Transition selections are made for each technique, and the likely recorded signals are modeled. For the suggested electronic transitions, the hyperfine splitting has been calculated; though the splitting of krypton transitions is comparably narrow, an understanding of the split profiles for bismuth will be crucial to accurately interpret future measurements.
Noting the particular difficulties associated with the analysis of a metal vapor, a series of apparatuses used to analyze a bismuth plasma are presented. The most useful of these proved to be a heat-pipe apparatus and a linear Hall-like discharge. From the heat-pipe apparatus, the suggested Bi I absorption analysis has been verified and the breakdown of bismuth vapor was characterized. The linear discharge was used to characterize bismuth plasma via various optical and probe-based measurements. Probe-based measurements include profiles of sheath potential, plasma potential, and electron temperature as well as the ion velocity distribution, ion current density, and plasma density at a single point. It is noted that while LIF poses significant difficulties for analysis of a bismuth thruster, high-resolution emission measurements provide a suitable path to optical velocimetry.
|Advisor:||Cappelli, Mark A.|
|School Location:||United States -- California|
|Source:||DAI-B 70/10, Dissertation Abstracts International|
|Keywords:||Bismuth, Hall thrusters, Krypton, Propellants|
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