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 ALFA ² 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.