Ionic Liquid Electrospray Beam Target Performance Characterization

Electrospray thruster technologies have been of interest to the space propulsion community for over a decade. The term is broadly used to describe propulsion technologies including colloid thrusters, ionic liquid ion sources, and liquid metal ion sources. At a top level, these technologies operate by the same mechanism – a strong electric field (~109 V/m) is applied to extract charged particles from emission sites to produce useful thrust. This technology offers promise for use on small satellites and for precision pointing on conventional satellites in a distributed manner due to the small form factor and mass (< 5 kg) of the complete system and low power consumption (< 50 W). While there has been substantial work to understand the beam physics and plume dynamics and the implications thereof on performance and lifetime1, there is a gap in an understanding of facility effects. Interactions with an electrospray plume and the vacuum chamber test facility has implications on both performance and lifetime. Therefore, any effort to characterize an electrospray thruster must be done so with an understanding of how the facility plays a role. In some ways, this is no different than the significant investment that has been made to understand the facility effects for the testing of plasma thrusters. However, there are different challenges involved with the management of positively charged, negatively charged, and neutral particles across a distribution of particle charge and mass as is observed when testing the technologies in a vacuum chamber. The focus of this paper is a characterization of the secondary particles from the impact with an electrospray beam target for ionic liquid thrusters. As part of this work, a purpose-built electrospray beam target was fully characterized. Results on secondary current and mass flux is presented as well as some initial results on secondary time-of-flight measurements. Additionally, modeling results are presented to inform the experimental observations. The results show that mitigating facility effects, while a challenge, can be done with the appropriately designed and biased beam target. If no consideration of facility effects is accounted for when testing electrospray thrusters, performance, reliability, and lifetime can be adversely affected and premature thruster failure may result. The work presented here improves our test capabilities to responsibly and successfully qualify and acceptance test electrospray thrusters for flight for long duration missions.