Elaine Petro

Abstract The operational lifetime of ionic liquid electrospray propulsion systems is limited by plume–extractor electrode interactions. Over time, propellant accumulation, surface erosion, and electrical shorts degrade the extractor and therefore restrict the total impulse throughput. Characterizing the secondary species generated by plume impacts with deposited ionic liquid is therefore essential to understanding and mitigating these degradation pathways. A surface analysis technique known as time-of-flight secondary ion mass spectrometry (TOF-SIMS), in the form of a custom electrospray laboratory diagnostic and an analytical-grade system, yields a comprehensive analysis of secondary ions formed from energetic ion beam impacts with ionic liquid thin-film substrates. Results revealed nearly identical positive secondary ion species for both EMI-BF 4 and EMI-Im thin films, whereas EMI-Im produced a more diverse set of negative ions consistent with the greater chemical complexity of its anion. The analytical-grade SIMS spectra revealed many relatively high mass-to-charge ratio secondary ions likely below the detection limit for the laboratory diagnostic, thus shifting the average <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>m</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>z</mml:mi> </mml:mrow> </mml:math> value to above the monomer mass for most spectra.

Electrospray thrusters promise compact, high specific impulse propulsion for small spacecraft, yet ground characterization remains confounded by secondary species emission and incomplete diagnostics of neutral products. To address these limitations, we perform energy-resolved mixed quantum/classical ab initio molecular dynamics of neutral 1-ethyl-3-methylimidazolium tetrafluoroborate colliding with Au extractor surfaces with impact energies from 10 to 100 eV to resolve fragment species spectra, charge states, kinetic energy partitioning, and scattering geometry. The simulations reveal a three-stage sequence with impact energy: the low energy regime, 10–20 eV, which favors ionic dissociation; intermediate energy regime, between 30 and 40 eV, opens a neutralization window; and high energy regime, greater than 50 eV, drives covalent fragmentation into many light products with mixed charge states. Fractional energy distributions show a transition from few-body, energy-concentrated outcomes in the low energy regime to many-body, energy-dispersed outcomes in the high energy regime. Deflection angle distributions exhibit a strong mass-to-angle anti-correlation such that heavier fragments favor small deflection, while lighter fragments are observed across the full deflection range but dominate larger deflection angles. The fraction of metastables peaks near 50 eV, coinciding with abundant neutral fragment production. Importantly, neutral bombardment still produces charged secondaries at the target even when the upstream ion plume is fully suppressed by a decelerating electrode. These findings provide a basis for de-biasing facility measurements by pairing tandem time-of-flight secondary ion mass spectrometry and a residual gas analyzer with suppression-bias corrections to inform the design of electrospray thrusters that reduce interception on extractor surfaces.

CRISPI (Compositional Regolith and Icy Surface analysis via Particle Impact) is a proposed ridealong in-situ mass spectrometer (MS) smallsat mission to provide synergistic astrobiology science at Ariel to the Uranus Orbiter and Probe (UOP) mission. By utilizing a high-resolution impact ionization MS based on the Europa Clipper’s Surface Dust Analyzer on a deployable smallsat platform, CRISPSI will provide in-situ mass data of the surface of Ariel, a moon of Uranus and one of the highest-priority astrobiology targets in the outer solar system. Ariel is specifically called out in the Origins, Worlds, and Life (OWL) Decadal Survey as a high-priority target because it is potentially a liquid-water-bearing icy ocean world that is known to have ammonia, an organic volatile species that indicates recent surface processing (possibly from cryovolcanism or other subsurface sources) and may act as an anti-freeze for water. However, the baseline UOP mission has few flybys of Ariel with limited compositional data provided only by optical spectrometers. CRISPI addresses this gap by enabling the direct and unambiguous detection of organics and salts present in and the quantification of ice-to-rock and salt-to-water ratios of &gt;1000 surface dust grains mapped to visible features on Ariel’s surface. In principle, CRISPI may also be used to study Miranda (another high priority astrobiology target) and the Uranus ring systems, which are poorly understood and are also specifically called out in the OWL Decadal Survey. Here we present results from the Cornell SmallSat Mission Design School (SMDS) that developed the CRISPI mission concept. We propose CRISPI to work in synergy with the UOP’s magnetometer, imagers, and optical spectrometers to extend its science reach, break ambiguities in optical spectrometer data, and provide unique and critical in-situ mass spectral data to assess Ariel’s habitability and search for organic signs of past or extant life.

Manufacturing variations and surface defects in electrospray ion sources contribute to performance degradation and reduced device lifetime, yet their quantitative impact on plume characteristics remains poorly understood. This work presents a coupled modeling approach that integrates 2D axisymmetric electrohydrodynamic solutions with fully 3D kinetic particle simulations to investigate how emitter-extractor concentricity errors and off-apex emission site formation affect plume divergence, thrust vector alignment, and extractor impingement. A meniscus distortion methodology is developed to approximate physically consistent fluid surfaces in non-axisymmetric electric fields while preserving the emission site topology within 200 nm of the apex. Simulations were conducted for a single EMI – BF4 ion source operating at 1900 V with concentricity errors from 0 to 80 μm and emission site offsets from 0 to 12 μm. Results demonstrate that emission site location dominates extractor impingement behavior: a 12 μm offset increases intercepted mass fraction from 0.15% to 4% and produces thrust misalignment of 33°, whereas an 80 μm concentricity error yields only 5.1◦ misalignment with no significant increase in intercepted mass. These findings indicate that surface profile control near the emitter apex, rather than emitter-extractor alignment, is the critical manufacturing parameter for minimizing off-axis emission. For emitters with 11 μm tip radius, surface roughness below 1 μm is recommended to prevent defect-induced emission site displacement.

The efficiency of ion emission from electrospray ionization sources is fundamentally constrained by the binding energy between the ionic liquid (IL) cation and anion. High binding energies favor the ejection of clusters and droplets, reducing the achievable specific impulse, while low binding energies promote the formation of a pure-ion beam. Density functional theory (DFT) provides accurate ion-pair energetics but is too computationally expensive for large-scale screening of IL propellants. In this work, we develop a machine learning (ML) framework capable of predicting DFT-calculated ion-pair binding energies directly from the SMILES representations of the constituent ions. Dimensionality-reduced features obtained via principal component analysis (PCA) were used to train two regressors: Ridge regression and kernel Ridge regression. Kernel Ridge regression achieved the better performance, with a coefficient of determination (R²) of 0.4, and a Spearman’s rank correlation coefficient of 0.9 with ten principal components. An R² value of 0.4 is still considered weak, which can be attributed to the size of the training set (only 20 data points). While there is room for improvement in the model’s predictive power, these results demonstrate that descriptor-based ML models can reliably rank IL binding energies and serve as efficient surrogates for first-principles screening of potential ionic liquid propellants for electrospray applications.

The influence of propellant properties on heat generation in Hall effect thrusters is evaluated and compared using two analytical plasma energy deposition models. Both approaches quantify discharge power losses to conduction to channel surfaces, radiation from excited neutral particles, and the plume. Each uses a propellant-specific energy deposition approach that accounts for the energy carried by each particle, specifically the ions, electrons, and neutrals. Initially developed for xenon, the models have been extended to krypton, argon, and water vapor. For each propellant, the ionization cost of electron-neutral collisions is evaluated and incorporated into the analytical models

The operational lifetime of ionic liquid electrospray propulsion systems is limited by plume-extractor electrode interactions. Over time, propellant accumulation, surface erosion, and electrical shorts degrade the extractor and therefore restrict the total impulse throughput. Characterizing the secondary species generated by plume impacts with deposited ionic liquid is therefore essential to understanding and mitigating these degradation pathways. A surface analysis technique known as Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), in the form of a custom electrospray laboratory diagnostic and an analytical-grade system, yields a comprehensive analysis of secondary ions formed from energetic ion beam impacts with ionic liquid thin-film substrates. Results revealed nearly identical positive secondary ion species for both EMI-BF4 and EMI-Im thin films, whereas EMI-Im produced a more diverse set of negative ions consistent with the greater chemical complexity of its anion. The analytical-grade SIMS spectra revealed many relatively high mass-to-charge ratio secondary ions likely below the detection limit for the laboratory diagnostic, thus shifting the average m/z value to above the monomer mass for most spectra. Finally, optical profilometry analysis reveals an estimated 0.5 nm/min sputter rate for an electrospray ion plume bombarding an ionic liquid thin film.

The operational lifetime of ionic liquid electrospray propulsion systems is limited by plume-extractor electrode interactions. Over time, propellant accumulation, surface erosion, and electrical shorts degrade the extractor and therefore restrict the total impulse throughput. Characterizing the secondary species generated by plume impacts with deposited ionic liquid is therefore essential to understanding and mitigating these degradation pathways. A surface analysis technique known as Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), in the form of a custom electrospray laboratory diagnostic and an analytical-grade system, yields a comprehensive analysis of secondary ions formed from energetic ion beam impacts with ionic liquid thin-film substrates. Results revealed nearly identical positive secondary ion species for both EMI-BF4 and EMI-Im thin films, whereas EMI-Im produced a more diverse set of negative ions consistent with the greater chemical complexity of its anion. The analytical-grade SIMS spectra revealed many relatively high mass-to-charge ratio secondary ions likely below the detection limit for the laboratory diagnostic, thus shifting the average m/z value to above the monomer mass for most spectra. Finally, optical profilometry analysis reveals an estimated 0.5 nm/min sputter rate for an electrospray ion plume bombarding an ionic liquid thin film.

Electrospray thrusters promise compact, high specific impulse propulsion for small spacecraft, yet ground characterization remains confounded by secondary species emission and incomplete diagnostics of neutral products. To address these limitations, we perform energy-resolved mixed quantum/classical (QM/MM) ab initio molecular dynamics (MD) of neutral 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF4, colliding with Au extractor surfaces with impact energies from 10 to 100 eV to resolve fragment species spectra, charge states, kinetic energy partitioning, and scattering geometry. The simulations reveal a three-stage sequence with impact energy: the low energy regime, 10 to 20 eV, which favors ionic dissociation, intermediate energy regime, between 30 to 40 eV, opens a neutralization window, and high energy regime, greater than 50 eV, drives covalent fragmentation into many light products with mixed charge states. Fractional energy distributions show a transition from few-body, energy-concentrated outcomes in the low energy regime to many-body, energy-dispersed outcomes in the high energy regime. Deflection angle distributions exhibit a strong mass-to-angle anti-correlation such that heavier fragments favor small deflection, whereas lighter fragments populate larger deflection angles. The fraction of transient metastables peaks near 50 eV, coinciding with abundant neutral fragment production. Importantly, neutral bombardment still produces charged secondaries at the target even when the upstream ion plume is fully suppressed by a decelerating electrode. These findings provide a basis for de-biasing facility measurements by pairing tandem time-of-flight secondary ion mass spectrometry and residual gas analyzer with suppression-bias corrections to inform the design of electrospray thrusters that reduce interception and contamination on extractor surfaces.

Electrospray thrusters promise compact, high specific impulse propulsion for small spacecraft, yet ground characterization remains confounded by secondary species emission and incomplete diagnostics of neutral products. To address these limitations, we perform energy-resolved mixed quantum/classical (QM/MM) ab initio molecular dynamics (MD) of neutral 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF4, colliding with Au extractor surfaces with impact energies from 10 to 100 eV to resolve fragment species spectra, charge states, kinetic energy partitioning, and scattering geometry. The simulations reveal a three-stage sequence with impact energy: the low energy regime, 10 to 20 eV, which favors ionic dissociation, intermediate energy regime, between 30 to 40 eV, opens a neutralization window, and high energy regime, greater than 50 eV, drives covalent fragmentation into many light products with mixed charge states. Fractional energy distributions show a transition from few-body, energy-concentrated outcomes in the low energy regime to many-body, energy-dispersed outcomes in the high energy regime. Deflection angle distributions exhibit a strong mass-to-angle anti-correlation such that heavier fragments favor small deflection, whereas lighter fragments populate larger deflection angles. The fraction of transient metastables peaks near 50 eV, coinciding with abundant neutral fragment production. Importantly, neutral bombardment still produces charged secondaries at the target even when the upstream ion plume is fully suppressed by a decelerating electrode. These findings provide a basis for de-biasing facility measurements by pairing tandem time-of-flight secondary ion mass spectrometry and residual gas analyzer with suppression-bias corrections to inform the design of electrospray thrusters that reduce interception and contamination on extractor surfaces.

Ionic liquids (ILs) have been gaining increasing focus in a variety of applications including emerging electric-propulsion concepts. A quantitative understanding of how IL ions fragment during high-energy collisions with background gases is therefore essential for interpreting mass spectra, predicting ion lifetimes in plasma and vacuum environments, and designing IL-based technologies. This work uses molecular dynamics (MD) simulations with a reactive force field to numerically model the collision-induced dissociation (CID) of isolated ions (both positive and negative) and ion clusters (2:1 and 1:2 clusters) of the prototypical ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF 4 ), colliding with a nitrogen (N 2 ) molecule, exploring all possible fragmentation channels arising from the breaking of both ionic and covalent bonds at collision energies ranging from 10 electron volts (eV) to 100 electron volts (eV) in the laboratory frame. The molecular dynamics results are compared with the observations from tandem mass spectrometry (MS2) experiments to assess the reliability of the MD results. The MD modeling predicts the dissociation onset collision energy of the [EMIM] + ion to be 20 eV (lab frame), while the [BF 4 ] − ion requires a collision energy of at least 40 eV (lab frame) to undergo dissociation. The primary fragmentation product of the [EMIM] + ion is found to be the 3-methylimidazolium cation, [C 4 H 7 N 2 ] +, with the cyanide anion, [CN] −, being the major fragment ion at higher collision energies (≥60 eV, lab frame). The [BF 4 ] − ion, on the other hand, dissociates to form the fluoride ion, [F] −, and the neutral BF 3 molecule, with the [BF 2 ] + ion being formed at higher collision energies (≥60 eV, lab frame). Both the 2:1 and 1:2 ion clusters are found to fragment at the lowest simulated collision energy of 10 eV (lab frame), with the fluoride ion, [F] −, being formed with rising abundance as the collision energy is increased. When compared, the mass spectra from MD modeling and experiments demonstrate a reasonable agreement, which suggests that reactive MD can be a reliable surrogate for CID studies of complex IL ions.

This paper presents an analytical framework for optimizing refueling strategies for space missions using electric propulsion assuming the availability of in-space refueling stations. The authors derive analytical expressions for key performance metrics, including normalized payload and structure mass, firing time ratio, and specific propellant consumption. They demonstrate that an evenly distributed refueling sequence maximizes the payload mass and that the optimal specific impulse decreases as the number of refueling stations increases. Additionally, the authors extend the concept of the Stuhlinger characteristic velocity to incorporate refueling. Finally, an optimization problem is formulated to determine the ideal refueling sequence that maximizes payload under constraints on the mission time within practical mission constraints. Using the SMART-1 mission as a case study, the authors illustrate the potential of their refueling optimization approach to significantly increase payload mass and reduce specific propellant consumption at the expense of a slightly longer mission time.

Gaining a comprehensive understanding of the electrospray plume composition and how it is affected by intermolecular collisions is critical to the performance prediction and advancement of the technology. The current study uses molecular dynamics modeling to identify and quantify the different types of processes that can ensue from the collisions between the monomer ions and the neutral molecules in the plume of an electrospray thruster operating with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([Formula: see text]). Collision cross-sections are calculated for the identified collisional events as a function of the relative velocity of impact. The cross-section profiles provide quantitative insights into the relative importance of the different types of collisional interactions. We predict that 1) recombination and charge exchange are only significant at very low impact velocities [[Formula: see text]], 2) ionic fragmentation bears relevance in the impact velocity range [Formula: see text], and 3) covalent fragmentation dominates at the higher impact velocities [[Formula: see text]]. The results can be used to track changes in the composition of ion and neutral species due to collisions in the plume and the introduction of new species into the plume. The summary of collisional outcomes via cross-sections allows for their incorporation in larger-scale [Formula: see text]-body and particle-in-cell simulations of both individual and arrays of emitters.

The design and capability of a novel electrospray propulsion time-of-flight secondary ion mass spectrometry (TOF-SIMS) diagnostic are presented to investigate secondary species emission from surface impingement of energetic molecular ion plumes. Designed on the basis of traditional SIMS principles, this diagnostic provides information on the relative intensity and chemical composition of charged secondary species given electrospray operational parameters like incident angle, primary ion energy, and target surface composition. The system consists of an externally wetted tungsten ion source operating with room temperature ionic liquid propellant, a target with a secondary species acceleration mesh, and a time-of-flight mass spectrometer with an electrostatic gate and microchannel plate detector. The results show that energetic primary plume impacts with a silver metallic surface induce both sputtering of the target (Ag+, Ag2+, and Ag3+) and molecular secondary ion emission in both polarities. Within these secondary species, peaks related to the ionic liquid primary ions (B+ and F-) were definitively detected. The remaining secondary ions include fragmentation products of the EMI+ cation, common salt contaminants (Na+, K+, and Cl-), and hydrocarbon species from ambient vacuum facility conditions. The fragment pattern observed is consistent with both the ionic liquid components and typical contaminants encountered in high-vacuum TOF-SIMS analyses. For electrospray propulsion, these secondary species contribute to lifetime-limiting processes intrinsic to thruster operation, like impingement and degradation of electrodes and emitters, while also contributing to facility effects that can corrupt ground-based testing and thruster qualification.

Abstract This study introduces a machine learning framework to predict the suitability of ionic liquids with unknown physical properties as propellants for electrospray thrusters based on their molecular structure. We construct a training dataset by labeling ionic liquids as suitable (+1) or unsuitable (-1) for electrospray thrusters based on their density, viscosity, and surface tension. The ionic liquids are represented by their molecular descriptors calculated using the Mordred package. The dataset is extremely imbalanced due to the scarcity of suitable candidates. To mitigate the issue of imbalanced data, we applied a combination of oversampling the minority class and undersampling the majority class. We evaluate four machine learning algorithms-Logistic Regression, Support Vector Machine (SVM), Random Forest, and Extreme Gradient Boosting (XGBoost)-with SVM demonstrating superior predictive performance. The SVM predicts 193 candidate propellants from a dataset of ionic liquids with unknown physical properties. Further, we employ Shapley Additive Explanations (SHAP) to assess and rank the impact of individual molecular descriptors on model decisions.

This work examines the emission of secondary charged species from a gold-plated target electrode when exposed to an ion-mode electrospray plume of EMI-BF 4 from a single externally wetted tungsten emitter. A decelerating electrode is used with the electrospray source to probe ion energies from 30-1800 eV in the positive and negative mode. Next, the ion plume was fully suppressed using the decelerating electrode, allowing only neutral constituents of the plume to travel to the target, and the emission of secondary charged species is observed. Time of flight mass spectrometry is used to confirm ion mode emission and proper deceleration of electrospray plume, and was also used to detect the creation of secondary charged species created by decelerating the plume.

This study explores the applicability of reactive molecular dynamics and the transition state theory to develop a thermal fragmentation model for the positive ionic liquid monomer, 1-ethyl-3-methylimidazolium (EMI+). The mean lifetime of the monomer is described as a function of temperature and the external electric field. The products of the thermal dissociation of EMI+ are analyzed to better understand the fragmentation mechanism and its dependencies on internal and external factors, such as internal energy thermodynamics and external electric field, respectively. The study also explores a first-order theoretical analysis of the electrospray emission process to estimate the post-emission temperature of EMI+. We predict that, to first order, the post-emission temperature of EMI+ is a function of the emitted current, assuming an ideal emission process. For a typical emitted current range, we find that the post-emission temperature of EMI+ can vary from 310 to 350 K. At an average ion temperature of 330 K, our theoretical fragmentation model predicts that the mean lifetime of the monomer ranges from 2.1×10−4 ns under an axial electric field of strength 0.1 V/Å to 5.0×105 ns in the field-free region. Finally, the monomer fragmentation model is incorporated into the single-emitter n-body model of the plume to assess the effects on the evolution of the ion beam. It is found that there is no monomer fragmentation in the acceleration region for monomer temperatures up to 375 K, while at 550 K, the fraction of dissociated monomers can be up to 10%.

Vacuum electrospray ionization (ESI) thrusters are an emerging electric propulsion (EP) technology that operate by firing ions from vacuum stable ionic liquids at high velocity. These ESI systems have the potential to be among the most efficient propulsion sources available while also providing a level of precise thrust control that is difficult or impossible to achieve with other EP sources. However, the angular properties of the ESI ejecta plumes are not well understood, particularly with respect to off-axis firing, molecular composition across the plume, and droplet accumulation, which can cause electrical shorts that disable the thruster. Here, we present a novel method for two-dimensional steering of vacuum electrospray ionization beams to better understand their angular properties. Utilizing an externally wetted tungsten needle with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), we employ a dual-axis goniometer to achieve in vacuo beam steering in both pitch and yaw. This setup enables detailed angular-dependent time-of-flight measurements of molecular species within the ESI plume as a function of plume angle. Our findings reveal significant variations in the relative abundance of monomers, dimers, trimers, and heavy species across different beam angles, with monomers being at a relative minimum and fragments, trimers, and heavy species being at a relative maximum at the beam center. The ability to accurately aim the ion beam dramatically enhances the signal-to-noise ratio for various diagnostic tools, underscoring the utility of this system for both scientific studies and practical laboratory applications. This two-dimensional steering capability offers a robust framework for future investigations into ESI plume dynamics and composition, enabling more precise characterizations that are critical for optimizing ESI-based propulsion systems and other applications.

Electrospray time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a diagnostic that reveals the mass-to-charge ratio and intensity of secondary ions generated by plume-surface interactions. This study examines how varying diagnostic operating conditions — such as impact energy & secondary beam acceleration voltage, exposure time, and surface composition — affect secondary ion production for an ionic liquid electrospray primary ion source. Increasing the impact energy from 2.5 to 5.5 keV did not alter the secondary ion composition but increased the ion yields in both polarities. However, the threshold impact energy for detecting secondary ions was a kilovolt higher in negative polarity, suggesting either different collision processes between polarities or effects of the multichannel plate bias. Prolonged exposure to the electrospray plume showed stable secondary ion composition over an hour time period, though the initial detection of intact BF4- ions indicated plume deposition on the target surface. Plasma cleaning of the target surface resulted in higher sputtered target yields and less of the smaller m/z secondary ions in positive polarity, suggesting that the vacuum environment and/or deposition of the primary plume strongly influences TOF-SIMS spectra.

This work introduces a novel liquid nanoparticle propellant for electrospray systems. Two different propellants, each made from different neutralization ratios, are investigated. Fluid properties for each propellant are listed and compared to the state-of-the-art ionic liquids. The firing properties of the liquid nanoparticle ion source (LNIS) are captured in the positive and negative modes with time of flight mass spectrometry (TOF). Performance estimates of thrust, specific impulse, and efficiency are reported.

Traditional low-temperature and high-temperature heat pipes dominate existing thermal control systems, but a significant gap persists in addressing thermal rejection efficiency within the intermediate-temperature range. Ceramics, with their exceptional chemical and thermal stability, offer compatibility with a broad range of working fluids, making them uniquely suited to bridge this gap. This work presents a novel experimental study of additively manufactured (AM) ceramic heat pipes. Grooved alumina envelopes were fabricated and tested with ethanol and Dowtherm A. Mass rate-of-rise experiments were conducted to quantify the effects of silicate glaze coatings on permeability and effective pore radius. Atmospheric heat pipe testing exhibited stable isothermal behavior under a heat flux of up to 5.65 W across the adiabatic section. These findings establish a foundation for leveraging AM ceramic technologies to enhance spacecraft thermal management.

The goal of this work is to explore the feasibility of a regenerative cooling approach that allows a high-power electric thruster to operate adiabatically and reject all excess heat through propellant pre-conditioning. Specifically, this study investigates regenerative cooling as a thermal management solution for high-power Hall Effect Thrusters (HETs) to enable stable operation under high thermal loads. Drawing on established methods in liquid rocket engines (LREs), the research explores the adaptation of regenerative cooling to HETs by evaluating heat transfer properties across various propellants and cooling channel geometries. The work includes a preliminary one-dimensional (1D) steady-state analysis of cooling performance in HET discharge channels, laying the foundation for future computational and experimental studies.

This paper presents a comprehensive framework for optimizing refueling strategies for space missions utilizing electric propulsion assuming the availability of in-space refueling stations. We derive analytical expressions for key performance metrics, including normalized payload and structure mass, firing time ratio, and specific propellant consumption. We demonstrate that an evenly distributed refueling sequence maximizes the payload mass and that the optimal specific impulse decreases as the number of refueling stations increases. Additionally, we extend the concept of the Stuhlinger characteristic velocity to incorporate refueling. Finally, an optimization problem is formulated to determine the ideal refueling sequence that maximizes payload under constraints on the mission time within practical mission constraints. Using the SMART-1 mission as a case study, we illustrate the potential of our refueling optimization approach to significantly increase payload mass and reduce specific propellant consumption at the expense of a slightly longer mission time.

The design and capability of a novel time-of-flight secondary ion mass spectrometry electrospray propulsion diagnostic is presented to investigate secondary species emission from surface impingement of high-velocity, energetic molecular ion plumes. Designed on the basis of traditional Secondary Ion Mass Spectrometry (SIMS) principles, this diagnostic provides information on the relative intensity and chemical composition of secondary species given electrospray operational parameters like incident angle, primary ion energy, and target surface composition. The system consists of an externally-wetted tungsten ion source operating with room temperature ionic liquid propellant, a target with a secondary species extraction mesh, and a time-of-flight mass spectrometer featuring an electrostatic deflection gate and a multichannel plate detector. Results show that energetic primary plume impacts with metallic surfaces induce molecular secondary ion emission in both positive and negative polarities. Likely sources of these secondary ions are considered - including hydrocarbon contamination of the target surface and charged fragments of the molecular primary ionic liquid ions. For electrospray propulsion, these secondary species contribute not only to lifetime limiting processes intrinsic to thruster operation like impingement and thus degradation of electrodes and emitters, but also contribute to facility effects corrupting ground-based testing and thruster flight qualification.

This paper presents the frequency response of a 6 MHz quartz crystal microbalance (QCM) when bombarded by ionic liquid ion source electrospray plumes of various energies using EMI-BF $$_4$$ . Time of flight mass spectrometry (ToF-MS) measurements confirm ion mode emission from the single, externally wetted tungsten emitter ion source and a retarding potential analyzer (RPA) was used to record the energy profile of the plume. The frequency change of the QCM is positively correlated with plume energy, with the most energetic plume (1900 eV) yielding the largest (positive) frequency change, and the lowest energy (30 eV) yielding a decrease in frequency. These measurements indicate removal of material on the quartz crystal surface at high energies, and deposition of propellant at low energies. X-ray photoelectron spectroscopy (XPS) was used to measure the equivalent homogeneous composition of the surface of each quartz crystal, which revealed traces of propellant across all samples and showed greater deposition rates when exposed to a low energy plume.

A design optimization framework for an active orbital debris removal mission to minimize the life cycle cost and mission time is presented. The general concept of operations consists of spacecraft capable of deorbiting 22 identical high-priority debris from sun-synchronous orbit via continuous low thrust from electrostatic thrusters. This modular subsystem framework is proposed and investigated with a fractional factorial design of experiments approach as preliminary analysis in order to inform effects of design variables as well as identify the scope of possible design and output space. The single-objective optimization minimizes the life cycle cost of the mission through a heuristic-based genetic algorithm, resulting in a 4.12-year, $354 million mission using 5.34 kW xenon Hall thrusters. Pareto fronts generated by a multiobjective weighted-sum genetic algorithm are presented to highlight the cost and time objective space for both a nominal and expensive xenon price per kilogram to show design space fluctuations with xenon price volatility. High-power 8 kW xenon Hall thrusters are proven to be in closest proximity to the utopia of the multiobjective design space with a cost of $687 million and mission time of 1 year. High-power krypton Hall thrusters prove to offer a comparable alternative to xenon systems.

Here we present a novel method for two-dimensional steering of vacuum electrospray ionization beams to better understand their angular properties. Utilizing an externally wetted tungsten needle with the ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF$_4$), we employ a dual-axis goniometer to achieve in-vacuo beam steering in both pitch and yaw. This setup enables detailed angular-dependent time-of-flight (TOF) measurements of molecular species within the ESI plume as a function of plume angle. Our findings reveal significant variations in the relative abundance of monomers, dimers, trimers, and heavy species across different beam angles, with monomers being at a relative minimum and fragments, trimers, and heavy species being at a relative maximum at the beam center. At higher angles, the relative monomer abundance increases, but at the plume extremities heavy species and trimer fragments begin to rise. The ability to accurately aim the ion beam dramatically enhances the signal-to-noise ratio for various diagnostic tools, underscoring the utility of this system for both scientific studies and practical laboratory applications. This two-dimensional steering capability offers a robust framework for future investigations into ESI plume dynamics and composition, enabling more precise characterizations that are critical for optimizing ESI-based propulsion systems and other applications.

This study introduces a machine learning framework to predict the suitability of ionic liquids with unknown physical properties as propellants for electrospray thrusters based on their molecular structure. We construct a training dataset by labeling ionic liquids as suitable (+1) or unsuitable (-1) for electrospray thrusters based on their density, viscosity, and surface tension. The ionic liquids are represented by their molecular descriptors calculated using the Mordred package. We evaluate four machine learning algorithms: Logistic Regression, Support Vector Machine (SVM), Random Forest, and Extreme Gradient Boosting (XGBoost), with SVM demonstrating superior predictive performance. The SVM predicts 193 candidate propellants from a dataset of ionic liquids with unknown physical properties. Further, we employ Shapley Additive Explanations (SHAP) to assess and rank the impact of individual molecular descriptors on model decisions.

In the search for life in our galaxy, and for understanding the origins of our solar system, the direct imaging and characterization of Earth-like exoplanets is key. In a step towards achieving these goals, the Superluminous Tomographic Atmospheric Reconstruction with Laser-beacons for Imaging Terrestrial Exoplanets (STARLITE) mission uses five CubeSats in a highly elliptical orbit as artificial guide stars to enable tomographic reconstruction of the atmosphere for extreme multi-conjugate adaptive optics (MCAO). Through the use of current and next-generation extremely-large ground-based telescopes, the STARLITE constellation at its ∼350,000 km apogee can provide brighter than -10 magnitude artificial guide stars from a 10 cm launching telescope in a sub-arcminute field of view for up to an hour. Careful selection and design of the ∼760 nm on-board laser will allow O₂ detection and characterization of exoplanet atmospheres. At a size of 12U, each satellite weighs only 19 kg and utilizes mostly commercially available off-the-shelf components to keep costs per satellite around $2M. In this paper, we will present the satellite mission concept and early system design for the STARLITE constellation.

A multiscale approach to multi-emitter electrospray-ion source modeling has been developed to propagate emission site conditions into spatial domains viable for plume evolution simulations. This framework uses the particle-in-cell method to maintain a feasible computational complexity for consumer-grade hardware. Source models for individual emission sites are informed by an n-body single-emitter model. Molecular effects such as ion-cluster fragmentation are included. This model is used to predict array-level properties such as plume divergence angle, and to demonstrate the computational feasibility of modeling many emitters in parallel. This framework is used to quantify the relationship between array dimensions and space-charge- induced beam divergence. It is shown that including interparticle forces in plume expansion results in an increase in plume half angle by 8.1 deg for 80% and 9.8 deg for 90% of the emitter current for a commercial electrospray thruster. These data indicate that, at the array scale, space charge has a significant effect on plume evolution and must be considered in analysis of these systems.

In this work, we employ combined numerical and experimental approaches to determine the chemical composition of secondary molecular species generated through particle-wall collisions for an EMI-BF4 ion beam. The numerical work builds upon previous molecular dynamics (MD) simulations of ionic liquid-surface interactions based on a reactive force-field model and n-body kinetic simulations of single emitter plumes. In the experimental work, a residual gas analyzer (RGA) is used to measure the collision byproducts of an ion electrospray thruster plume. The respective mass spectra show good qualitative agreement, providing experimental validation of the MD framework to characterize propellant-surface interactions for a wide range of ionic liquid ion source applications.

Additively manufactured porous ceramic electrospray emitters developed with microscale 3-D printing have shown potential to operate similarly to electrospray emitters fabricated via subtractive manufacturing methods. However, the emission behavior of additively manufactured emitters requires further characterization. In this work, we develop a porous ceramic emitter via two-photon polymerization and characterize its emission performance with 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) using direct current-applied voltage measurements and time-of-flight mass spectrometry. 3-D geometrical characterization of the emitters is performed via scanning electron microscope and laser scanning confocal microscope imaging.