Self-Healing Field-Emission Neutralizers for Electric Propulsion

Electric propulsion (EP) thrusters have the potential to enhance or enable Discovery-class missions. However, a significant challenge in scaling micro ( 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.

The devices may find use in other space- and earth-borne applications requiring low-power, cold electron emission in environments that would be hostile to traditional Spindt-type emission cathode. Re-generable field emitters could be used in electron microscopy, electron beam lithography, flat-panel video displays, etc.

The proposed neutralization technique would be especially well suited (but not limited) to use in FEEP systems, where the FEEP ion emitters can be re-purposed as electron emitters for spacecraft neutralization. In this arrangement two identical FEEP thrusters could share duties, with one performing neutralization and the other ion emission swapping roles when the electron field-emitters need to be re-generated, and greatly reducing system complexity. Furthermore, the architectural similarity between FEEP thrusters and the proposed neutralizer technique could enable a unique hybrid system: an array of emitters could be operated in electron-emission mode as the neutralizer for a meso-scale Hall or ion thruster during portions of a mission requiring high thrust-to-power, then switched over to ion-emission mode to function independently as a FEEP thruster when high Isp is desired.

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