Microthrusters based on the T5 and T6 hollow cathodes

Abstract

Small spacecraft with limited on-board resources would benefit greatly from the development of a low power, low cost microthruster able to offer propellant savings over conventional alternatives and enable higher energy missions. Such a thruster would also be beneficial in the development of all-electric spacecraft whereby the normally separate reaction control system and primary electric propulsion system were able to operate from a common propellant management system.

In recent years experiments on a T6 hollow cathode have demonstrated the possibility of such a device however the performance, in particular thrust efficiency, falls far short of what would be termed a viable thruster. Nevertheless the means by which hollow cathode discharges are able to produce thrust corresponding to very high velocity propellants is not at all understood, nor is the means by which to improve performance. The relevance of the very high energy ion signatures typical of hollow cathode discharges, synonymous with hollow cathode erosion, has
also not been established.

Indirect thrust measurements were made at the University of Southampton on two separate types of hollow cathode, derived from the T5 and T6 gridded ion thrusters, with unique design modifications, primarily of anode geometry. Testing was conducted with argon and xenon and measurements were made via a deflected pendulum micro-thrust balance and supporting architecture constructed specifically for the work. Ion energy measurements were made using a hemispherical energy analyzer in a separate experiment at NASA Jet Propulsion Laboratory on a third XIPS cathode, derived from the XIPS 25cm ion thruster, with xenon and krypton.

These measurements provided unique insight into the influence of terminal parameters such as discharge current, mass flowrate and cathode/anode geometry on thrust production and downstream ion energy distributions. Significant improvements in thrust efficiency have been made with the T5 cathode and in doing so, have taken hollow cathodes a significant step forwards as a viable and competitive propulsion technology.

An analysis of the thrust production is made, and on this basis conclusions are drawn on the existence of electrothermal, electromagnetic and electrostatic mechanisms specific to the cathode and operating regime, as well as their roll in other associated hollow cathode phenomenon. The main conclusions of the work are presented and recommendations made for future experimental work.

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