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Completed TRL 3 (started at 2, targeting 3)
High specific impulse (Isp) electrospray propulsion is an enabling technology for many future NASA missions. Since electrosprays produce thrust by applying an electric field to a conductive liquid propellant, which extracts and accelerates charged droplets or ions, they can provide unprecedented levels of thrust precision and efficiency. An electrospray that operates in the purely ionic regime can achieve high Isp and is referred to as an Ionic Liquid Ion Source (ILIS). The low extracted current of a single ILIS emitter (~0.5-5uA) means that it produces extremely low thrust, requiring the multiplexing of many emitters to achieve performance requirements. Large-scale manufacture of high spatial density emitter arrays can be achieved through computer numerical control (CNC) machining of a porous glass material. To meet NASA’s high-Isp micropropulsion requirements, the lifetime of an ILIS electrospray thruster must exceed 500 hours, maintain an Isp greater than 1500 s, and consume less than 10 W of power. ILIS propulsion is well suited to achieve these requirements given its scalability, high thrust density, and low power.
The objective of my proposed research is to advance the Technology Readiness Level (TRL) of electrospray thrusters by investigating the underlying phenomenon that causes anomalous neutral mass loss, which detrimentally affects high-Isp electrospray thruster performance. Electrospray ion-induced secondary electron emission (ESI-SEE) is a likely root cause of the decomposition and outgassing of ionic liquid propellant. In addition to causing neutral mass loss, ESI-SEE may also result in several other effects detrimental to thruster life and performance. Backstreaming electron current produces a presently unquantified error in actual emitted ion current, which is critical for evaluating thruster performance. Precipitate left after decomposition of propellant clogs porous emission sites, eventually resulting in thruster end-of-life. Secondary electrons may also contribute to neutralization of charged constituents throughout the thruster’s plume. Neutral particle flux is ignored by all reported plume divergence measurement methods, which rely on the angular distribution of plume current flux. Neutral propellant transport will produce error in plume divergence measurements, which also strongly affects overall thruster performance.
I propose developing an experiment apparatus for obtaining the first secondary electron yield measurements for electrosprayed ionic liquid species impinging on conductive materials. The results of this experimental effort will provide quantification of an important physical phenomenon that will enable propulsion researchers to develop improved thrusters, diagnostics, and facilities for assessing electrospray thruster performance.
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