Direct two-dimensional goniometric steering of vacuum electrospray ion beams for angular time-of-flight studies

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.