Theory of Power Generation From Spacecraft Charging

Space plasmas, though tenuous, can lead to the development of large magnitude potentials on spacecraft surfaces relative to the ambient plasma in a process known as spacecraft charging. This effect poses a threat to spacecraft when surfaces charge differentially as it increases the risk of electrical arcing. Spacecraft engineers have primarily focused on the mitigation of charging and not its exploitation. The potentials and current collected by the spacecraft present an opportunity to harvest power from the space environment. Although the current density and floating potential due to incident charged species in the orbit-motion-limited model are identical for all spacecraft surfaces with similar geometries, material property differences between surfaces or charge control devices break the symmetry and allow for net current to flow. Differences in the secondary electron yield, in particular, produce strong differences in floating potential even when the spacecraft is eclipsed or far from the sun. This article presents a theory for computing the power available to harvest from spacecraft charging given the parameters of ambient space plasmas and surface material properties. Given a load connecting two differentially charged surfaces, optimal power is harvested by matching the load resistance to the input plasma impedance. While this holds for any model of the ambient plasma, the orbit-motion-limited theory yields closed-form solutions to the charging equations for spherical and planar surfaces. These solutions provide an upper bound on generated power that scales with the density and temperature of the cold electron population. An environment-specific optimal anode-to-cathode area ratio exists that minimizes the input impedance to the harvesting circuit. Deployable anodes are recommended to satisfy this condition and increase the amount of power harvested. Energy harvesting from spacecraft charging can be used in the Jovian magnetosphere to generate on the order of 10 mW/m(2) of power.