Feasibility study of a micro-electrospray thruster based on a porous glass emitter substrate

Ma, Chao, Bull, Thomas Gregory and Ryan, Charles (2017) Feasibility study of a micro-electrospray thruster based on a porous glass emitter substrate. 35th International Electric Propulsion Conference, Georgia Institute of Technology, Atlanta, United States. 08 - 12 Oct 2017. 15 pp . (In Press)

Abstract

Electrospray thrusters utilize a strong electric field to extract and accelerate charged particles contained at a needle tip in a liquid state[1].Electrospray thrusters are one of the few types of electric propulsion systems that can offer high specific impulses (i.e. >1000 seconds) whilst still being scalable down to fit within a 1UCubeSat form structure. These small electrospray thrusters, here termed micro-electrospray thrusters are attracting considerable interestd due to this Nanosatellite system suitability[2].The thrust though from an individual down scaled micro-electrospray emitter is relatively small, of the order of0.1μN[3].Given this small value, it is necessary to ‘multiplex’ the thrust by having an array of hundreds or indeed thousands of emitters[4–7].To create these large arrays many different manufacturing methods have been investigated,including micro electro mechanical system (MEMS)methods[8,9], electrochemical etching[10,11],laser ablation[12–14],computer numerical control (CNC) machining[3,15,16]and additive manufacturing[17].Many of these techniques result in an electrosprayemitter design with an emitter tens of micrometres in heightand with emitter diameters ofthe order oftenmicrometresif not smaller. This results in a ‘postage stamp’sized emitter array, which in itself is considerably smaller than perhaps what is required onboard a CubeSat. Alternatively somewhat larger emitters may produce greater charged particle current per emitter, andtherefore greater thrust.To investigate the feasibility of a larger emitter design, thispaper studies athrusterusing anemittermanufactured fromsintered porous glassusingaCNC machining technique.The emitter manufactured is of a pyramidal shaped design, with an emitter height of 5mmandside surface anglesrangingfrom10 ̊to30 ̊.Five different prototy peemitter shapes are manufactured, investigating the feasibilityofmanufacturingsingleemitters using the method. Initial experimental results are illustrated, demonstratinggood operationparticularlyin a bipolar, alternating voltage,mode. Initial Time-of-flight measurements would seem to indicate that the emitter was operating in a purely ionic regime, with a thrust of the order of9.25μNper emitter and a specific impulse of the order of 4550secondsat 3443 V with 60μA emission currentfrom theon e emitter. This value of thrust is particularly high from one emitter, with onereas on suspected being the occurrence of multiple emission sites at the emitter tips during the electrospray process. Prototype designs ofarray of emitters, and also more spike like emitters, are presented with the manufacturing demonstrating the reasonable promise of this very low cost, simple and quick to iterate manufacturing process.

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