Date of Award

4-2025

Degree Name

Doctor of Philosophy

Department

Mechanical and Aerospace Engineering

First Advisor

Kristina Lemmer, Ph.D.

Second Advisor

Nicholas Taylor, Ph.D.

Third Advisor

Muralidhar Ghantasala, Ph.D.

Fourth Advisor

Charlie Ryan, Ph.D.

Keywords

Electric propulsion, electrospray, electrospray thruster, ionic liquid ion source, single-emitter, time-of-flight mass spectrometry

Abstract

Ionic liquid ion sources (ILIS) are being developed as one solution to fill a technology gap in low power, high efficiency, high specific impulse space propulsion systems. With a sufficiently intense electric field applied between an emitter electrode and an extractor electrode, charged particles are extracted from the fluid surface and accelerated out of the thruster. In this work, three experimental campaigns examine emission behavior of a single emitter ILIS made of porous material that allows passive propellant feed via capillary action of the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4).

Repeated measurements of the emitter and extractor current as a function of applied emitter voltage within hours of each other and using the same emitter produced significantly different results. Repeated two-dimensional spatially resolved Faraday probe measurements of the ion current density distribution in the ionic liquid ion source plume were also performed. The ion current spatial distribution changed between and during measurements, indicating that emission sites are dynamic when an electrostatic potential is applied. Variance in the time delay between emitter voltage application and the emission of ions was identified using high-speed beam current measurements during emission onset transience. These results demonstrate that ion emission from a passively fed ILIS at the single emitter scale can be stochastic when controlling for input parameters such as emitter voltage, emitter and extractor geometry, and propellant. Inconsistent emitter and extractor current for constant applied emitter voltages was also observed during the second experimental campaign.

The effect of the vacuum facility on luminescent glow at the emitter was investigated using microscopy imaging and optical emission spectroscopy. Images and optical spectra were measured for varied distances between the ILIS and a downstream plate, plate material, and emitter voltage. No relationship between the plate distance or material and luminescence intensity was observed. All conditions had a strong positive correlation between extractor current and luminescence intensity. The OES data and images together indicate that the luminescence resulted from emitted ions impinging on the extractor with sufficient energy to fragment the molecular EMI cation.

An orthogonal acceleration time-of-flight mass spectrometer (oa-ToF-MS) was developed to investigate molecular fragmentation in the ILIS plume. Using the developed system, mass spectra peaks corresponding to molecular fragments of EMI and BF4 were successfully including the protonated methyl-imidazolium cation, the ethyl cation, the deprotonated ethyl cation, the methyl cation, and the fluoride anion were also identified. A peak at m/z 196.9, hypothesized to be a deprotonated EMI (m/z = 110)-BF4 cluster, was measured for the first time during negative polarity emission of an ILIS.

Access Setting

Dissertation-Open Access

Download

Share

COinS