Electrospray propulsion time-of-flight secondary ion mass spectrometry diagnostic.

Abstract

The design and capability of a novel electrospray propulsion time-of-flight secondary ion mass spectrometry (TOF-SIMS) diagnostic are presented to investigate secondary species emission from surface impingement of energetic molecular ion plumes. Designed on the basis of traditional SIMS principles, this diagnostic provides information on the relative intensity and chemical composition of charged secondary species given electrospray operational parameters like incident angle, primary ion energy, and target surface composition. The system consists of an externally wetted tungsten ion source operating with room temperature ionic liquid propellant, a target with a secondary species acceleration mesh, and a time-of-flight mass spectrometer with an electrostatic gate and microchannel plate detector. The results show that energetic primary plume impacts with a silver metallic surface induce both sputtering of the target (Ag+, Ag2+, and Ag3+) and molecular secondary ion emission in both polarities. Within these secondary species, peaks related to the ionic liquid primary ions (B+ and F-) were definitively detected. The remaining secondary ions include fragmentation products of the EMI+ cation, common salt contaminants (Na+, K+, and Cl-), and hydrocarbon species from ambient vacuum facility conditions. The fragment pattern observed is consistent with both the ionic liquid components and typical contaminants encountered in high-vacuum TOF-SIMS analyses. For electrospray propulsion, these secondary species contribute to lifetime-limiting processes intrinsic to thruster operation, like impingement and degradation of electrodes and emitters, while also contributing to facility effects that can corrupt ground-based testing and thruster qualification.