Direct detection of neutral species from ionic liquid electrospray thrusters in the purely ionic regime

Recent experiments on ionic liquid electrospray thrusters revealed lower efficiency than expected from time-of-flight (ToF) measurements, despite operating in the purely ionic regime (PIR). This discrepancy was attributed to excessive propellant consumption caused by neutral particles generated during ion emissions. In this study, ToF spectrometry was performed using a channel electron multiplier (CEM) with an externally wetted emitter and two propellants: 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium dicyanamide. ToF measurements confirmed that the thruster operated in the PIR. While the ion species composition remained unchanged with increasing CEM gain, the zero levels of ToF signals differed between conditions with and without ion emissions. In addition, the CEM signal increased with rising emitter current, even when the ion beam was completely blocked by the deflector gate. These findings indicate that neutrals generated in both acceleration and field-free regions can be directly detected using the CEM. Considering the lower detection efficiency of neutrals compared to that of ions, the number of generated neutral particles was higher than expected, which could explain the excessive propellant mass loss. Such neutral species are overlooked when a metal plate or a Faraday cup is employed as a collector for ToF measurements.