Modeling and Performance of Electrodynamic Low-Work-Function Tethers with Photoemission Effects

Abstract

A low-work-function tether is a long conductor coated with a low-work-function material that orbits around a planet with both the magnetic field and ionosphere. Depending on the work function of the coating and the tether temperature , the photoelectron emission can be relevant within the cathodic tether segment. Thus, this mechanism needs to be added to the thermionic emission considered in previous works. An emission model for low-work-function tethers, including a typical solar photon spectrum, a Fowler–DuBridge law for the photoelectron yield of the coating, and a Richardson–Dushman law for the thermionic emission, is presented, and used to organize the thermionic and photoelectric dominated regimes of low-work-function tethers within the plane. For and , the photoemission and thermionic emission can be of the same order and have similar efficiency as the electron collection. The emission model is combined with orbital-motion theory for all the plasma and emitted particles, and the longitudinal bias and current profiles throughout a low-work-function tether are determined for typical low-Earth-orbit environmental values. Results for the average current are presented. The study highlights the main electrical, mechanical, and optical properties that should be considered in the design of low-work-function tethers, and it briefly discusses some promising materials.