Electric propulsion (EP) thrusters have the potential to enhance or enable Discovery-class missions. However, a significant challenge in scaling micro (< 100 W) EP devices up as well as scaling macro (> 1 kW) EP devices down is the lack of a compatible neutralizer technology in the meso scale ( < 1 kW). Traditionally, the technology used for spacecraft neutralization has been the hollow cathode, though hollow cathodes require an unsatisfactory fraction of a propulsion system's propellant and power in the meso-scale regime. In fact, they require such a large amount of propellant and power that system efficiency is reduced by 50-100%. In addition, using a hollow cathode causes undesirable specific impulse reduction. The most promising technology for meso-scale neutralizers is field emission (FE), which requires the use of nano-scale sharp emitters and high electric fields to establish a beam of electrons. The drawback of FE devices is that the nano-scale emitters become damaged when operated in elevated pressure environments (10-5 Torr), causing catastrophic failure. The research proposed here is to develop field-emission cathodes for use in meso-scale EP that eliminate tip degradation not through attempts to minimize tip wear, but instead by incorporating self-assembling nanostructures that can repeatedly re-generate damaged emitter tips in space and fully restore the functionality of a damaged or degraded cathode. The procedure is the equivalent of having a MEMS fabrication and repair lab on-board the spacecraft. The re-generable emitters proposed here have been successfully demonstrated in the laboratory in work by Makela, et. al. dating back to 2007. The re-generable neutralizers could enable highly efficient, high-Isp, low-mass propulsion systems operating between a few Watts and 1 kW by either scaling existing micro technologies up or scaling existing macro technologies down.
