arXiv subjects

Elaine Petro

Publications and source records attributed to Elaine Petro.

5 recordsLinked to original sources

arXiv2026

Inferring Thermal Dissociation Kinetics of Ion Clusters from Molecular Dynamics

Electrospray ionic liquid ion sources operating in the pure ion regime emit metastable, low solvation number ion clusters whose post-emission dissociation modifies plume composition, energy distributions, and diagnostic observables. While field-free measurements support first-order, thermally activated decay on microsecond timescales, the acceleration region is characterized by rapidly varying electric fields and nanosecond residence times such that dissociation becomes strongly field-enhanced. In this work, microcanonical ensemble molecular dynamics (MD) trajectories are used to infer dissociation kinetics of EMI-BF4 ion clusters under controlled internal energy and applied electric field. Positive and negative dimers, trimers, and tetramers are simulated at temperatures between 600 to 1000 K and uniform fields of 10^6 to 10^9 V/m. Dissociation lifetimes are extracted using a connectivity-based fragmentation criterion, and product channels are classified to obtain pathway-resolved branching probabilities. Across all solvation numbers, lifetimes are field independent in the weak field limit, but collapse by orders of magnitude above 10^8 V/m. Cluster polarity dependence is most prominent in the temperature-controlled regime and diminish as electrostatic work dominates. The MD simulations further reveal that field-driven changes in dissociation topology, including a transition in trimer breakup from neutral pair evaporation to charged core ion ejection, and three competing tetramer pathways involving single neutral loss, double neutral emission, and core ion ejection. The MD dataset is reduced to compact parameterizations of {τ_n}(E,T), yielding transferable kinetic inputs for multiscale plume transport models.

physics.plasm-ph↗

arXiv2026

On the Two-Dimensional Structure and Asymmetries of Ionic Liquid Electrospray Plumes

Vacuum electrosprays play important roles in electrospray propulsion, a type of spacecraft electric propulsion wherein charged ions or droplets are accelerated at high velocity to provide spacecraft thrust, and they also have important implications for surface etching and mass spectrometry. Here we present the first fully two-dimensional time-of-flight (TOF) mass spectrometry survey of a vacuum electrospray plume, generated by a tungsten needle externally-wetted with the ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF$_4$). We find that the plume exhibits clear two-dimensional compositional variation, structure, and asymmetry, with heavy particles and energetic neutrals being more prevalent in the center and a ring-shaped distribution for the monomers (the lightest molecular ions) peaking at 10$^\circ$ off beam-axis with a relative minima in the center. Using a new spatial baseline characterization method, we find a population of energetic neutrals firing 3.4$^\circ$ off beam-axis. In particular, we find that by comparing different parts of the plume, the estimated propulsive efficiency from any one sampled point may vary by as much as a factor of 6. We also find that high mass droplets, which are often assumed as absent in many studies of externally-wetted needles, may carry away significant propellant mass at lower effective velocity and reveal a cone-jet mode of operation at 470 nA output current. We thus find that whole-plume compositional surveys may more accurately assess plume composition and propulsive efficiency, and a significant portion of the `missing mass' in electrospray propulsion sources presumed to be operating in the pure ion regime can be potentially explained by limited sampling of the spatially non-uniform ion plume.

physics.flu-dyn↗

arXiv2025

Ab initio Simulations of EMI-BF4 Neutral-Surface Interactions in Electrospray Thrusters

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.

physics.chem-ph↗

arXiv2024

Direct Two-Dimensional Goniometric Steering of Vacuum Electrospray Ion Beams for Angular Time-of-Flight Studies

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.

physics.ins-det↗

arXiv2024

Propellant Discovery For Electrospray Thrusters Using Machine Learning

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.

physics.chem-ph↗