ElECtRosPRAY thrusters electrostatically emit and accelerate ions and/or charged droplets for thrust in the Isp range of 150–6000 seconds. A common choice of propellant for electric propulsion applications is the ionic liquid 1-ethyl-3-methylimidazolium bis (triflouromethylsulfonyl) imide (EMI-Im), which exhibits the favorable properties of low vapor pressure, high conductivity, and low viscosity. Electrospray devices offer unique benefits including passive propellant flow, unpressurized propellant storage, and a wide range of thrust-to-power ratio for devices. The primary drawback of electrospray thrusters is the short lifetimes of existing devices. 1 Current efforts at the University of California, Los Angeles (UCLA) Plasma and Space Propulsion Laboratory (PSPL) are focused on investigating lifetime and performance of electrospray thrusters, 2 specifically the Colloid Microthruster Technology (CMT) for the Laser Interferometer Space Antenna (LISA) project.

As part of this effort, several mechanisms contributing to lifetime reduction of electrospray devices have been identified and are presented in. 1, 3–6 The primary concern for electrospray lifetime is overspray, where propellant at wide plume angles impinges on the extractor and accelerator grids/electrodes. Addressing this failure mechanism requires a rigorous understanding of the plume profiles, specifically the mass flux and current density profiles. While electrospray plume current density profiles for devices operated in dropletmode using EMI-Im propellant have been previously measured, 7 the existing data are not resolved to high half-angles that are of significance for lifetime investigation efforts. Additionally, the only measurements of mass flux of EMI-Im were conducted for an externally-wetted emitter operated in a low flow rate regime, 8 and indicated negative flux beyond 20◦ in the high half-angle region that is of significance for lifetime investigation. Using data-fitting techniques on these measurements to extrapolate the properties at high half-angles has proved challenging as uncertainty overwhelms the data fit values beyond the range of the experimental data. Current density and mass flux measurements in the large half-angle regions (impingement regions) can greatly improve understanding of electrospray performance and lifetime. The objective of this study is to obtain and analyze charge-and mass-flux measurements of an ionic liquid electrospray plume to investigate implications on thruster performance and lifetime.