Ionic liquid ion source emitter arrays fabricated on bulk porous substrates for spacecraft propulsion

Abstract

Ionic Liquid Ion Sources (ILIS) are a subset of electrospray capable of producing bipolar beams of pure ions from ionic liquids. Ionic liquids are room temperature molten salts, characterized by negligible vapor pressures, relative high conductivities and surface tensions lower than water. Compared with the colloid form of electrospray, renowned for its applications to spectroscopy, ILIS yield highly monoenergetic beams composed entirely of ions. In this respect they are similar to Liquid Metal Ions Sources, but offer the ability to emit both positive and negative ions from a benign propellant that remains in the liquid state over a wide range of temperatures. When applied to spacecraft propulsion these sources are very power efficient and yield high specific impulses. Furthermore. the low flow rates and negligible vapor pressures of ionic liquids allow for passive feeding systems which can remain exposed to the vacuum of space. This configuration would remove the need for pressurized propellant tanks or valves, both of which are difficult to miniaturize for small satellites. However; the thrust produced from each emitter is very low, less than 0.1 [mu]N. As a result, compact arrays of active ILIS have been sought since their discovery. If arrays of modest packing density (~ 5 emitters/mm²) could be achieved, ILIS as thrusters would offer a scalable form of propulsion capable of providing useful thrust levels to small satellites with performance comparable to established, but difficult to miniaturize, plasma based ion engines. This research has sought a technique for creating arrays of ILIS from bulk porous substrates as part of an overall process for microfabricating complete thrusters. The thesis includes a survey of potential fabrication methods considering both suitability for forming arrays of ILIS and the ability to integrate each technique within a thruster packaging process. Electrochemical etching is highly selective and can proceed at rates which are limited by mass transport conditions. In this thesis we show how this etching regime can be exploited to smoothly remove material from the surface of a bulk porous metal substrate without damaging the internal pore structure. Dry film photoresists have been identified as a suitable alternative to spin on techniques for porous materials and have been applied within an electrochemical etching process. A two step process for forming arrays of ILIS has been motivated using numerical simulations of the etching process to predict emitter profiles and investigate the impacts of non-uniform etching conditions. These concepts have been applied experimentally using a custom built, automated, etching station capable of repeatedly producing arrays of 480 emitters spaced 500 pm apart on a 1 x 1 cm porous nickel substrate pre-mounted, and aligned, within a silicon thruster package. The emitters are typically 165 [mu]m tall with rounded tips suitable for operation as ILIS. Pulsed voltage conditions were found to significantly enhance wafer level uniformity enabling fabrication of functional emitters within a few hundred [mu]m of the substrate boundary. The structures have been smoothed and rounded, making them suitable for use as ILIS, during a secondary etch process using electrolytes doped with nickel chloride to suppress transient effects. These doped solutions enabled a few [mu]m of material to be removed selectively from the porous surface while maintaining smooth features. These arrays have been mounted and aligned with electrostatic grids to demonstrate their emission capabilities. Propellant has been fed to the emitters by capillarity within the porous bulk and then extracted at potentials as low as 850 V. Beam currents exceeding several 100 [mu]A at both positive and negative polarities have been measured using both EMIIm and EMI-BF₄ ionic liquid propellant. Two complete devices were tested yielding large beam currents and very high transmission fractions (- 88-100 %) from both attempts. We estimate that these devices can supply 10's of [mu]N of thrust at modest operating potentials, ~ 1.5 kV. with a specific impulse of roughly 2000-3000 s. When completely packaged, the thrusters measure 1.2 x 1.2 x 0.2 cm, weigh less than 1 g and require less than 0.65 W of operating power. These characteristics would be ideal for a small satellites where volume, mass and power are all at a premium, while the thrust levels would be sufficient to enable a variety of orbit variation and attitude control maneuvers. For example, applied to a CubeSat, this type of thruster system, including PPU, would occupy roughly 10 % of the spacecraft volume and mass while enabling de-orbiting from an 800 km altitude in roughly 100 days, compared with many years when left to decay naturally.