Pulse propagation along conductors in low-density, cold plasmas as applied to electrodynamic tethers in the ionosphere.

Abstract

This research characterizes the general propagation behavior of high-voltage electromagnetic pulses along conductors in low-density, cold plasmas. As a specific application, this characterization uses electrodynamic tethers in the ionosphere. Electromagnetic pulses are produced along the tether-plasma system as it transitions from open- to closed-circuited states and as it is driven by radio-frequency voltage sources. These perturbations take a finite amount of time to propagate along the tether and, as they do so, they affect the surrounding ionospheric plasma. This interaction in turn affects the tether's transmission-line characteristics. The dynamic evolution of the sheath is examined as the pulse front travels past a given section of tether and disturbs the local sheath. Present tether transmission-line models assume, as a first-order approximation, that the plasma-sheathed tether can be modeled as a simple rigid coaxial transmission line. This has proven acceptable for tethers with low induced or driven voltages. An improved model is needed, however, when steady-state plasma-sheath dynamics cannot be assumed, such as for longer deployed tether lengths, which have higher induced emf, or higher driven voltages. A dynamic circuit model of the plasma-sheathed tether is developed with knowledge gained from theoretical analyses, experimental results, and particle-in-cell computer simulations. Using this dynamic-sheath model as their basis, lumped-element transmission-line parameters for capacitance and inductance per unit length are derived for the plasma-immersed tether where it was found that capacitance is a function of voltage but inductance is approximately constant. These parameters are included with per-unit-length resistance and induced--emf elements to form the complete lumped-parameter model. The tether circuit model is implemented in the SPICE circuit simulation program. This implementation allows examination of the tether's transmission-line characteristics as well as pulse propagation and morphology. Previously developed models of satellite and Orbiter interactions with the plasma based on Tethered Satellite System mission data can be used with this circuit model as the endpoints of the complete electrodynamic-tether system.