Development of a micro hollow cathode for use as an ion micropropulsion electron source

Abstract

Recent interest in small-scale satellites has encouraged the development of novel miniatUlised propulsion systems. A variety of technologies exist which could be applied to this role, generally focused around chemical and electIic thruster designs. The propulsion system must be durable, low in mass and must be able to efficiently propel the spacecraft to a range of velocities, on the order of kilometres per second. Electrostatic thrusters demonstrate high combined levels of power efficiency and specific impulse in relation to other propulsion technologies including other electric systems. Electron bombardment ion thrusters are therefore an excellent candidate for use on small-scale satellites. The reduction in size of existing technologies presents design challenges, due to difficulties faced in fablication and the effects on operation when scaling to such small sizes. The goal of this research project has been to develop and test a miniarurised electron source to establish the effects of novel materials and design on functionality. Electron sources are key components of electron bombardment ion thrusters, used for both ionisation and beam neutralisation. Initial research conducted during this study showed thermionic filament and field emission sources were difficult to apply to inert gas ion propulsion systems. This was due to their lack of robustness at the higher pressures required to efficiently ionise the propellant. A study into micro hollow cathodes was therefore conducted due to their demonstrated performance at larger scales. Three types of micro hollow cathode were designed, fabricated and tested. The most complex design developed during this project had a body fOlmed from a low-diameter metal tube with a turned orifice and an internal carbon matrix impregnated with compounds designed to promote electron emission. The other two models were a simple metal body and a surface impregnated body, and were plimarily used to compare performance with the more complex model. All 3 types exhibited similar initiation voltage properties, with the impregnated insert cathode showing slightly higher initiation voltages at low flow rates. The keeper discharge could be established in each case, but transfer to the anode discharge could not be perfOlmed. The keeper current and voltage showed that the impregnated insert device operated at the lowest power for similar flow rates and keeper distances, suggesting impregnated carbon inserts deserve more study.

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