Highly ionic beams of several hundred μA per cm 2 have been measured from porous glass ionic liquid electrospray sources fabricated using a conventional mill. The thrust output from three prototype devices, two emitting the ionic liquid EMI-Im and one emitting EMI-BF 4, was measured directly using a precise balance. Thrusts up to 50 μN were measured when emitting EMI-Im in a bipolar, alternating potential configuration at less than 0.8 W input power and with propellant supplied from an internal reservoir. Measurements of mass spectra via Time of Flight spectrometry, angle resolved current distributions, ion fragmentation and energy deficits have been applied to accurately calculate thrust and mass flow rates indirectly from the same devices. For two of three cases, calculated and directly measured thrusts were in agreement to within a few μN at input powers from 0.1 W to 0.8 W . Emissions of EMI-BF4 are shown to yield nearly purely ionic beams supporting high propulsive efficiencies and specific impulses, ∼ 65 % and > 3200 s respectively at 0.5 W . Conversely, greater polydispersity was observed in EMI-Im emissions, contributing to reduced specific performance, ∼ 50 % propulsive efficiency and ∼ 1500 s specific impulse at 0.5 W .

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.

Microfabrication, wafer-level integration, and characterization of internally fed arrays of electrospray thrusters for spacecraft propulsion are discussed. 5 µm inner diameter, 100 µm long capillaries and 150-to-300 µm diameter annular extractor electrodes are integrated vertically via a polymer based wafer bonding process, allowing high yield and post testing disassembly of the bonded stack. The small inner diameter of the capillaries allows passive, capillary force driven delivery of the propellant to the emission site, and therefore potentially eliminating the need for an active pump system. The fabricated thruster chips were successfully tested in pumpless liquid delivery configuration under unipolar and bipolar excitation.

Miniaturized electrical thrusters based on electrospray (or colloid) emitters could revolutionize the spacecraft industry by providing efficient propulsion capabilities to micro and nano satellites (1-100 kg). We report on our recent advances in the development of this technology within the MicroThrust (www.microthrust.eu) European consortium. We present the design and operation of the currently fabricated and next generation emitter arrays, describing their microfabrication process and measured performance. The emitters are out-of-plane internally fed capillaries micromachined in monolithic silicon. They are 100 µm tall and have an inner diameter of 5-10 µm. We operate the devices in both unipolar and bi-polar modes and find that the latest devices operate in a mixed regime, with the emitted spray a composition of ions and droplets. Their specific impulse is consequently in the few hundred seconds, highlighting the need for higher impedance, smaller emitters. Onset voltages are in the range of 800-850V for 200 µm inner diameter extractors, current levels for 19 emitter arrays of 2-3 µA. Preliminary analysis hints to plume half-angles of 35-40 • , although these values depend on the operation mode and beam composition. Nomenclature g Standard gravity (9.80665 m/s 2 )

Electric propulsion systems using electrospray ion sources are a scalable and high specific impulse technology which could enable small spacecraft to perform high ΔV missions. This paper presents an overview of a recently developed source design which achieves 10's of μN of thrust at less than 1 W of input power with a specific impulse up to ∼3000 s. Demonstration devices, fabricated conventionally without microfabrication methods yet with an active area of ∼1 cm 2 , emit positive and negative ion beams from an ionic liquid passively supplied from a coupled porous reservoir. Directly measured thrusts (up to ∼28 μN ) from two simultaneously operating thruster modules are shown to be consistent with summing the calculated total force from each module. The two particle beams are configured to be at opposite polarity, in progression towards a charge neutralized system without a dedicated neutralizer. Influences of reservoir pore size and filling state are discussed in the context of performance and lifetime. Specifically, recent results have demonstrated that increasing the reservoir pore size can induce significant droplet or heavy particle populations within an otherwise ionic beam. Large reductions in specific impulse and propulsive efficiency due to these transitions are discussed here. For example, the calculated specific impulse of a negative EMI-BF4 beam could be reduced from ∼2800 s to ∼700 s by changing the reservoir pore size alone. Meanwhile, capillary actions within the reservoir aid in containing liquid via a negative Laplace pressure, thereby preventing lifeending liquid to extractor grid bridges/shorts. Finally, the technology status is reviewed through highlighting critical developments required to arrive at a functional, and applicable, propulsion system.