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Secondary Species formed from ionic liquid electrospray ion plume impacts with propellant thin films

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.