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

Electrospray 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. Both an active current-balance control circuit and a passive self-balancing configuration are discussed and evaluated experimentally. Tests are performed with a highly ionic electrospray source which includes a pair of machined porous glass electrospray emitter arrays in a single holder. In the passive configuration, unbalanced charge collection on a common floating extractor yields rapid charging toward 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 neutralization during stable periods of emission yet insufficient during potential alternations. Methods to improve this approach are presented, 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 were in excellent agreement with commanded levels and computations based on measured currents (up to ) and voltages (up to 2100 V).