Experimental Testing on the Effect of Emitter Radius of Curvature on Electrospray Ion Emission

Over the past few decades, electric propulsion (EP) has emerged as a key enabling technology for satellite maneuvering. One NASA mission using EP was the LISA Pathfinder, whose goal was to test new EP systems for use on a future space-based gravitational wave detection satellite constellation. The EP system utilized electrospray thrusters, which provided exceptionally precise control over attitude and position. The successful use of electrosprays in spaceflight indicates a promising future for the technology, along with a wider range of applications. This technology works by applying electric potential to ionic liquids, accelerating the propellant to create thrust. Compared to other propulsion technologies, electrospray offers a uniquely high specific impulse, which implies an extremely efficient propellant usage for a given thrust. However, a critical trade-off of this technology is the sacrifice of high thrust for the allowance of high precision. Thrusts can be produced in the micronewton range, allowing satellites to move extremely small distances with great accuracy. Electrospray systems are useful tools that, with further research, could also be deployed on cubesats and other nanosatellites to aid in high-precision measurements. An ionic liquid ion source (ILIS) is a type of electrospray device that has three distinct architectures: colloidal, porous, and externally wetted. For this study, externally wetted emitters were chosen due to their ease of construction and ease of use in a laboratory setting. It is also believed that they have similar emission characteristics to the other two architectures, so findings can be extrapolated. Externally wetted emitters consist of a needle coated with an ionic liquid. An electric potential is applied between the liquid and an extractor plate placed near the tip of the needle. This causes the formation of a Taylor cone, which allows ions to evaporate from the liquid. This ion emission creates a thrust depending on a variety of factors such as the charge-to-mass ratio of the emitted ions and the current emitted. One factor that is hypothesized to play an important role in determining emission characteristics is the radius of curvature of the externally wetted needle. This paper seeks to quantify and explain how the radius of curvature of the needle affects the emitted current. Electrochemical processes were used to sharpen and then roughen a pre-cut tungsten wire with a diameter of 500 microns. The radius of curvature, typically in the range of tens of microns, of the needle was able to be estimated by viewing the tip of the needle through a microscope. The radius of curvature was then able to be adjusted through additional electrical and chemical processes. MATLAB code was created to accurately and empirically analyze the radius of curvature of the needle using image processing. The code first creates multiple pixel-to-distance conversion ratios by using microscope images of the tungsten needle on a scaled slide. It then uses edge detection, circle-fitting functions, and the aforementioned conversion factors to determine the radius of curvature of the tungsten needle. The code uses multiple conversion factors and measurements to provide an absolute minimum and maximum value for the radius of curvature of the needle tip. A radius of curvature was able to be obtained for all tungsten needle images that have been run through the code. The accuracy of the code will be verified using a reference slide containing scale lines and a circle with known measurements. Once the accuracy of the code is verified, needles with different radii of curvature will be measured by the code. These needles will then be fired in an externally wetted ILIS electrospray device using EMI-BF4 as a propellant. The current emitted from these needles in the device will be measured while applying different electric potentials to the device. Current-Voltage (I-V) curves will be created and analyzed to determine the effect of radius of curvature on ion emission in our electrospray device. These I-V curves will be used to analyze the validity of published "startup voltage" equations. "Startup voltage" equations provide a model for determining the minimum electric potential application required to produce a current in the emitter using variables such as radius of curvature, distance between the needle tip and extractor, and various properties of the ionic liquid. These results will contribute to the field of electric propulsion and enhance our knowledge of the electrospray phenomena by providing empirical validation of startup voltage models and improving our understanding of how emitter geometry influences ion emission in electrospray thrusters.