Direct optical visualisation of ionic liquid ion source Taylor cones

Abstract

Ionic liquid ion sources are used in electrospray spacecraft ion thrusters, but these sources and consequently these thrusters suffer from limited lifetimes and off-axis plumes. Limited lifetimes have been partially attributed to this off-axis emission impinging on the downstream grounded extractor electrode, an effect likely exacerbated by the formation of multiple emission sites. Porous electrospray emitters are of interest due to the ionic emission they enable. In larger capillary emitters, the emission behaviour is well understood, with Taylor cones forming across the capillary orifice and forming a largely axial plume, but their plumes often consist of larger solvated species and have other operational challenges. This understanding is not present for porous electrospray emitters, where clear visualisation of a Taylor cone has yet to be documented. As a result, it is unknown where on an emitter aTaylor cone may form, what size it takes and even prediction of the voltage at which onset occurs has proved elusive. Detailed computational simulations of electrospray are available in literature but are unable to report Taylor cone sizes with confidence, leaving uncertainty in the set-up of such simulations. This paper sets out the case for direct optical visualisation of porous electrospray emitter Taylor cones, covering predictions of Taylor cone size as well as previous visualisation efforts. Initial results from this direct optical visualisation approach are given and explored, including what is believed to be one of the first clear image of a Taylor cone on a porous emitter operating in a vacuum.

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ORCID for Euan Donovan-Hill: orcid.org/0000-0001-5038-0804

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