Electrical model and optimal design scheme for low work-function tethers in thrust mode

A low work-function tether (LWT), a subclass of electrodynamic tether made of a conductor partially coated with a low work function material, can exchange momentum and energy with planetary magnetospheres without any consumable. If fed by an onboard power source to reverse the natural direction of the current given by the motional electric field, a LWT can produce an useful thrust in drag compensation and re-boost scenarios. In the considered scheme, the LWT has an anodic bare segment for passive electron collection, followed by an insulated segment, a power source, and a cathodic segment that emits electrons passively through thermionic and photoelectric effects. Current and voltage profiles along the LWT are obtained and used to compute the system efficiency, i.e. the electrical power to mechanical power conversion rate, and the minimum electrical power to avoid space-charge effects in the cathodic segment. The design conditions to reach a high-efficiency regime are presented. For a given orbit and required thrust, optimal values for the tether geometry, including the fractional lengths of all three tether segments, are found. The results are applied to the preliminary design of a LWT system that would compensate the aerodynamic drag on the International Space Station. The analysis shows that the mission can be performed by a LWT fed with a power source of 4.7 kW, length, width and thickness equal to 5.1 km, 2 cm and 30µm, work function 1.5 eV, and temperature 600 K.This work was initially supported by Agencia Estatal de Investigación (Ministerio de Ciencia, Innovación y Universidades of Spain) under the project ESP2017-82092-ERC (AEI) and continued thanks to funding received from the European Unions Horizon 2020 research and innovation programme under grant agreement No 828902 (E.T.PACK project). GSA work is supported by the Ministerio de Ciencia, Innovación y Universidades of Spain under the Grant RYC-2014-15357