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Research Thrusts |
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Engineering Education Innovation
As the world continues to change with globalization and technological advances so must engineering education p...
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Advanced Power System
In the face of an impending energy crisis, the Advanced Power Systems research center is exploring alternative...
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Space Systems
The Space Systems Research group is creating innovative electric propulsion systems to make space travel more ...
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Mechanics of Multi-scale Materials
The Mechanics of Multi-scale Materials research group uncovers the relationships of structures across the full...
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Multi-scale Sensors and Systems
The Multi-scale Sensors and Systems Research Group specializes in the design, fabrication, integration, and te...
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Sustainable Manufacturing and Design
Many of the campus research efforts on sustainability are coordinated by the Sustainable Futures Institute (SF...
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Research Projects
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A Vaporizing Liquid-metal Anode for High-power Hall Thrusters |
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Project Date:
2006 |
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Sponsor:
Air Force Office of Scientific Research
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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.
Bismuth has many attributes that make it well suited for development as a propellant. When compared
to more traditional propellants such as xenon, bismuth holds significant advantages. Attractive physical attributes follow from the atomic structure and size of bismuth atoms. Bismuth is significantly more massive then xenon (209 amu vs 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 12.1 eV yielding a cost of 0.092 eV/amu . Density is also an important advantage. Since bismuth is a solid at standard conditions the of the "propellant tank" can be reduced and it need not be a pressure vessel.
Beyond physical advantages, the economics of using bismuth is also of critical interest. For instance, bismuth retails for about $8/kg as opposed to $7,700/kg for xenon which translates to a huge savings in propellant cost: a negligible $110-per-day to operate a 50-kW Bi thruster compared with $106,000-per-day for an equivalent xenon device (without reclaimation). There are significant ground-test facility cost savings as well, as bismuth doesn't require the use of expensive cryogenic pumps. Since bismuth is a solid at room temperature, any exhausted bismuth will hit the tank wall and solidify, turning the entire vacuum chamber into an effective pumping surface. Additionally, the layer of bismuth that is deposited on the chamber walls will also absorb some of the residual gas. With that in mind, operating a 50kW bismuth hall thruster would require only enough pumping speed to keep up with the cathode mass flow (assuming a xenon cathode) Although immature as a Hall thruster propellant, the use of bismuth is not without precedent. Soviet work performed in the 1980's and only recently reported in the open literature evaluated bismuth anodelayer thrusters. TsNIIMASH researchers reported thrusters with power up to 140 kW and specific impulse as high as 8,000 seconds operating with anode efficiencies exceeding 70%. Papers reporting on this early work give few details on bismuth flow control method or apparatus.
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