Deep Dielectric Charging of Spacecraft Polymers by Energetic Protons

The majority of research in the field of spacecraft charging concentrates on electron charging effects with little discussion of charging by protons. For spacecraft orbiting in the traditional LEO and GEO environments, this emphasis on electrons is appropriate since energetic electrons are the dominant species. However, for spacecraft in orbits within the inner radiation belts, or for interplanetary and lunar space probes, proton charging effects may also be of concern. To examine bulk spacecraft charging effects in these environments, several typical highly insulating spacecraft polymers were exposed to energetic protons with energies from 1 to 10 MeV to simulate protons from the solar wind and from solar energetic proton events. Results indicate that effects in proton-charged dielectrics are distinctly different than those observed due to electron charging. In most cases, the positive surface potential continued to increase for periods on the order of minutes to a day, followed by long-time-scale decay at rates similar to those observed for electron charging. All samples charged to positive potentials, with substantially lower magnitudes than for equivalent electron fluence. Possible explanations for the different behavior of the measured surface potentials from proton irradiation are discussed; these are related to the evolving internal charge distribution from energy-dependent electron and proton transport, electron emission, charge migration due to dark current and radiation-induced conductivity, and electron capture by embedded protons.