A Multiple-Cathode, High-Power, Rectangular Ion Thruster Discharge Chamber of Increasing Thruster Lifetime
Ion thrusters are high-efficiency, high-specific impulse space propulsion systems proposed for deep space missions requiring thruster operational lifetimes of 7-14 years. One of the primary ion thruster components is the discharge cathode assembly (DCA). The DCA initiates and sustains ion thruster operation. Contemporary ion thrusters utilize one molybdenum keeper DCA that lasts only ~30,000 hours (~3 years), so single-DCA ion thrusters are incapable of satisfying the mission requirements. The aim of this work is to develop an ion thruster that sequentially operates multiple DCAs to increase thruster lifetime. If a single-DCA ion thruster can operate 3 years, then perhaps a trip le-DCA thruster can operate 9 years. Initially, a multiple-cathode discharge chamber (MCDC) is designed and fabricated. Performance curves and grid-plane current uniformity indicate operation similar to other thrusters. Specifically, the configuration that balances both performance and uniformity provides a production cost of 194W/A at 89% propellant efficiency with a flatness parameter of 0.55. One of the primary CDC concerns is the effect an operating DCA has on the two dormant cathodes. Multiple experiments are conducted to determine plasma properties throughout the MCDC and near the dormant cathodes, including using "dummy" cathodes outfitted with plasma diagnostics and internal plasma property mapping. Results are utilized in an erosion analysis that suggests dormant cathodes suffer a maximum pre-operation erosion rate of ~5-15 ?m/khr (active DCA maximum erosion is xxviii 70 ? m/khr). Lifetime predictions indicate that triple-DCA MCDC lifetime e is approximately 2.5 times longer than a single-DCA thruster. Also, utilization of new keeper materials, such as carbon graphite, may significantly decrease both active and dormant cathode erosion, leading to a further increase in thruster lifetime. Finally, a theory based on the near-DCA plasma potential structure and propellant flow rate effects is developed to explain active DCA erosion. The near-DCA electric field pulls ions into the DCA such that they bombard and erode the keeper. Charge-exchange collisions between bombarding ions and DCA-expelled neutral atoms reduce erosion. The theory explains ion thruster long-duration wear-test results and suggests increasing propellant flow rate may eliminate or reduce DCA erosion.
J. L. Rovey, "A Multiple-Cathode, High-Power, Rectangular Ion Thruster Discharge Chamber of Increasing Thruster Lifetime," University of Michigan, Jan 2006.
Mechanical and Aerospace Engineering
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