Design and fabrication of the thruster heads for the MicroThrust MEMS electrospray propulsion system

Microfabricated electrospray thrusters are widely acknowledged as one of the most promising technologies for the propulsion of small spacecraft. Their relative simplicity, high efficiency (> 70%), low footprint (M < 500g, V < 10cm3) and large potential specific impulse (> 3000s) enable the creation of a miniature system capable of providing up to 5km/s _V to 3U CubeSats. We report here on our latest efforts in the development of such a thruster system, completed within the MicroThrust (www.microthrust.eu) project. While a companion paper will present early test results of the thrusters, this paper will focus on their design and fabrication. We use MEMS microfabrication to manufacture internally fed capillary emitters from silicon. This permits the high fluidic impedance required to get the necessary low flow rates associated with pure ionic mode operation, in addition to allowing the fabrication of large arrays of perfectly aligned, nearly identical emitters. We present for the first time the wafer-level integration of an acceleration stage, with individual electrodes operating on up to 127 emitters on a single chip. By adding the accelerator, we increase both the specific impulse and thrust generated by the emitters, while also increasing the thrust efficiency by electrostatic focusing the spray. We have fabricated chips with varying emitter density (213 and 125 emitters per cm2) and have successfully tested passively fed emitter arrays, obtaining up to 35 uA of current at +875V for a 91 emitter array.