Time-of-flight characterisation of alternative propellants for porous Electrospray thrusters

Abstract

Electrospray propulsion is a type of small satellite propulsion which could have a significant impact on the space industry due to, among other reasons, its high specific impulses. High specific impulses are enabled by the evaporation of high charge-to-mass ratio ions from the surface of a liquid under the influence of a sufficiently strong electric field. This process of ion evaporation is called electrospray ionisation and is enabled by the use of room temperature molten salts, termed ionic liquids, typically comprised of a cation and anion. However, the fundamental physics of ion emission is not well understood, especially the factors determining the charge-to-mass ratios of these ions. A better understanding of the factors determining ion emission would allow for a more
informed selection of these ionic liquids, therefore enable better electrospray thruster design. In order to investigate ion cluster emission, a single emitter porous electrospray thruster was tested with seven different ionic liquids. Three different instruments were used to characterise the thruster: a full plume current collector, a retarding potential analyser and a time-of-flight mass spectrometer. The last instrument contained a large current collector, a ‘reflecting’ gate and a flight length of 550 mm.

Eleven different emitters were tested with the seven ionic liquids in order to characterise a broad liquid property range. Three of the ionic liquids were called metal/metalloid ionic liquids, ionic liquids comprised of EMI+ cations and anions comprised of a central metal/metalloid atom surrounded by halogens. These were: EMI-BF4, EMIFeCl4 and EMI-SbF6. The remaining four ionic liquids were comrpised of ions which contained more than one charge, termed multiply-charged ionic liquids. These were: (EMI)2-Co(SCN)4, C6(mim)2-(Im)2, C6(mim)2-Co(SCN)4 and (C6mim)3-Dy(SCN)6. Five out of seven of these ionic liquids have not been previously tested.

Current-voltage tests showed a large range of currents. EMI-FeCl4 emitted an especially large amount of current, with over 50 μA recorded for a single emitter. EMIBF4, EMI-SbF6, (EMI)2-Co(SCN)4 and C6(mim)2-Co(SCN)4 were found to emit similar currents, 10 to 20 μA, despite a large difference in conductivity. C6(mim)2-(Im)2 and (C6mim)3-Dy(SCN)6 were found to emit the least current but were also found to have great high voltage stability, emitting at voltages exceeding 4000 V. However, further current-voltage characterisation is required to reliably confirm these characteristics.

Time-of-Flight tests showed that the emitted ion cluster sizes were similar, with most of the plumes being comprised of monomers, dimers and trimers. The charge-to-mass ratios were found to be similar for all the ionic liquids tested, despite having large differences in conductivities and viscosities. It was also shown that the average charge-to-mass ratio did not change with volumetric flow rate, remaining constant for most tests completed. An ion cluster emission model was introduced to predict the ion cluster distributions of the plume, however this was found to be inaccurate. This showed that the ionic liquid properties do not provide a reliable method for predicting charge-to-
mass ratio of the ion clusters emitted.

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Identifiers

ORCID for Szymon Dworski: orcid.org/0000-0003-4787-9289

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