Scaling Laws for Very Low-Current Lab6 Thermionic Cathodes for Electric Propulsion Systems for Small Satellites

Low-power Hall thrusters are ideal propulsion systems for small satellites. The operation of such electric propulsion thrusters rely of the ability of electron emission by a thermionic cathode for discharge maintenance and plume neutralization. Designing low-current cathodes remains a trial and error process, while a comprehensive set of scaling and dimensioning laws for such devices is yet to be developed. In this contribution, very low-current (0.1-1 A) cathodes with LaB6 knife-edge emitters of various geometrical dimensions (emissive area, emitter orifice diameter and aspect ratio, keeper orifice diameter and aspect ratio) are systematically tested in the attempt of constructing a set of scaling laws that link geometrical characteristics with operational parameters and cathode discharge plasma properties. Langmuir probes and optical emission spectroscopy are used to collect information on plasma density, electron temperature and plasma potential along the cathode discharge to bolster the understanding on the relation between discharge plasma properties and cathode operational parameters. Moreover, particular attention is paid to the cathodes' stability regions in terms of discharge mode and mode transition across a range of emission currents and xenon and krypton mass flow rates. Such scaling laws allow for a rapid design of new devices and for the prediction of operational characteristics when tested in standalone mode in diode and triode configuration with an external anode and keeper electrode.