Search

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. [ABSTRACT FROM AUTHOR]

Resistivity values were experimentally determined using charge-storage methods for six samples remaining from the construction of the internal discharge monitor flown on the Combined Release and Radiation Effects Satellite (CRRES). Three tests were performed over a period of three to five weeks each in a vacuum of ∼5 × 10-6 torr with an average temperature of ∼25 °C to simulate a space environment. Samples tested included FR4, polytetrafluoroethylene (PTFE), and alumina with copper electrodes attached to one or more of the sample surfaces. FR4 circuit-board material was found to have a dark-current resistivity of ∼1 × 1018 Ω · cm and a moderately high polarization current. Fiber-filled PTFE exhibited little polarization current and a dark-current resistivity of ∼3 × 1020 Ω · cm. Alumina had a measured dark-current resistivity of ∼3 · 1017 Ω · cm, with a very large and more rapid polarization. Experimentally determined resistivity values were two to three orders of magnitude more than found using standard American Society for Testing and Materials (ASTM) test methods. The 1-min wait time suggested for the standard ASTM tests is much shorter than the measured polarization current-decay times for each sample indicating that the primary currents used to determine ASTM resistivity are caused by the polarization of molecules in the applied electric field rather than charge transport through the bulk of the dielectric. Testing over much longer periods of time in vacuum is required to allow this polarization current to decay away and to allow the observation of charged-particle transport through a dielectric material. Application of a simple physics-based model allows separation of the polarization current and dark-current components from long-duration measurements of resistivity over day-to month-long time scales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport. [ABSTRACT FROM AUTHOR]