Kristina M. Lemmer’s research while affiliated with Western Michigan University and other places

Luminescence at the face of ionic liquid ion sources and nearby facility surfaces is a commonly reported radiative phenomenon that requires thorough examination. In this study, we present magnified images of a single emitter porous-media ionic liquid electrospray in profile, which provides spatial information on the origin of the luminescence. To determine what role facility interactions play in luminescence at the electrospray face, we varied the distance and material of a downstream beam target on which the ion plume terminated. The effect of applied emitter voltage on luminescence was also examined. Analysis of luminescence images and corresponding telemetry data demonstrate that the formation of luminescence near the emitter tip is not only a function of the surrounding test facility, but also dependent on the electrospray device, itself. The data also indicate the majority of luminescence observed within our experimental system is formed primarily from high-energy collisions between emitted propellant ions and propellant accumulated on the orifice of the extractor electrode.

Luminescence spectroscopy was used to examine the dynamics of propellant dissociation near the emitter tip of a single-emitter porous electrospray thruster loaded with the ionic liquid EMI-BF4. Luminescence spectra from CH, C2, CN, NH, BH, Hα, and Hβ were observed and confirmed by comparison with simulated spectra. Analysis of the CH (A ²Δ, v′ = 0) spectra yielded a rotational temperature 3082 ± 30 K while the C2 (d ³Πg− a ³Πu) Swan system yielded rotational temperatures 6252 ± 92 K and 5914 ± 75 K for Δv = 0 and Δv = +1, respectively. Examination of the integrated spectral signals from acquired CH (A ²Δ), BH (A ¹Π), and Hα spectra showed a strong correlation with measured extractor current in both positive and negative polarity mode. The evidence suggests the formation of these electronically excited species is due to dissociative excitation induced by high-energy collisions between emitted ions and propellant accumulated on the extractor orifice. A weak broadband signal was also observed and is likely due to dissociative excitation of the anion, BF4⁻, leading to the formation of electronically excited BF2. Analysis of the neutral gas within the test chamber with a mass spectrometer confirmed the presence of BF2, providing strong evidence the observed broadband signal is the result of BF2.

Ionic liquid ion sources are a promising form of efficient thrust for power- and mass-constrained satellites. Flight performance requirements necessitate large arrays of emitters; however, the fundamental behaviors of the ion emission process are obscured by the simultaneous operation of multiple emitters. One such behavior is the stochastic nature of Taylor cone formation and ion emission from the emitter. To examine these phenomena, a single-emitter electrospray using a conventionally machined porous borosilicate emitter cone was operated using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Repeated performance curves of the same emitter, measuring the emitter and extractor electrode currents as a function of the emitter voltage, vary significantly between tests. Ion current density spatial distribution maps of the plume and time-resolved emitter current and plume current while retarding potential energy analyzer sweeps are also presented. High-speed current measurements on a collector plate downstream of a different single-emitter electrospray were collected to examine the variation in the onset delay time of the Taylor cone. The onset delay time decreased with increasing emitter voltage and was effectively eliminated using bipolar switching. In monopolar mode, the onset delay rate was not sensitive to changes in the emitter voltage switching frequency across the range of tested values.

To completely characterize the evolving state of a plasma, diagnostic tools that enable measurements of the time-resolved behavior are required. In this study, a gridded ion source with superimposed oscillations was utilized to verify the functionality of a high-speed retarding potential analyzer (HSRPA), at frequencies equivalent to the low frequency oscillations occurring in Hall effect thrusters (HETs). The verification of this device provides an effective alternative to existing diagnostics for measuring time-resolved ion energies. Retarding potential analyzers (RPAs) have established themselves as a fundamental diagnostic in the field of electric propulsion (EP), enabling the measurement of ion energy distributions within the plumes of EP thrusters. The work presented here has demonstrated the capability of a standard RPA in conjunction with high-speed circuitry and data fusion techniques to produce time-resolved ion energy distribution functions (IEDFs) at higher frequencies and beam potentials than have previously been investigated. Tested frequencies ranged between 20 and 80 kHz with 10 V peak-to peak oscillations at a mean beam potential of 570 V. In addition, measurements were conducted with several waveforms, functioning as the superimposed oscillation, including a sine wave, triangle wave, and noisy sine wave. Data from the HSRPA were successfully reconstructed into time series utilizing two data fusion techniques: the empirical transfer function method and shadow manifold interpolation. Time-resolved IEDFs were produced at all frequency set points and waveforms. This investigation has demonstrated the HSRPA effectiveness at producing time-resolved measurements under conditions similar to those occurring in HETs.