Optical emission characterization of a single emitter electrospray thruster interacting with surfaces

Abstract

Electrospray thrusters are a promising small satellite electric propulsion system, capable of emitting ions with alternating polarity. It has been observed that these emitted ions can give rise to glowing near metal surfaces. Secondary species emissions (SSE) are assumed to be the main driving mechanism for this observed light emission. In this study, we explored the compositions of this glow via optical emission spectroscopy (OES) on a porous, single emitter ionic liquid electrospray thruster using 1-Ethyl-3-Methyimidazolium Tetrafluoroborate (EMI-BF₄) as the propellant. The thruster emitter was biased from ± 2400 V to ± 3400 V while keeping the emission voltage (ΔV_emission) at 2400 V by changing the extractor potential. The thruster was operated in anion and cation modes against a stainless steel target plate, which was grounded and then biased ± 100 V and ± 200 V. Excited CN, CH, H, and Na, which may be triggered by the fragmentation and excitation of EMI⁺and dissociation of NaBF₄ impurities, were consistently observed within the spectra, being the strongest near the target. The line intensity ratios are used as a metric for comparison of different experimental conditions. The changes in line ratios might point out different secondary electron energy distribution functions for the polarity of the landing ions at elevated beam energies (E_beam). The different ratios were obtained with ± 3.4 keV, while ± 2.4 keV caused very similar excitations on the spectra for either anions or cations hitting the target. Based on the current measurements on the target plate and collector plate, cations yielded 2.3 - 8.5 secondary electrons while the yield was 0.15 - 0.44 for the anion emission. The increase in the number of electrons emitted from the surface may contribute to increasing line intensities at higher ion landing energies.

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