Extractor electrode clogging in FEEP propulsion and in-space mitigation demonstration

Since the first flight of a Field emission electric propulsion (FEEP) thruster in 2018, more than 270 FEEP based propulsion systems have been launched, with system powers between 40 and 100W, and including a propulsion system version that allows for direct thrust vectoring capability without moving parts. These propulsion systems produce thrust by accelerating electrostatically accelerated ions extracted by Taylor-cone based field evaporation from a passively fed, liquid metal propellant. In addition to ions emitted, this type of emission is known to additionally emit a certain ratio of quasi-neutral droplets, of which a certain fraction accumulates on the extractor electrode, leading to shrinkage of the emitter to extractor spacing over long duration firings if not counteracted. This work introduces a mathematical model to model this shrinkage behavior based on ion emitter parameters. This model is compared to ground test data available and applied to an in-space application. We then present the repeated in orbit demonstration of the previously developed mitigation method to clean the extractor electrode covering a total thrust operation time of approximately 1800 hours of firing, and present design updates to the propulsion systems to enhance cleaning capability.