A Numerical Optimization Study of a Novel Electrospray Emitter Design

The low thrust and high specific impulse of electric propulsion has been brought to the forefront for CubeSat and small spacecraft applications. Electrospray thrusters, which operate via electrostatic principles, have seen much research, development, and application in recent years. The small sizes of the spacecraft that utilize electrospray thrusters has focused development into the miniaturization of this technology to the micro-scale. Miniaturization introduces design challenges that must be addressed, including power supply mass and footprint requirements. This consequence requires investigation into the effects of design choices on the thruster onset voltage, defined as the voltage at which ion emission begins. This study focuses on the second design iteration of novel electrospray thruster technology being developed by the University of Tennessee Space Institute. The thruster design consists of an emitter chip with capillaries, etched into borosilicate glass with an indium tin oxide extraction grid deposited on the top surface, which are fabricated using ultrafast laser machining in the Center for Laser Applications. Emitter capillaries open into wider, cylindrical voids defined as the electrospray cavities. An onset-voltage-optimization study is performed via electrostatic simulations by determining design choices that minimize the onset voltage for three design parameters: the electrospray-cavity aspect ratio, the effects of indium tin oxide erasure as a consequence of the laser machining process, and the choice of an ionic liquid propellant. The results show that electrospray cavities with smaller dimensions lead to a decrease in onset voltage, but optimal aspect ratios can be determined. Erasure of the extraction grid leads to an increase in onset voltage, which is greater for smaller electrospray cavity dimensions, and is most profound for small electrospray cavity heights. Therefore, the amount of erasure should be minimized as much as machining processes will allow, since smaller electrospray-cavity widths and heights are a desirable design choice to minimize onset voltage. The results also show that, although more investigation is required, ionic liquid propellants with lower surface tensions yield smaller onset voltages. This work details the methodology of the optimization study and provides best practices in preliminary electrospray emitter chip design for electrospray thrusters.