This dissertation, "Development of a Direct Evaporation Bismuth Hall Thruster," is hereby approved in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in the field of Mechanical Engineering-Engineering Mechanics.
Thrust, I sp and efficiency measurements were taken on a segmented Hall thruster designed to run on both bismuth and xenon in order to ascertain the effect anode current attachment as well as anode power density. Overall, very little change in thrust, specific impulse and efficiency were measured across the operating spectrum when running on xenon. Using a unique dual-propellant distributor, this work reports on experiments to use a xenon discharge as a "jump start" mechanism to provide waste heat necessary to initiate direct bismuth evaporation. Using the shim electrodes and magnetic fields for temperature control, the thruster is operated entirely on bismuth after a xenon warm-up stage.
Bismuth metal vapor Hall thrusters may have superior performance and economic characteristics when compared to xenon. From increased efficiency to reduced propellant and testing costs, bismuth seems to have a bright future. Of paramount importance when developing a practical bismuth device is the mechanism by which the propellant flow is controlled. This paper reports on an effort to use waste heat from the thruster to control the evaporation of a reservoir of liquid bismuth maintained within the discharge chamber. Research done thus far indicates that mass flow control can be achieved via a segmented anode configuration that serves as a thermostat to control input power into the bismuth reservoir. Thermal modeling has indicated that sufficient thermal gradients can be maintained between anode segments. Laboratory testing on xenon development thrusters validates the scheme to control reservoir temperature through discharge current sharing.
Using bismuth in place of gases such as xenon for Hall thruster propellant could potentially offer both physical and economical gains. As research continues to develop Hall thrusters that are fueled with bismuth, it will become advantageous to maintain one propellant supply rather than multiple supplies for the anode and cathode. The recent development of a bismuth Hall thruster at Michigan Tech, operated using a xenon LaB 6 cathode, provided a motive to explore the feasibility of developing an entire bismuth system. This paper provides a background on the development and operation of a bismuth vapor LaB 6 cathode. I. Introduction ISMUTH has many attributes that make it well suited for development as a Hall thruster propellant. Attractive physical characteristics follow from the atomic properties of bismuth. Bismuth, with an atomic mass of 209 amu, is significantly more massive than the more traditional xenon (131 amu). The large, heavy atoms thus have a lower neutral diffusion velocity and a larger electron-impact cross-section, resulting in a greater probability of ionization and increased propellant utilization. Not only is the ionization probability greater for Bi than Xe, but the energy cost-per-kg of mass flow to create a bismuth plasma is only 37% that of Xe: Bismuth's first ionization level is 7.3 eV, resulting in an ionization cost of 0.035 eV/amu, compared to xenon's
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