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

TL;DR

The paper tackles the problem of limited lifetime in ionic liquid electrospray thrusters due to plume–extractor interactions that generate secondary species on surfaces. It employs a dual TOF-SIMS approach, combining a lab ES I-TOF-SIMS diagnostic and a high-resolution IONTOF TOF-SIMS5 to analyze SSE when EMI-BF$_4$ and EMI-Im thin films are bombarded by quasi-pure ion plumes, at a maximum impact energy of $E_{impact,max}=|\\pm V_{source}|+|\\mp V_{target}|$ around $4$ keV. The results show that positive SSE spectra are dominated by the EMI$^+$ cation with similar patterns across ILs, while negative SSE diverges with EMI-Im producing a richer set of fragmentation products; high-mass clusters extend well above the monomer, indicating complex chemical pathways. These findings provide experimental grounding for understanding and mitigating lifetime-limiting degradation due to plume-surface interactions and facility effects in ionic-liquid electrospray propulsion.

Abstract

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.

Figures (11)

• Figure 1: Structural diagrams of the cation EMI$^+$ ($m/z$ = 111 amu), as well as anions BF$_4^-$ ($m/z$ = 87 amu) and Im$^-$ ($m/z$ = 280 amu).
• Figure 2: Electrospray operation involves molecular ion plumes impacting extractor electrodes and downstream facility surfaces, causing the formation of secondary species. These secondary species can backstream to the emitter and introduce great measurement uncertainty.
• Figure 3: Electrospray TOF-SIMS diagnostic, where a single electrospray ion source directs a polydisperse molecular ion plume at a target of interest to induce sputtering and the formation of secondary ions to then be analyzed via a linear time-of-flight mass spectrometer.
• Figure 4: Ionic liquid thin film spincoat process, where 3 mL of an ionic liquid/methanol mixture was dispensed on a silver-coated silicon wafer.
• Figure 5: Primary TOF showing a plume composed of primarily ions, with a portion of continuous small droplets for both EMI-Im and EMI-BF$_4$ propellants.