Ionic liquid electrospray beam target performance characterization.

Abstract

Electrospray thruster ground testing, with well understood facility effects, is of critical importance to qualify the technology for long duration flight missions. While there has been substantial work to understand the beam physics and plume dynamics of electrospray thrusters and the implications thereof on performance and lifetime, work to understand the impact of facility effects has been neglected until recently. Interactions between an electrospray plume and the vacuum chamber test facility have implications on both performance and lifetime. Therefore, any effort to characterize electrospray thruster performance and lifetime must be done so with an understanding of facility effects. In some ways, this is no different than the significant investment that has been made to understand the facility effects for plasma thruster testing. However, there are different challenges with the management of positively charged, negatively charged, and neutral propellant particles across a distribution of particle charge and mass when testing electrospray thrusters in a vacuum chamber. The focus of this paper is to characterize the significance of secondary particles from the impact of ionic liquid electrosprays with a beam target, and the influence of a novel beam target design and biasing. Results on secondary current and mass flux measurements are presented with some initial results on secondary time-of-flight measurements from the beam target. Additionally, beam target modeling results are presented to support the experiments and interpretation of the results. The results revealed secondary particles with an average charge-to-mass ratio as low as 31 C/kg, and that an improperly biased beam target, or no beam target, can artificially inflate emitted current due to electron back streaming by as much as 20%. The experimental and modeling results suggest an optimized beam target and screen voltage of -100 V and -200 V, respectively. If no consideration of facility effects is included in testing electrospray thrusters, performance, reliability, and lifetime can be adversely affected, and premature thruster failure may result. The work presented here improves our understanding of facility effects and our capabilities to mitigate them to successfully qualify and acceptance test electrospray thrusters for flight.