Application of AF-NUMIT2 to the Modeling of Deep-Dielectric Spacecraft Charging in the Space Environment

A 1-D numerical iteration software tool (NUMIT) designed for laboratory modeling has been refined, developed, and greatly enhanced, so that it can be used to model and predict the deep charging of dielectrics on spacecraft. The new tool, called Air Force NUMIT version 2 (AF-NUMIT2), can now model isotropically incident electron fluxes from realistic energy spectra that change with time. The model has been extended to incident electron fluxes an order of magnitude lower than before (>= 10 keV), and aspects of the model have been checked against Monte Carlo simulations. Electron energy flux spectra, as measured by Combined Release and Radiation Effects Satellite, have been used in dozens of simulations of Kapton charging over multiple days. Kapton has been modeled with both dark conductivities and radiation-induced conductivities extending over two orders-of-magnitude. The result has been a surprisingly consistent relationship between maximum internal electric-field strengths and conductivity, regardless of the particular incident electron spectra.