Microfabricated electrospray emitter arrays with integrated extractor and accelerator electrodes for the propulsion of small spacecraft

Abstract

Microfabricated electrospray thrusters could revolutionize the spacecraft industry by providing efficient propulsion capabilities to micro and nano satellites (1–100 kg). We present the modeling, design, fabrication and characterization of a new generation of devices, for the first time integrating in the fabrication process individual accelerator electrodes capable of focusing and accelerating the emitted sprays. Integrating these electrodes is a key milestone in the development of this technology; in addition to increasing the critical performance metrics of thrust, specific impulse and propulsive efficiency, the accelerators enable a number of new system features such as power tuning and thrust vectoring and balancing. Through microfabrication, we produced high density arrays (213 emitters cm−2) of capillary emitters, assembling them at wafer-level with an extractor/accelerator electrode pair separated by micro-sandblasted glass. Through IV measurements, we could confirm that acceleration could be decoupled from the extraction of the spray—an important element towards the flexibility of this technology. We present the largest reported internally fed microfabricated arrays operation, with 127 emitters spraying in parallel, for a total beam of 10–30 µA composed by 95% of ions. Effective beam focusing was also demonstrated, with plume half-angles being reduced from approximately 30° to 15° with 2000 V acceleration. Based on these results, we predict, with 3000 V acceleration, thrust per emitter of 38.4 nN, specific impulse of 1103 s and a propulsive efficiency of 22% with <1 mW/emitter power consumption.

Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

“…capillaries [3,4,25,26], silicon tips roughened through etches or CNT growth [1,17], electrochemical etching of porous metals [16,27], use of Rosensweig instabilities in ionic liquid ferrofluids [28] and laser ablation of porous glass [5]. Arrays of externally wetted [17] and porous [2,5] ILIS have, encouragingly, been demonstrated to support a few hundred μA per cm 2 of highly ionic emissions.…”

“…First, compared with precise silicon microfabrication [26], the tendency for porous emitters to achieve high and ionic currents is particularly remarkable considering the relative imprecision and range in apex radii typical of processes which have successfully been applied (eg. few to 10's of μm in [27]).…”

“…Low polydispersive efficiencies (<50%) can result from only a few percent of large droplets within an otherwise ionic beam [40]; greatly penalizing the power efficiency when targeting a high specific impulse through the PIR. This relatively severe loss mechanism has led to significant focus on achieving the PIR [26,41], or more recently and with promising results, obtaining extremely small (∼ few kDa) monodisperse droplet beams [20]. In this regard, ILISs which achieve the PIR through externally wetted [7] or porous [16] electrospray emitters are a promising avenue for high efficiency, high specific impulse devices.…”

“…However, relative to the apex radius, these asymmetries lie far from the anticipated emission site loci and the influence is not expected to be large. Generally, these inconsistencies in fabrication may be regarded as a clear shortfall when compared with the precision achievable from silicon microfabrication [17,21,26]; however, the tip curvature ranges are comparable with other porous sources [5,16,27].…”

A 10 nN resolution thrust-stand for micro-propulsion devices

“…1,2 Several miniature thruster technologies, such as, cold-gas thrusters, 1,3 electrospray propulsion using liquid metals 4 or ionic liquids, [5][6][7][8][9][10][11] micro resisojets, 12,13 and micro pulsed plasma thrusters 13 are under investigation as prospective candidates targeting stable thrusts in the range of a few µN to several hundred µN. Several thrust-stands have been reported over the last few years for measuring thrust from these microthrusters with sub-µN resolutions.…”