Electrospray Facility Effects and Their Impact on Life-Limiting Mechanisms

Electrospray, a non-plasma-based form of electric propulsion is not immune to the facility effects seen with plasma-based thrusters. While electrospray facility effects may be inconsequential with regards to performance, they can be substantial enough to dominate life-limiting mechanisms. Busek produces multiple types of electrospray thrusters that operate from droplet-dominated emission to ion-dominated emission and through similar phenomena, both suffer from facility effects that can dictate thruster life.  For droplet mode thrusters, where electrospray beam interception on electrostatic grids is typically <0.5%, grid currents measured has been determined to be ~50% accounted for through back-streaming of ion-induced secondary electrons and reflected droplets. Back-streaming propellant contaminates grids and can result in thruster failure. With high specific impulse thruster variants, back-streaming of droplet mass is less of an issue; however, sustained glow discharges are commonly observed at the extraction grid plane and even coupling to the target if placed at short distance. Busek has developed electrospray beam targets tailored to suppression of the properties of the incoming beam species and performed detailed measurement of ion-induced secondary electron yield from thruster grid and target materials. Data collected is intended to better inform life models that aim to predict thruster life when direct testing of required mission life plus the typical 50% margin, 6 years of continuous operation, is problematic and cost prohibitive.