Electrospray propulsion: Modeling of the beams of droplets and ions of highly conducting propellants

Abstract

This article presents a model for the beams of charged droplets and ions typical of electrospray thrusters. The model is applied to a typical highly conducting propellant operating in the mixed droplet-ion emission regime but can be extended to simulate other emission regimes. A key feature of the model is the separation of the beam in two zones: a small inner region including the droplet emission area, where the equations of motion of all droplets are integrated simultaneously while retaining direct droplet-on-droplet Coulomb interactions and a much larger outer region where the beam is treated as a continuum. In addition to the geometry of the electrodes, inputs to the model include the distribution of diameters and charge-to-mass ratios of the droplets and their initial positions and velocities. Most of this information is determined from experiments. The analysis of the numerical solution confirms that lateral oscillations of the slender jet producing the droplets is an important factor in the expansion of electrospray beams, and that droplets in flight are the main sources of ion emission in these propellants. In addition to improving the knowledge of the physics of electrospray beams, the model is a significant tool for designing the extractor and accelerator electrodes of electrospray thrusters and for minimizing the deposition of beam particles on the surfaces of spacecrafts.