Theory of Plasma Contactor Neutral Gas Emissions for Electrodynamic Tethers

An existing anisotropic anodic plasma contactor model is extended to include neutral gas emissions. Within the framework of the contactor model, the inclusion of external ionization leads to an integro-differential equation. By choosing the ratio of the contactor radius (ao) to the neutral gas-electron ionization mean free path at the contactor exit (Awo) as a suitable expansion parameter, an analytical expression for the current-voltage characteristic can be found for (ao/X^o) < 1, which corresponds to weak external ionization. The performance of this contactor model is examined via a dynamic simulation model developed for electrodynamic tethers. Detailed models of the geomagnetic field and the ionosphere are used to create a realistic environment. The highly nonlinear contactor voltage-current characteristic is incorporated into a circuit equation, which includes radiation impedance, and is solved along the tether's orbit. Results show that it is more effective to fully ionize the contactor gas internally than to partially ionize it externally and that, based on the specific power for the tether system, the optimum gas to use is argon. In addition, the effect of the radiation impedance on tether system performance is examined.