McKenna JD Breddan, Richard E Wirz “Machine learning electrospray plume dynamics,” Engineering Applications of Artificial Intelligence 133 (2024); https://doi.org/10.1016/j.engappai.2024.108095

Machine learning models are applied to simulated electrospray particle data to investigate plume dynamics from emission to final particle properties. A limited set of final particle properties are successfully regressed exclusively from emission property inputs. Random Forest model feature rankings for final plume angle reveal that particle charge has dominant influence when emission velocity is strictly axial, while lateral emission velocity has dominant influence when particles are emitted with an off-axis velocity component. In addition to providing correlations between initial and final particle properties, the machine learning models also identify correlations between different final particle properties. These correlations reveal opportunities for experimental approaches and diagnostic design by determining experimental measurements that offer insight into desired final particle properties.

NM Uchizono, RE Wirz, AL Collins, C Marrese-Reading, SM Arestie, JK Ziemer, “A diagnostic for quantifying secondary species emission from electrospray devices,” Review of Scientific Instruments, 92(2) (2023); pdf

Measuring the polydisperse beam of charged species emitted by an electrospray device requires accurate measurements of current. Secondary species emission (SSE) caused by high velocity nanodroplet or molecular ion impacts on surfaces contributes to substantial uncertainty in current measurements. SSE consists of both positive and negative species, so mitigating measurement uncertainty requires different considerations than plasma diagnostic techniques. The probe and analysis methods described herein distinguish between current contributions from positive SSE, negative SSE, and primary species. Separating each contribution provides positive and negative SSE yield measurements, and corrected current measurements that reflect the true primary current. Sources of measurement uncertainty in probe design are discussed, along with appropriate mitigation methods. The probe and analysis technique are demonstrated on an ionic liquid electrospray operating in droplet emission mode to obtain an angular distribution of positive and negative SSE yields for an ionic liquid electrospray.

McKenna J.D. Breddan, Richard E. Wirz, “Electrospray plume evolution: Influence of drag,” Journal of Aerosol Science, Volume 167 (2023); https://doi.org/10.1016/j.jaerosci.2022.106079

The University of California, Los Angeles (UCLA) Plasma, Energy, & Space Propulsion Laboratory (PESPL) presents the Discrete Electrospray Lagrangian Interaction (DELI) Model for simulating the evolution of electrospray plumes. This publication describes the DELI Model, verification of its Coulomb collision module, model validation with atmospheric plume data, and novel comparisons of simulated plumes evolved with different fractions of the drag force. DELI Model results reproduce experimentally-observed droplet clustering events that yield plume expansion as a result of Coulomb repulsion. Furthermore, the presented comparison of identical emitted species evolved to steady state with different fractions of applied drag force demonstrates decreased mean droplet velocity and increased plume expansion with increased drag force. Publication results serve as a useful tool in examining electrospray plume evolution in atmospheric and vacuum regimes. Keywords: Electrospray; Plume evolution; Coulomb expansion; Drag deceleration

Collins, A.L., Wright, P.L., Uchizono, N.M. et al., “High angle mass flux of an electrospray plume,” J Electr Propuls 1, 32 (2022); https://doi.org/10.1007/s44205-022-00031-w

High-resolution mass flux measurements of an electrospray plume are reported at high (>30°) angles, which are relevant to direct impingement of downstream electrodes. Interrogation of the plume edge greatly reduces uncertainty in electrospray device lifetime estimation related to mass flux to electrode surfaces. An angularly-actuated Thermoelectric Quartz Crystal Microbalance (TQCM) provides resolution down to 2 pg cm² s⁻¹, allowing the highest resolution mass flux measurements of an electrospray plume to be reported herein. In-situ microscopy of the electrospray meniscus revealed changes to the electrode lines-of-sight of approximately 2°–3° due to the increasing meniscus tip height with beam current. Using the TQCM measurements and previous QCM results, a data-driven model is proposed for estimating electrode impingement as a function of beam current and aperture line-of-sight, which quantitatively captures the rapid increase in mass flux at higher beam currents or at low angles. The results show it is possible to guarantee negligible electrode impingement within a specified range of throttle levels.

Huh, H., & Wirz, R. E., “Simulation of electrospray emission processes for low to moderate conductivity liquids,” Physics of Fluids, 34(11) (2022); https://doi.org/10.1063/5.0120737

An electrohydrodynamic numerical model is used to explore the electrospray emission behavior of both moderate and high electrical conductivity liquids under electrospray conditions. The Volume-of-Fluid method, incorporating a leaky-dielectric model with a charge relaxation consideration, is used to conserve charge to accurately model cone-jet formation and droplet breakup. The model is validated against experiments and agrees well with both droplet diameters and charge-to-mass ratio of emitted progeny droplets. The model examines operating conditions such as flow rate and voltage, with fluid properties also considered, such as surface tension, electrical conductivity, and viscosity for both moderate and high conductivity. For high conductivity and surface tension, the results show that high charge concentration along with the meniscus and convex cone shape results in a higher charge-to-mass ratio of the emitted droplets while lower conductivity and surface tension tend towards concave cone shapes and lower charge-to-mass droplets. Recirculation flows inside the bulk liquid are investigated across a range of non-dimensional flow rates, δ, and electric Reynolds numbers, ReE. For high conductivity liquid emission at the minimum stable flow rate, additional recirculation cells develop near the cone tip suggesting the onset of the axisymmetric instability.

Wright, P., Wirz, R., “Transient Flow in Porous Electrosprays,” (2022); https://doi.org/10.21203/rs.3.rs-1865748/v1

Porous ionic electrospray emitters have received significant interest for space propulsion due to their performance and operational simplicity. We have developed a diffusion equation for describing the transient flow response in a porous electrospray emitter, which allows for the prediction of the settling time for flow in the porous emitter. This equation accounts for both the change in liquid storage at exposed pores on the emitter with pressure, and viscous diffusion through Darcy’s law. Transient flow solutions are provided for the most common emitter topologies: pillar, cone, and wedge. Transient flow solutions describe the settling time and magnitude of current overshoot from porous electrosprays. Comparing diffusion of pressure to the onset delay model for electrospray emission shows that diffusion is most relevant at higher voltages and when a porous reservoir is used. Accounting for multiple emission sites on the wedge geometry shows that emission sites settle in proportion to emission site spacing to the power -1.74. Applying the diffusion equation to published results shows good agreement between analytical predictions and experimental data.

Uchizono, N., Wright, P., Collins, A., et al., “Emission spectra of glows produced by ionic liquid ion sources,” Appl. Phys. Lett. 121, 154101 (2022); https://doi.org/10.1063/5.0096595

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.

Enomoto, T., Parmar, S.M., Yamada, R. et al., “Molecular Dynamics Simulations of Ion Extraction from Nanodroplets for Ionic Liquid Electrospray Thrusters,” J Electr Propuls 1, 13 (2022); https://doi.org/10.1007/s44205-022-00010-1

Molecular dynamics (MD) simulations were performed for ion extraction from electrospray thrusters to investigate relevant extraction processes numerically. To approximate the electrospray jet tip, a simulation domain consisting of 4-5 nm-sized ionic liquid droplets was used. The extracted ion angles and kinetic energies from EMI–BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and EMI–Im (1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide) droplets were quantified by applying uniform electric fields of 1.3–1.7 V nm−1. The MD simulations are in great agreement with simulations presented in the literature and consistently show a greater preference for monomer emission than reported experimentally. At field strengths above 1.5 V nm−1, apparent droplet fracturing and breakup lead to an increase in ion angular velocity distributions. Greater mobility of EMI–BF4 ions than EMI–Im was also observed, indicative of the crucial role of cation-anion hydrogen bond strengths in ion extraction and beam composition between different propellants.

Uchizono, N. M., Marrese-Reading, C., Arestie, S. M., Collins, A. L., Ziemer, J. K., & Wirz, R. E., “Positive and negative secondary species emission behavior for an ionic liquid electrospray,” Applied Physics Letters, 121(7) (2022); https://doi.org/10.1063/5.0102592

Ionic liquid electrosprays can emit a polydisperse population of charged droplets, clusters, and molecular ions at high velocity. Secondary species emission (SSE) is a term that encompasses the many concurrent impact and emission phenomena that occur when electrosprayed primary species strike a surface, resulting in a diverse population of secondary electrons, ions, clusters, and droplets. This letter examines the spatial dependency of SSE behavior across an [EMI]Im electrospray beam using microscopy of the target surface, and experimental quantification of SSE yields as a function of plume angle. Microscopy of the beam target confirms our prediction of shock-induced desorption when operating at elevated beam voltages. SSE yield measurements show that, upon impact with a surface, incident primary species that consist of entirely positive charge will produce both positive and negative SSE. Furthermore, results show that the SSE yields for an ionic liquid electrospray have strong spatial and energy dependencies. These findings have significant implications for understanding and predicting ionic liquid electrospray thruster lifetime and performance, and focused ion beam applications.

Uchizono N.M., Collins A.L, Marrese-Reading C., Arestie S.M., Ziemer J.K., Wirz R.E., “The Role of Secondary Species Emission in Vacuum Facility Effects for Electrospray Thrusters,” J. Applied Physics 130, 143301 (2021); https://doi.org/10.1063/5.0063476

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 0.86ugs^-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.

Thuppul A., Collins A.L., Wright P.L., Uchizono N.M., Wirz R.E., “Mass Flux and Current Distributions of Electrospray Plumes,” J. Applied Physics 130, 103301 (2021); https://doi.org/10.1063/5.0056761

Performance and lifetime analysis of electrospray thrusters requires accurate knowledge of the mass and charge distributions of the plume. Mass flux and current density distributions were measured for a single capillary electrospray emitter using EMI-Im and found to be substantially different across a wide range of flow rates and emission voltages. Mass flux measurements yield an n~3 super-Gaussian profile across all flow rates and voltages, while current density measurements change shape from n~1.5-2.5 –super-Gaussian profiles monotonically with decreasing flow rate, where n=1 is Gaussian and higher n values correspond to increasingly more flat-top “super-Gaussian” profiles with steeper drop-off toward higher angles. For increasing flow rate, the mass flux profile grows while maintaining its shape, whereas the current density profile exhibits higher kurtosis, i.e., plumes that distribute proportionately more charge to higher angles. Additionally, higher extraction voltages exhibited tilted emission that led to highly off-axis plumes, ~10 degrees, for both mass flux and current density. Lifetime and performance assessments of electrospray thrusters must consider that mass flux and current density in the plume display different distribution shapes and trends in shape across changes in extraction voltage and flow rate.

Uchizono N.M., Collins A.L., Thuppul A., Wright P.L., Eckhardt D.Q., Ziemer J.K., Wirz R.E., “Emission modes in electrospray thrusters operating with high conductivity ionic liquids,” Aerospace, Special Issue: Electric Propulsion, 2020, 7(10), 141; https://doi.org/10.3390/aerospace7100141

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.

Magnusson J.M., Collins A.L., Wirz R.E., “Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters”, Aerospace, Special Issue: Electric Propulsion, 2020, 7(11), 153, https://doi.org/10.3390/aerospace7110153

To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼9×10⁵ cm s⁻¹. The mean secondary electron yield, 𝛾, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1 kV. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼1000 u and velocities greater than ∼10⁶ cm s⁻¹. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.

Thuppul A., Wright P.L., Collins A.L., Ziemer J.K., Wirz R.E., "Lifetime Considerations for Electrospray Thrusters", Aerospace, Special Issue: Electric Propulsion, 2020, 7(8), 108, https://doi.org/10.3390/aerospace7080108

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.