Charge Neutralization and Direct Thrust Measurements from Bipolar Pairs of Ionic- Electrospray Thrusters

lectrospray (ES) thrusters are unique in their ability to emit both positive and negative beams, each contributing similarly to thrust. Spacecraft charging could be prevented without a dedicated neutralizer if currents of simultaneously emitted opposing polarity beams were matched; despite a lack of implicit coupling between them. We discuss and evaluate experimentally both an active current-balance control circuit and a passive self-balancing configuration. Our highly ionic ES source includes a pair of machined porous glass ES emitter arrays in a single holder. In the passive config- uration, unbalanced charge collection on a common floating extractor yields rapid charging towards artificially imposed 200 V limits without means for charge suppression, showing the unsuitability of that architecture. Active balancing of emitted currents was fully effective at charge neutraliza- tion during stable periods of emission, yet insufficient during potential alternations. We present methods to improve this approach, including demonstrating beam current modulation at up to 450 Hz. Direct thrust measurements when simultaneously emitting opposing polarity beams of the ionic liquid 1-ethyl-3-methylimidazolium tetrauoroborate are presented. Direct thrust measurements up to 35 μN were in excellent agreement with commanded levels and computations based on measured currents (up to 190 μA) and voltages (up to 2100 V).