Wafer-level integrated electrospray emitters for a pumpless microthruster system operating in high efficiency ion-mode

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

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

Highly ionic beams of several hundred microampere per squared centimeter 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 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide and one emitting 1-ethyl-3-methylimidazolium-tetrafluoroborate, was measured directly using a precise balance. Thrusts up to 50μN were measured when emitting 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide 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 the three cases, calculated and directly measured thrusts were in agreement to within a few micronewtons at input powers from 0.1 to 0.8 W. Emissions of 1-ethyl-3-methylimidazolium-tetrafluoroborate were shown to yield nearly purely ionic beams supporting high propulsive efficiencies and specific impulses of ∼65% and greater than 3200 s, respectively, at 0.5 W. Conversely, greater polydispersity was observed in 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide emissions, contributing to reduced specific performance, ∼50% propulsive efficiency, and ∼1500s specific impulse at 0.5 W

Microfabrication of Capillary Electrospray Emitters and ToF Characterization of the Emitted Beam

Microfabrication, assembly, and characterization of internally fed arrays of electrospray emitters for spacecraft propulsion are discussed. Several different emitter geometries were fabricated, and the sprayed beams from these emitters are characterized for their ion/droplet composition. It is shown that with smaller inner diameters, ionic mode of operation can be achieved more easily, due to the increase in the hydraulic impedance. Up to 2000 s Isp was measured from capillaries with 5 μm inner diameter and 100 μm with 750V extraction voltage using EMI-BF4. Due to the small dimensions of the microfabricated capillaries, the onset and ionic mode operation voltages can be significantly lower thanmacroscopic emitters. The major failure mode for the emitters was the liquid overflow from the capillary tip, which can be resolved through coatings that wet the ionic liquids differently. A detailed study of the wetting behavior of two ionic liquids is also presented for various materials compatible with silicon microfabrication technology

A ToF-MS with a Highly Efficient Electrostatic Ion Guide for Characterization of Ionic Liquid Electrospray Sources

We report on the development of a time-of-flight (ToF) mass spectrometer with a highly efficient electrostatic ion guide for enhancing detectability in ToF mass spectrometry. This 65-cm long ion guide consists of 13 cascaded stages of Einzel lens to collect a large fraction of emitted charges over a wide emission angle and energy spread for time-of-flight measurements. Simulations show that the ion guide can collect 100% of the charges with up to 23° emission half-angle or 30% energy spread irrespective of their specific charge. We demonstrate this ion guide as applied to electrospray ion sources. Experiments performed with tungsten needle electrospraying the ionic liquid EMI-BF4 showed that up to 80% of the emitted charges could be collected at the end of the flight tube. Flight times of monomers and dimers emitted from the needles were measured in both positive and negative emission polarities. The setup was also used to characterize the electrospray from microfabricated silicon capillary emitters and nearly 30% charges could be collected even from a 40° emission half-angle. This setup can thus increase the fraction of charge collection for ToF measurement and spray characteristics can be obtained from a very large fraction of the emission in real time

Progress Towards a Miniaturized Electrospray Thruster for Propulsion of Small Spacecraft

Miniaturized electrical thrusters based on electrospray (or colloid) emitters could rev- olutionize 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 of than 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

Development of a ToF setup with an ion - guide for characterization of electrospray microthrusters

Electrospray microthrusters are a very promising micro-propulsion system which can potentially provide very high specific impulse (Isp ~ 3000s). In this paper, we report on the development of a time-of-flight (ToF) measurement setup for characterization of the spray composition of the electrospray sources. This setup consists of an electrostatic ion-guide built with 13 cascaded single-stage Einzel lenses along with a 65 cm long flight tube to focus the emitted charges on a Faraday cup detector. The distributed lens ensures a large fraction of the emitted beam can be collected, even when the beam divergence is high. Simulations of the focusing capabilities of the ion guide have been performed and are compared with a single stage Einzel lens, with excellent agreement. The efficiency of the lens has been characterized with microfabricated capillary electrospray devices and with electrochemically etched tungsten needles

MicroThrust MEMS electrospray emitters – integrated microfabrication and test results

With the growth of interest in small satellites (<10kg), there is a particular need to provide a propulsion element for this class of spacecraft. Microfabricated electrospray thrusters offer a solution to this problem. By using ionic liquids as the propellant solely ions can be emitted, resulting in a large specific impulse. The thrust from an individual emitter is though a fraction of a μN. However by using well-established MEMS technologies thousands of capillary emitters can be manufactured within an area of a few cm2, increasing the thrust to the mN level. We report on results from the Microthrust FP7 Project 1, where the aims are to manufacture and test a complete breadboard thruster system based upon microfabricated thruster chips, alongside the design of a flight system that could enable a CubeSat to leave earth orbit. Prior to this project we had developed a number of manufacturing processes for specific thruster elements. We report here on a new generation of microfabricated emitters, and their relative performance. The emitters consist of 70 μm high etched-Silicon capillaries with outer diameters tapering to less than 10 μm. Previous designs included 5 μm silica microspheres within the 18 to 24 μm internal diameter of the emitter to increase the hydraulic impedance. However the filling factor of these microspheres in individual emitters differed; therefore a new generation of emitters having more similar impedance and with 5 - 10 μm internal diameters and hole depths of 100 μm have been manufactured. Previously the etched-Silicon extractor chip was aligned to the emitter chip using 200 μm ruby spheres. Due to assembly difficulties this has been replaced with a polymer-based wafer bonding interface, allowing for simplified assembly and a wafer-scale fabrication process. These emitters have been tested in both uni-polar and bi- polar mode, using the ionic liquid 1-ethyl-3- methylimidazolium tetrafluoroborate (EMI-BF4). The tests herein have been achieved without an acceleration stage. The Time-of-Flight data shows a mixed ion-droplet regime, approaching a Purely Ionic Regime (PIR) at low flow rates

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 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

Experimental progress towards the MicroThrust MEMS electrospray electric propulsion system

This paper describes the experimental progress towards an operational microfabricated electrospray thruster, as part of the EU FP7 “MicroThrust” Project. Microfabrication of an electrospray multiplexed thruster allows the seamless manufacturing of arrays of emitters, combining high specific impulse with sizeable thrust. The resulting thruster can thus be extremely efficient with a thrust approaching ≈100µN, depending on array size. We are working within the European FP7 project MicroThrust consortium to develop the complete MEMS electrospray array thruster for 10-100 kg class satellites to enable these small spacecraft to perform large delta-V missions (5 km/s). This includes the manufacture of MEMS Silicon electrospray emitters, and also a passive propellant feeding system, a miniaturized high voltage power supply, and the investigation of possible missions. We report here on the experimental testing of the micromachined Silicon capillary arrays, constituting arrays of either 91 or 127 emitters. A detailed description of the manufacture of the arrays will be described in a separate companion paper. To increase the specific impulse and thrust, and to reduce the plume angle, an accelerator has been integrated into the thruster Silicon chips. The thrust, and specific impulse has been measured using a Time-of- Flight (ToF) system, whilst the emitter current has been measured as the applied voltage was varied. The plume angle has been discerned using a translating Faraday cup