Ionic Liquid Ion Sources Research Articles

Inferring ion cluster lifetimes from energy-resolved mass spectra of an EMI-BF4 electrospray plume

The fragmentation of ion clusters within the accelerating fields of ionic liquid ion sources (ILISs) is well documented and degrades ILIS performance and lifetime. Some of the most popular ILIS liquids, such as EMI-BF4 (1-Ethyl-3-methylimidazolium tetrafluoroborate) and EMI-Im (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), emit clusters with lifetimes as low as ∼1 ns. Studies of fragmentation within the accelerating field typically rely on measuring the plume energy distribution averaged over all plume species and comparing those measurements with numerical simulations to estimate ion cluster lifetimes. Here, for the first time, we estimate EMI-BF4 cluster lifetimes by analyzing the energy distributions of individual plume species. We use this novel analysis method to estimate mean lifetimes of positive EMI-BF4 ion clusters from previously published experimental data. We find that the mean lifetime ranges from τ=3.7ns to τ=124ns for [EMI+][EMI-BF4] dimers and ranges from τ=1.5ns to τ=23ns for [EMI+][EMI-BF4]2 trimers. Fitting those data to an analytical fragmentation model, we estimate the binding energy and temperature as ΔGS0=0.49eV and T=394K for dimers and ΔGS0=0.40eV and T=365K for trimers. Comparing our results with previous studies supports the conclusion that clusters are emitted with a wide distribution of internal energies, contrary to the common assumption of single internal energy for each species.

Tandem Energy-Analyzer/Mass-Spectrometer Measurements of an Ionic Liquid Ion Source

Ionic liquid ion sources (ILISs) produce charged particle beams by using a strong electric field to extract and accelerate molecular ions and ion clusters from liquid propellant. The wide variety of emitted species and the fragmentation of metastable ion clusters lead to broad distributions of energy and mass among plume species. Quantifying those distributions is critical to understanding ILIS physics and ILIS performance in a particular application. Here, we use an electrostatic energy analyzer in tandem with a mass spectrometer (EA/MS) to measure mass spectra for various energy ranges in the plume of an ILIS operating with the electrospray propellant EMI-BF4. Those spectra were used to calculate energy distributions for major plume species, probed at the entrance of the EA/MS instrument. Those measurements suggest that 12% of the plume (by current fraction) is composed of species formed by ion clusters that fragment more than once prior to reaching the instrument. For the first time, we estimate the composition of the plume within the electric field of the ion source from experimental measurements. That analysis shows that at least 10% of plume current was emitted as trimers, and more than 3% was emitted as larger species.

Ion evaporation-induced tip streaming from liquid drops of ionic liquids

Ion evaporation from charged surfaces of ionic liquids has aroused significant interest due to its wide range of applications in various fields, such as ionic liquid ion sources (ILIS), electrospray thrusters, and high-precision etching, among others. This study delves into the transient electrohydrodynamics of ionic liquid drops undergoing ion evaporation with the assistance of an externally applied electric field. A transient electrohydrodynamic (EHD) model is developed to make up for the dearth of direct observation (visualization). A structure morphologically resembling the traditional cone-jet is depicted, featuring charged jets emitted from the drop's ends that subsequently break into tiny droplets. In contrast to the traditional cone-jet, this structure forms through a distinct mechanism, with charge depletion induced by ion evaporation playing a crucial role. A comprehensive exploration of parameter influences reveals the following key findings: (a) the normalized radius of the jets increases with the augmentation of the dimensionless number Ξ, which is defined as the ratio of the electric potential energy of ions to their activation energy for evaporation; (b) enhancing the electric Bond number Bo, which quantifies the ratio of Maxwell stress to surface tension, expedites the temporal evolution of tip streaming.

Temperature effects on electrospray current from an externally wetted EMI-Im ionic liquid ion source

Ionic liquid ion sources are expected to be used in a wide range of applications such as space electric propulsion and focused ion beam micromachining. It is known that the backstreaming of secondary charged species generated by ion beam impacts can cause unexpected temperature rise and chemical changes in ionic liquids. This paper reports on results of heating experiments using a sharp needle emitter wetted with an ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide, at temperatures in a range from room temperature to 120 °C. Current measurements show that positive and negative electrospray currents from the heated emitter increased as the temperature increased. Time-of-flight (TOF) mass spectrometric measurements reveal that the beam composition changed significantly with increasing temperature, indicating that charged droplets as well as ions were emitted from the heated emitter. The TOF data show that a significant fraction of the current is due to droplets at higher temperatures. On the basis of the results obtained, the size and charge of the emitted droplets are discussed. The beam is roughly estimated to contain charged droplets with a diameter of around 20 nm at 120 °C.

(Invited) Ionic Liquid Electrospray Ion Sources for Space Propulsion

The number of microspacecraft launched has significantly increased since 2013, and more than 300 nano/microsatellites (< 50 kg) were launched in 2017. Although some microspacecraft already have a propulsion system, a commonly used propulsion system is a cold-gas thruster, which has a low delta-v (a few 10s of m/s), and thus, high delta-v (> 1 km/s) thrusters are required for more advanced missions. One of the candidates for such thrusters is ion thrusters, which were already mounted on two 50 kg-class microsatellites and successfully operated in space. However, it is not easy to miniaturize ion thrusters so that they can be mounted on even 10 kg-class nanosatellites unless high-pressure gas-feeding systems are removed. Moreover, ion thrusters require electron sources to neutralize ion beams, which consume additional propellants and power. Since most recently launched microspacecraft are less than 10 kg, more compact thrusters are required. Ionic liquid electrospray ion sources are attractive devices for such electric propulsion, especially for microspacecraft. Electrospray thrusters typically consist of many emitters, an extractor electrode, and an accelerator electrode optionally. When a high voltage between the emitter and the extractor is applied, the ionic liquid is transported to the emitter tips, and a strong electric field deforms the ionic liquid into a conical shape known as a Taylor cone. When the force of the electric field is stronger than the surface tension pressure of the ionic liquid, ions evaporate directly from the liquid surface. The extracted ions are then accelerated by the potential difference between the emitter and extractor to produce the thrust. Since the ionic liquid is composed of only cations and anions without solvent, its vapor pressure is negligible; therefore, it can exist in the liquid phase in vacuum, simplifying the propellant feed system and reducing the size of the entire propulsion system. Moreover, the direct acceleration of ions without discharges can be possible, and no neutralizers are required, which also contributes to the power and size reduction together with high efficiency. The key component of electrospray thrusters is the emitter, where there are typically three types of structures: externally-wetted (or needle-shaped), internally-wetted (or capillary), and porous ones. Each has ion emission characteristics depending on the structure. Here, we have fabricated these three types of emitter structures with a wide range of scale: 1 to 100 micrometers. Using these emitters, we have conducted ion emission experiments and beam diagnostics. We obtained a high current density over a few 10 mA/cm2 with 1-micrometer scale emitters, a high current of approximately 1 mA with porous emitters, and a high efficiency with externally-wetted emitters. The emitter structures as mentioned earlier and experimental results will be presented at the conference site.

Effects of the temperature of a protic ionic liquid on ion beam production by vacuum electrospray

Ionic liquid ion sources generate ion beams from ionic liquids by vacuum electrospray. Electrospray characteristics generally depend on the physical properties of the liquids used. A key factor affecting physical properties is temperature. In this study, ion beam production was investigated using a protic ionic liquid, propylammonium nitrate (PAN), at temperatures ranging from 22 to 60 °C. An ion beam was produced using a needle emitter equipped with a cartridge heater, thermocouple, and sharpened glass rod externally wetted with PAN. The experimental results showed that the heating of the emitter increased the ion beam current. This will be due to an increase in the conductivity and a decrease in the viscosity of PAN with increasing temperature. Furthermore, the abundance of larger cluster ions increased, whereas that of smaller cluster ions decreased with increasing temperature. It turned out, however, that higher heating of the emitter stopped ion beam production. Two hypotheses for the beam stop are proposed and discussed.

Emission spectra of glows produced by ionic liquid ion sources

Electrospray devices, such as ionic liquid ion sources, often exhibit glows during operation in high vacuum facilities. The lack of electron excitation mechanisms during electrospray operation prompts the question: “What causes glow in an electrospray device?” Our optical emission spectroscopy results show that electrospray glow exhibits a broad spectral response between 350 and 800 nm with emission lines corresponding to atomic metal constituents of impinged surfaces, neutral and ionized atomic constituents of the ionic liquid propellant, and molecular line shapes that may also be dissociation products of the ionic liquid. We have previously defined secondary species emission to describe the many complex interfacial phenomena that occur when electrosprayed species impact surfaces downstream of the emitter. Our analysis of the optical emission spectra shows that these glows are only possible in the presence of secondary species emission. Therefore, the answer to the proposed question: high-velocity impacts that generate secondary species are the root cause of glow for electrosprays in high vacuum facilities.

Quantification of ionic-liquid ion source beam composition from time-of-flight data

Ionic-liquid ion sources produce beams of charged particles through evaporation and acceleration of ions and charged droplets from the surface of an ionic liquid. The composition of the emitted beam can impact the performance of ion sources for various applications such as focused beams for microfabrication and space propulsion. Numerical inference is considered for quantification of the beam composition of an ionic-liquid ion source through determining the current fraction of different species along with providing uncertainty in inferred values. An analysis of previously presented data demonstrates the ability to quantify the presence of ion clusters, including the distinct presence of heavy ion clusters such as heptamers. Quantification of beam composition will be an important technique for quantitative comparison of different time-of-flight data.

Energy properties and spatial plume profile of ionic liquid ion sources based on an array of porous metal strips

An ionic liquid ion source (ILIS) is a kind of high brightness ion source capable of providing high-speed positive or negative ion beams. This paper presents a miniaturized ILIS based on an array of porous metal strips. The porous emitter array, integrated with seven 10 mm long strips, is fabricated using wire electrical discharge machining (WEDM) combined with electrochemical etching. The assembled ILIS is 30 mm × 30 mm × 17.5 mm in size and weighs less than 25 g. A series of experiments, including an I–V characteristic test, a retarding potential analyzer (RPA) test, and a spatial plume distribution test, have been conducted in vacuo to characterize the performance of the ILIS. Results show that the emitted current is up to about 800 μA and ion transparency is as high as 94%. Besides, RPA curves reveal that the total fragmentation rate of the emitted particles accounts for 48.8% in positive mode and 59.8% in negative mode. Further, with the increase in applied acceleration voltage, the voltage loss rises while the energy efficiency decreases. It is also found that the plume perpendicular to the strips has a higher divergence than the one parallel to the strips. A numerical simulation by COMSOL reveals that the electric field distribution between the two electrodes results in such a spatial plume profile.

The role of secondary species emission in vacuum facility effects for electrospray thrusters

Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86 μg s−1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.

Electrostatic Levitation on Atmosphere-Less Planetary Bodies with Ionic-Liquid Ion Sources

This paper presents an initial feasibility study on the use of ionic-liquid ion sources for electrostatic actuation on atmosphere-less planetary bodies. The natural surface charging of atmosphere-less planetary bodies has been studied as the cause of the transport of regolith across the surface and for electrostatic levitation of a spacecraft in proximity to small asteroids. The low magnitude of the natural surface electric field (order ) severely limits the capability of a vehicle to leverage electrostatic levitation as its maneuvering strategy, particularly on large planetary bodies, such as the Moon. Ionic-liquid ion sources are considered as an actuator for charging of both the vehicle as well as the local surface of the planetary body. By irradiating the surface with ions, the surface electric field can be increased far beyond its natural value and could enable electrostatic levitation on planetary bodies as large as the Moon with current technology. A low-fidelity analytical model of the charging process is developed to estimate requirements on the local surface charge density and limitations on the levitation height and translational speed of the vehicle. Experiments are conducted in a laboratory environment to demonstrate the feasibility of using ionic-liquid ion sources for combined vehicle and surface charging by creating a 1 mN electrostatic force through charge transport while requiring only 0.2 mW of input power. These experiments are in reasonable agreement with the low-fidelity model that describes the fundamental physics of this concept.

Electrochemical Reactions of Ionic Liquid in Vacuum and Their Influence on Ion-Beam Production by Electrospray

Electrochemical reactions at interfaces between metal and ionic liquid are of great practical importance in ionic liquid ion sources, which can produce negative and positive ion beams by electrospray in vacuum. This is because electrochemical reactions involved in ion beam production can cause emitter degradation, leading to beam instabilities. Here we investigate ion beam production using a tungsten needle wetted with ionic liquid. We also investigate electrochemical reactions of ionic liquid at tungsten electrodes in vacuum with a quadrupole mass spectrometer, which allows us to measure volatile products formed via electrochemical reactions. Two types of ionic liquids were tested: propylammonium nitrate (PAN), which is protic and hydrophilic, and 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide ([EMIM][TFSA]), which is aprotic and hydrophobic. Beam measurements show that a negative ion beam containing large cluster ions can be produced continuously using each ionic liquid. Electrochemical measurements with PAN show that volatile products are formed via electrochemical oxidation and reduction, thereby indicating that ion beams of PAN can be produced continuously in both negative and positive ion modes. Experiments with [EMIM][TFSA] show that the electrochemical reductions of both [EMIM]+ and [TFSA]− occur and thus form volatile products, indicating that such reactions enable its continuous negative ion beam production.

Measurement of the dissociation rates of ion clusters in ionic liquid ion sources

Ionic liquid ion sources utilize electric fields to evaporate and accelerate ions and ion clusters to ∼1 keV energies. Ion clusters may dissociate after evaporation, which is not a well-characterized phenomenon and has relevant consequences in many applications. We measure the dissociation rate-constants of ion clusters for several ionic liquids. It is found that ion cluster dissociation occurs on timescales of the order of 1–5 μs and follows a constant-rate equation in the region outside the ion source. Using the measured rate-constants, we estimate the post-emission ion cluster temperatures. We also qualify the way the electric field enhances the rate-constants. Finally, our work supports the hypothesis that ion clusters with many degrees of freedom have lower dissociation rates.

Ion beam generation from a protic ionic liquid source with an externally wetted tungsten needle

Ionic liquid ion sources (ILISs) are similar to liquid metal ion sources and use ionic liquids (i.e., room temperature molten salts) instead of liquid metals. Aprotic ionic liquids have been used in most cases, whereas protic ion liquids have hardly been used thus far. In this study, propylammonium nitrate (PAN), which is a protic ionic liquid, was tested as the liquid material for an ILIS. Beam generation was investigated in DC operation using a tungsten needle externally wetted with PAN. Experimental results showed that a stable ion beam was generated without special measures and that the ion beam contained cluster ions with m/z values on the order of 103. Additionally, electrochemical experiments were performed in vacuum to investigate electrochemical reactions involved in beam generation. Residual gas analysis revealed that H2O and O2 were electrochemically produced at a tungsten electrode immersed in PAN. These products will probably result from electrochemical reactions of water in PAN because protic ionic liquids contain a small amount of water even in vacuum. The stability of the ion beam can be explained by the effect of water in PAN.

Fabrication of porous emitters for ionic liquid ion source by wire electrical discharge machining combined with electrochemical etching

Ionic liquid ion source (ILIS) is a promising ion source, which can be applied to space propulsion, microfabrication, and surface modification. Fabrication of high-quality ILIS emitters is one of the key technologies for the application of ILIS. A new method is proposed for the fabrication of porous emitters with a designed shape. This method uses wire electrical discharge machining (WEDM) combined with electrochemical etching, and the porous emitter is fabricated by two steps. First, the porous metal is machined by WEDM to get the external geometry of the emitter. Then, electrochemical etching is employed to remove the recast layer. A series of experiments has been conducted to find the appropriate machining parameters. Experiments reveal that sharp porous emitter tips for the ILIS can be fabricated by WEDM combined with electrochemical etching at 5 V etching voltage. Moreover, the apex curvature radius of the emitter is controllable by adjusting the etching time. It is found that the apex curvature radius varies from 4.5 μm to 18.4 μm when increasing etching time from 40 s to 120 s at 5 V etching voltage. Those emitters have been applied to ILIS tests, and their I-V characteristics are investigated. Furthermore, this method has been used to machine dense fields of emitters. A 1 cm2 emitter array chip integrated with 676 emitters has been successfully machined, and the I-V characteristic curve of the emitter array chip is also achieved.

Development of a microthrust balance and ion beam measurement system: Characterizing a dual-mode thruster for spacecraft

There is a rapid progression toward spacecraft miniaturization due to the prospect of reduced launch costs, the requirement for satellite constellations, and the integration of microelectromechanical systems (MEMS). Satellite propulsion is required for increased mission flexibility and orbital debris mitigation, but down-scaling of such devices has proven non-trivial. Some electrostatic thrusters are both challenging and promising given their increase in performance with small geometry, but the relatively low thrust produced through ion acceleration limits their mission applicability. For this reason, it may be desirable to produce thrusters that are able operate across a wide range of specific impulses, depending on maneuver time, thrust bit requirements, and several other factors that influence propulsion system selection. At RAPPEL, a bimodal thruster that can operate at high specific impulse with electrospray propulsion or high thrust using cold gas expansion has been developed. Inevitably, such a propulsion system comes with complications related to performance characterization. A novel and economical data acquisition system has been established to measure the performance of both modes in high vacuum. For the cold gas mode, a torsional thrust balance was built in-house to measure thrust levels between 1−15mN, and can be easily modified for lower ranges. The natural frequency of the thrust stand was measured to be 5.6Hz. The vacuum system sustained pressures below 5mbar while the cold gas nozzle operated, and 1.3×10−5mbar during electrospray operation. A 3-D printed linear cold gas nozzle was used to validate the thrust stand. Its thrust was close to 9mN at a mass flow rate of 20mg/s of nitrogen, producing a specific impulse of just under 45s. Due to the noise present in thrust stand measurements at the sub-mN force-level produced by electrosprays, the thrust stand was built to also accommodate a complementary ion beam current measurement system. The power processing unit, used to generate square-wave ±2500V signals using optodiodes, was built to operate a bipolar ionic liquid ion source (ILIS). The rise and fall times of the push/pull switch were close to 1ms and 30ms, respectively. The electrical power system used to operate the electrospray and measure its performance is examined in detail, followed by the results from validation of both data acquisition systems.

A low complexity time-of-flight mass spectrometer with ion size measurement based on secondary particle yield

A mass spectrometer has been designed to distinguish ions of different mass but identical mass to charge ratio. Separation of mass from charge is based upon secondary particle emission when each ion impacts the detector. The yield of secondary particles is a function of kinetic energy rather than energy-per-charge. Combined with an electrostatic energy gate and time-of-flight, the mass can be determined from kinetic energy, energy-per-charge, and mass-per-charge. The design is of moderate precision but simpler than available alternatives – particles need not be extracted from the vacuum chamber for sizing nor the detector kept at cryogenic temperatures. The prototype is demonstrated on an aluminum ablation plume and an ionic liquid ion source.

Etching of glass, silicon, and silicon dioxide using negative ionic liquid ion sources

Ionic liquid ion sources have been proposed as a new type of ion source for focused ion beam and broad ion beam applications. In this paper, the ionic liquid EMI-BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) was used as an ion source to generate negatively charged ions and irradiate glass (Pyrex 7740), silicon, and silicon dioxide targets. The results indicate that negative EMI-BF4 ion beams can prevent issues related to surface charge accumulation on dielectric substrates, achieving etching selectivities of SiO2:Si of at least 1.55. The etching rate increases on glass, silicon, and silicon dioxide at higher ion landing energies. It is shown that the negative EMI-BF4 beam has a higher yield than traditional metal gallium ion beams, likely due to the chemical reactivity of fluorine radicals. This effect is also noticeable when compared to results using positive EMI-BF4 beams.

Validation of a time-of-flight mass spectrometer using an ionic liquid ion source

A low-cost time-of-flight mass spectrometer has been developed to characterize the plasma plume resulting from pulsed laser ablation. In this paper, an ionic liquid ion source using EMI-BF4 is used to experimentally validate the spectrometer, as well as a model of the spectrometer. The mass spectrometer is shown to experimentally measure the energy-per-charge distribution of the ion source, and match with literature. In addition, a relation between alignment error and measurement error predicted by theory is confirmed, further validating the model. Now that it has been validated, the spectrometer can confidently be used to determine the two-dimensional distribution of speed and energy-per-charge of a pulsed laser ablation plasma, or other pulsed plasma sources.

Sputtering of silicon and glass substrates with polyatomic molecular ion beams generated from ionic liquids

The effect of irradiating 1-ethyl-3-methylimidazolium positive (EMIM+) or dicyanamide negative (DCA–) ion beams using an ionic liquid ion source was characterized concerning its sputtering properties for single crystalline Si(100) and nonalkaline borosilicate glass substrates. The irradiation of the DCA– ion beam onto the Si substrate at an acceleration voltage of 4 and 6 kV exhibited detectable sputtered depths greater than a couple of nanometers with an ion fluence of only 1 × 1015 ions/cm2, while the EMIM+ ion beam produced the same depths with an ion fluence 5 × 1015 ions/cm2. The irradiation of a 4 kV DCA– ion beam at a fluence of 1 × 1016 ions/cm2 also yields large etching depths in Si substrates, corresponding to a sputtering yield of Si/DCA– = 10, and exhibits a smoothed surface roughness of 0.05 nm. The interaction between DCA– and Si likely causes a chemical reaction that relates to the high sputtering yield and forms an amorphous C-N capping layer that results in the smooth surface. Moreover, s...