Design of a Simple, Modular Diagnostic Suite for Electrospray Propulsion Systems

With the rise of research into micropropulsion for spacecraft, testing facilities for these propulsion systems are becoming increasingly in demand. Although there are some large facilities that can be used upon request, there are challenges to measure key parameters such as extremely low thrust in a routine fashion to ensure rapid testing. To facilitate testing, many labs create their own in-house diagnostic instruments, frequently designed to fit into smaller vacuum chamber facilities. These instruments are designed and built one at a time and must be dismantled and rebuilt for experiments using different instruments. The purpose of this paper is to offer an alternative that can be easily modified for various types of concurrent testing in a small laboratory setting. Specifically, a modular design that includes both a time-of-flight (TOF) system and a microbalance for both direct and indirect thrust measurements (challenging due to the order of magnitude of thrust levels expected to be in the micronewton range in electrospray propulsion systems) [1]. TOF systems measure the time it takes the charged particles to travel between the thruster, or an electrostatic gate, and a collector plate that are located at a known distance from each other. The Bradbury Nielsen Gate (BNG) used in this diagnostic suite operates by quickly directing alternating high voltages into the gate creating an electrostatic field that deflects the particle beam from the thruster away from the collector plate [2,3]. Measuring the current that is captured at the collector is used to determine the time at which the particles have completed their flight path. The thrust, mass-flow rate and specific impulse can then be calculated using these time and distance measurements. The collection plate was designed based on the one used by Peiffer [4] with fins placed at a 45° angle from the top surface of the plate to recapture any particles or secondary electron emissions that escape the collector from the bombardment of the emitted beam. On the other hand, direct thrust measurements can come from various instruments including varying types of thrust stands or using a microbalance to measure the change in force applied by the thruster when active vs not active. Types of thrust stands include torsional or pendulum thrust stands that measure the rotation of an arm to determine the applied force, magnetically levitated thrust stands that infer the force applied to a levitated platform from its angular displacement, and impingement-type thrust stands that measures the force applied by the plume of the thruster on an impingement plate [5]. Microbalances can be used by placing the thruster directly on the balance and measuring the difference in the applied force prior to thruster operation and during operation. This resulting force is the most direct way to measure the thrust. The microbalance can also be used, in conjunction with a lightweight plate, as an impingement-type thrust stand if the thruster is pointed towards the plate and the entirety of the plume is captured. The versatility of the microbalance is the reason it was chosen for this modular diagnostic set-up. In the current proposed set-up, the TOF system can either integrate the microbalance or operate without it. The benefit of setting up the TOF system with the microbalance, and one of the reasons for designing a collection plate to trap all the emitted particles, is that both a direct measurement of the thrust, measured by the microbalance, and an indirect measurement can be taken simultaneously. This allows for a comparison of the two thrust measurements, along with measuring the mass flow and current-voltage characteristics of the propellant and calculation of the specific impulse during a single experiment run, eliminating the need to perform multiple experiments to obtain results for each measurement. On the design side, the use of rods to hold the various pieces of the testing instruments allows for easy modification of the set-up by sliding on and off the desired equipment. Holders for the thruster, BNG and collector plate that fit onto these rods are made from PEEK and can be placed at any height. This design also allows for new instruments to be added in the future such as a wire probe that could be fitted to move between the rods on stepper motors if more information on plume characterization is desired. The PEEK holders could also be modified to hold different sized thrusters or collector plates in future experiments. The final paper will present the design, testing of the diagnostic suite and the system will be validated with an electrospray prototype. [1] Ziemer, J. K., “Performance of Electrospray Thrusters,” IEPC, 2009. [2] Mallalieu, P., and Jugroot, M., “Development of a Time-of-Flight Diagnostic System for Electrospray Thrusters,” IECP, 2022. [3] Yoon, O. K., Zuleta, I. A., Robbins, M. D., Barbula, G. K., and Zare, R. N., “Simple Template-Based Method to Produce Bradbury-Nielsen Gates,” American Society for Mass Spectrometry, 2007. [4] Peiffer, L., and Tajmar, M., “Novel Compact Mass Balance for Simultaneous Thrust and Mass Efficiency Measurements for FEEP Thrusters,” IEPC, 2024. [5] Lyne, C. T., Liu, M. F., and Rovey, J. L., “A Brief Review of Electrospray Propulsion Diagnostics,” AIAA SciTech Forum, 2023.