Analysis of oscillatory local neutralization according to fragmentation in the ionic liquid electrospray thruster plume

An electrospray thruster is an electrostatic propulsion system that employs ionic liquids as propellant. Its ion beam consists of single ions and ion clusters, each cluster comprising a single ion attached to one or more neutral ionic liquid molecules. Due to its meta-stability, the ion cluster can fragment into a less massive ion and neutral molecules during emission. This fragmentation alters the overall plume composition, significantly affecting ion oscillation, which plays a key role in plume neutralization. This study aims to analyze changes in ion oscillation caused by fragmentation and to investigate plume neutralization through ion oscillation. Plume simulations are conducted for seven cases using the particle-in-cell–Monte Carlo fragmentation hybrid method, including cases with and without fragmentation, as well as varying current density ratios of monomers to dimers. The results confirm that the plume consists of oscillating ions and diffusing ions, allowing the identification of the oscillation region. Oscillating ions are primarily found near the center of the plume. The oscillation frequency of primary monomers increases due to the presence of secondary monomers generated by fragmentation, contributing to the overall ion oscillation frequency. The degree of neutralization is quantified using the normalized charge density. In this study, neutralization refers to local quasi-neutrality, where the plume locally achieves and maintains nearly zero net charge density. It is found that neutralization at the center of the plume, corresponding to the oscillation region, is more effective than at the boundary. In cases with fragmentation, the increased oscillation frequency further enhances neutralization near the center of the plume.