The Effects of Geometry and Material Variations on the Onset Voltage of Electrospray Emitters

Electric propulsion is becoming an increasingly desirable technology as space missions become more complex and more commonplace. Electrospray propulsion specifically is proving promising as a low-thrust, high specific impulse technology with the ability to execute very precise thrust maneuvers in applications such as attitude adjustments to small satellites. Electrospray devices operate by electrostatic acceleration of charged droplets and/or ions to produce thrust. The current technological trend is to drive these devices towards the micro-scale. To become a viable space propulsion technology, the manufacturability, the required footprint, and the power requirements of electrospray devices are areas which must be addressed. With regards to manufacturability, this work proposes a new electrospray device design which utilizes liquid-filled through-channels bored in a dielectric material, as opposed to the free-standing needles in a more traditional electrospray device. This is achieved through technology at the Center for Laser Applications at the University of Tennessee Space Institute utilizing picosecond laser pulses shaped into a Bessel beam. This approach has shown potential to be both easier to manufacture and more robust than creating free-standing needles. In addressing required footprint and power requirements, this work performs a comprehensive study on the effects of geometric and material variations on the onset voltage of electrospray emitters. This study provides insight towards the creation of a design configuration that can minimize onset voltage and maximize thrust density. The effects of the thickness of the dielectric material, relative permittivity of the dielectric material, extractor distance, radius of the ionic liquid channel, and emitter pitch on the onset voltage are examined. Results show that, in general, tall, small-radius channels surrounded by a material with a low relative permittivity value and with emitter pitch values resulting in a ratio of emitter pitch to emitter height of 2.5 or greater give the lowest onset voltages. The relationships between the various parameters are complex, however. These complexities are discussed in detail within this work.