D TL C IPL-TR-93-2055 9. SPONSORING.i MONITORING AGENCY .4AN 8 D 10 .SPONSOR!,NG MONITORING AGENCY REPORT NUMBER 11. SUPPLEMENTARY NOTES *Boston College, Chestnut Hill, MA Reprinted from XVth International Symposium on Discharges and Electrical Insulation in Vacuum-Darmstadt-1992 12a, DISTRIUTION AVAILABILITY STATEMENT j1 2 b. DISTRIBUT'IN CODE Approved for public release; Distribution unlimited 13. ABSTRACT (Maximum2 10 wcrds) Sixteen samples of standard insulating materials with electrodes were eposed to the full variety of the Earth's space radiation beus tor 14 months. Spontaneous discharges were recorded for each sample and are compared to the radiation levels which were simultaneously monitored. Samples with the most exposed insulator surface pulsed most frequently. Pulsing correlated with electron flux, but not at all with proton flux. The pulse rate per unit electron flux was initially small, rose continuously for 7 mutahs, and then fell slightly during the last seven months. A computer model predicts the charging of the inmulators by the high energy electron flux; It took I to 6 months for the electric fields to approach steady state levels. Most of the pulses were less than 50 volts on 50 ohms. The pulsing rate decays when the satellite leaves the electron belts; the decay became more rapid after 7 months. Pulsing during the first six months had different characteristics than later pulsing. 14. SUBJECT TERMS 15. NUMBER OF PAGES Electrical Discharging Radiation Insulating 5 16. PRICE CODE 17. SECURITY CLASSIFICATION 118. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ASSTRAC0. L ON OF ASTRA UNCLASSIFIED UNCLASSIFIED I UNCLASSIFIED SAR !NSN 7540-01,280-1500 Szarda'd '-orr'9 298 (Rev 2-89) IC
A key parameter in modeling differential spacecraft charging is the resistivity of insulating materials. This parameter determines how charge will accumulate and redistribute across the spacecraft, as well as the time scale for charge transport and dissipation. ASTM constant voltage methods are shown to provide inaccurate resistivity measurements for materials with resistivities greater than ~10 17 Ω-cm or with long polarization decay times such as are found in many polymers. These data have been shown to often be inappropriate for spacecraft charging applications, and have been found to underestimate charging effects by one to four orders of magnitude for many materials. The charge storage decay method is shown to be the preferred method to determine the resistivities of such highly insulating materials. A review is presented of methods to measure the resistivity of highly insulating materials-including the electrometerresistance method, the electrometer-constant voltage method, and the charge storage method. The different methods are found to be appropriate for different resistivity ranges and for different charging circumstances. A simple, macroscopic, physics-based model of these methods 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. The model largely explains the observed differences in resistivity found using the different methods and provides a framework for recommendations for the appropriate test method for spacecraft materials with different resistivities and applications..
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