Silicon Emitter Needle and Array Design for Indium Electrospray Arrays for Spacecraft Propulsion

Abstract

A design for silicon microfabricated emitter arrays was developed for electrospray thrusters for indium propellant in compact architectures with scalable thrust, low mass and volume, high specific impulse and high efficiency operation. The design elements include tip height, height uniformity across the array, tip radii, axial groove depth, tip angle, emitter shank sidewall angle and number of emitters. They were derived from commercial liquid metal ion source designs and modeling, fabrication and test results. The most critical results of the emitter array design analysis suggest that microfabricated silicon emitter array emitters should have a height greater than 280 microns with a height uniformity of +/-10 microns, a tip half angle of 49° for uniform turn-on voltages and current across the array, low beam divergence and high mass utilization efficiency. Elements of the design were fabricated and demonstrated in single emitter and in 400 element emitter arrays to validate the design, fabrication and performance capability. The design height and uniformity of arrays was demonstrated for 85% of the emitters in a prototype array. Required tip and sidewall angles and groove depths have been microfabricated. Single microfabricated silicon emitters demonstrated better performance in current and voltage characteristics than commercially available liquid metal ion sources. Microfabricated emitters and arrays demonstrated the required current and stability performance to enable the MEP thruster development for indium propellant.