Abstract-We present the possibility of a low work-function material, calcium aluminate electride, being used for a coating on a bare electrodynamic tether system. Analyses suggest that the coating would eliminate the need for an active cathodic device like a hollow cathode and, consequently, eliminate the need for an expellant to the hollow cathode, thus resulting in an electrodynamic tether system that requires no consumables. Applications include on-orbit power generation and deorbiting debris from low Earth orbit in a simple and trouble-free manner.

This paper discusses the development and testing of a C12A7 electride hollow cathode. C12A7:e − is a crystalline ceramic in which electrons clathrated in subnanometer sized cages act as a conductive medium. C12A7:e − has a predicted work function of 0.6 eV and hence is of interest in electron emission devices. A 6-mm diameter hollow cathode was fabricated with a C12A7:e − insert and operated for over 50 h on xenon with no signs of degradation. It was successfully started at room temperature without a heater and steady state operating temperatures below 650°C were observed during some tests. In addition, the C12A7:e − cathode was operated on iodine for over 20 h at discharge currents up to 15 A.

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Hollow cathodes are critical devices in the successful operation of electric propulsion thrusters. Cathodes featuring C12A7:electride as an ultra-low work function thermionic emitter are hypothesized to have faster start-up times and lower operating temperatures than conventional hollow cathodes. C12A7:electride is a crystalline ceramic in which electrons clathrated in sub-nanometer sized cages act as a conductive medium. Due to its unique atomic structure and large size, C12A7:electride has a predicted work function of 0.6 eV. As a result, C12A7:electride is an attractive option for a thermionic emission material in a hollow cathode. Calculations predict equivalent levels of current emission as LaB6 or CeB6, but at much lower temperature. C12A7:electride is stable at temperatures below its re-crystallization temperature (∼1000 °C) and is not consumed during operation. C12A7:electride has been fabricated at CSU using a simplified, one-step approach that results in a more conductive material than previously reported. The material has been integrated into a novel hollow cathode design that takes advantage of its unique properties to decrease start-up time and power consumption. Our paper will discuss preliminary results obtained with several cathode configurations. In addition, results from the experimental determination of basic material properties such as conductivity and work function will be presented. Strategies to further enhance electron emission through surface modifications will also be discussed.