Stochastic Emission in Passively Fed Single-Emitter Porous-Media Ionic Liquid Ion Sources

Abstract

Ionic liquid ion sources are a promising form of efficient thrust for power- and mass-constrained satellites. Flight performance requirements necessitate large arrays of emitters; however, the fundamental behaviors of the ion emission process are obscured by the simultaneous operation of multiple emitters. One such behavior is the stochastic nature of Taylor cone formation and ion emission from the emitter. To examine these phenomena, a single-emitter electrospray using a conventionally machined porous borosilicate emitter cone was operated using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Repeated performance curves of the same emitter, measuring the emitter and extractor electrode currents as a function of the emitter voltage, vary significantly between tests. Ion current density spatial distribution maps of the plume and time-resolved emitter current and plume current while retarding potential energy analyzer sweeps are also presented. High-speed current measurements on a collector plate downstream of a different single-emitter electrospray were collected to examine the variation in the onset delay time of the Taylor cone. The onset delay time decreased with increasing emitter voltage and was effectively eliminated using bipolar switching. In monopolar mode, the onset delay rate was not sensitive to changes in the emitter voltage switching frequency across the range of tested values.