Lithium Purification System Development for Long-Life Lithium-Fed Lorentz Force Accelerators
Lithium is an attractive metal propellant for advanced nuclear-electric propulsion missions because its low molecular weight results in high Isp and high conductivity for advanced MPD thrusters. The ALFA2 NASA program investigated a liquid lithium MPD thruster to support a > 5 year mission. Unfortunately, lithium is corrosive to most metals, leeches elements from alloys, destroys dielectric insulators, and is prone to contamination. Therefore, long-duration deep space missions using lithium will only be possible if these adverse effects can be mitigated or eliminated. For the ALFA2 NASA mission, cathode erosion and work function poisoning due to impurities (notably oxygen) was observed and severely limits lifetime. Using a lithium purification and filtration system with highly porous gettering materials in series with cold trap filters is a possible way of reducing lithium contamination levels; thereby improving thruster lifetime. A proof-of-concept experiment was carried out to examine cold filtering and evaluate several candidate getter materials for purification. The long-term goal is to develop an active filtering and purification system for the ALFA2 thruster, or other lithium/liquid metal fed thrusters, to mitigate contamination effects for long-duration (>5 years) space missions. Starfire Industries successfully demonstrated: (1) reduction in impurity content by filtration, (2) regeneration of the filter assembly, (3) getter operation and regeneration, and (4) established a baseline for designing a prototype for a laboratory-scale thruster. These advances increase the regenerable filtration and gettering concept from TRL 1 to TRL 3. In addition, significant mass reduction is realized when compared to getter-only purification schemes that would likely perform poorer than the described system.
R. A. Stubbers et al., "Lithium Purification System Development for Long-Life Lithium-Fed Lorentz Force Accelerators," 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, American Institute of Aeronautics and Astronautics (AIAA), Jan 2007.
Mechanical and Aerospace Engineering
NASA Small Business Innovation Research
Keywords and Phrases
Cathode Erosion; Nuclear-Electric Propulsion; Lithium
Article - Conference proceedings
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