On the Two-Dimensional Structure and Asymmetries of Ionic Liquid Electrospray Plumes

Abstract

Vacuum electrosprays play important roles in electrospray propulsion, a type of spacecraft electric propulsion wherein charged ions or droplets are accelerated at high velocity to provide spacecraft thrust, and they also have important implications for surface etching and mass spectrometry. Here we present the first fully two-dimensional time-of-flight (TOF) mass spectrometry survey of a vacuum electrospray plume, generated by a tungsten needle externally-wetted with the ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF$_4$). We find that the plume exhibits clear two-dimensional compositional variation, structure, and asymmetry, with heavy particles and energetic neutrals being more prevalent in the center and a ring-shaped distribution for the monomers (the lightest molecular ions) peaking at 10$^\circ$ off beam-axis with a relative minima in the center. Using a new spatial baseline characterization method, we find a population of energetic neutrals firing 3.4$^\circ$ off beam-axis. In particular, we find that by comparing different parts of the plume, the estimated propulsive efficiency from any one sampled point may vary by as much as a factor of 6. We also find that high mass droplets, which are often assumed as absent in many studies of externally-wetted needles, may carry away significant propellant mass at lower effective velocity and reveal a cone-jet mode of operation at 470 nA output current. We thus find that whole-plume compositional surveys may more accurately assess plume composition and propulsive efficiency, and a significant portion of the `missing mass' in electrospray propulsion sources presumed to be operating in the pure ion regime can be potentially explained by limited sampling of the spatially non-uniform ion plume.