Introduction
Electrospray (ES) thrusters generate low thrust via the electrostatic extraction of charged species from an electrified liquid. These systems are characterized by the propellant used and emission mode of the ion beam. The most common of these, field emission electric propulsion (FEEP), utilize liquid metals as its ion source to produce positively charged ions and/or droplets and have previously been studied here at NGPDL. Recent developments include nonmetallic ion sources such as ionic liquids (ILs), room-temperature molten salts, that support emission of heavy ions that contribute to high propulsive efficiencies and specific impulses. ILs additionally exhibit low vapor pressure that allow for exposure in vacuum conditions. 

Though ionic liquid ion sources (ILIS) have demonstrated their applicability within micropropulsion [1], further efforts are required to study higher thrust capabilities by coupling multiple ES emitters in parallel, a process known as multiplexing. High-fidelity modeling of the electrohydrodynamics of ILIS emitters are thus employed to elucidate the varying failure modes and coupling conditions. 

 

Methods
A passively fed porous geometry operating solely in ion emission, known as the purely ionic regime (PIR), is selected and shown in Figure 1. Porous-based geometries, through selection of substrate porosity and pore size, allow for further control of beam composition than traditional capillary-based geometries [2] while a passively fed ILIS allows for flow and emission current to be characterized by the extractor voltage.

Figure 1. Porous-based electrospray emitter configuration

 

Initial efforts will focus on capturing the meniscus dynamics at the emission site shown in Figure 2. This is accomplished by utilizing a two-phase approach where the upstream Stokesian flow conditions alongside the applied potential are used to resolve the electric stress and surface charge evolution at the liquid-vacuum interface via discrete boundary elements. 

Figure 2. Meniscus at emission site, where τ represents the components of eletctric stress, p and Q are inlet pressure and flow, μ and ϵ are liquid viscosity and dielectric, and j is ion emission.

 

Future Work
This upstream continuum model will be used to investigate coupling conditions for a kinetic approach at and beyond the meniscus interface to simulate ion emission and far-field transport.

Investigators
Amin Taziny, Dr. Ronald Chan

Acknowledgments
Financial support for this project is gratefully provided by the Air Force Office of Scientific Research. 

References
[1]  G. Anderson et al., “Experimental results from the ST7 mission on LISA Pathfinder,” Phys. Rev. D, vol. 98, no. 10, p. 102005, Nov. 2018, doi: 10.1103/PhysRevD.98.102005.

[2]  G. Lenguito and A. Gomez, “Pressure-Driven Operation of Microfabricated Multiplexed ElectroSprays of Ionic Liquid Solutions for Space Propulsion Applications,” Journal of Microelectromechanical Systems, vol. 23, no. 3, pp. 689–698, Jun. 2014, doi: 10.1109/JMEMS.2013.2283728.