Computer simulation on temperature-dependent internal charging of complex dielectric structure

Abstract

Some dielectric structures on satellites would experience temperature variation in a relatively large range, giving rise to a considerable change in its conductivity and consequently resulting in a significant influence on the dielectric internal charging. However, due to the limitation to the model of conductivity versus temperature and the tool for three-dimensional (3D) simulation of internal charging, this temperature dependence has not attracted much attention. Therefore, the conductivity of a satellite used modified polyimide is measured in a temperature changeable vacuum environment under high electric field (in MV/m). Keithley 6517 B is used to capture the mild electrical current in a relatively long measuring time (several hundred seconds). According to the Arrhenius temperature dependence and considering the conductivity enhancement due to high electric field, good agreement is obtained between fitted data and measured results by setting the activation energy to be 0.40 eV. In addition, the radiation induced conductivity (RIC) is taken into account by using the Fowler model. The conductivity at room temperature is found to be comparable to the RIC from the condition with 2 mm aluminum shielding. Using the derived results, the internal charging simulation in three dimensions is carried out for a selected part of a structure in this material, where Geant4 is used to derive the distribution of charge deposition and radiation dose in three dimensions. The incident energetic electrons are assumed to follow the exponential distribution under geosynchronous orbit severe radiation condition where the flux of electrons with energy larger than 2 MeV is assumed to be 1.0×109 m-2·-1·sr-1. It is found that the internal charging will become more serious as the temperature decreases. The charging time is about 1 h at temperature 330 K, whereas this time is increased to 10 h for temperature below 250 K. The most serious charging domain appears around the boundary line of the grounding surface close to the radiation source, where the electric field strength exceeds 107 V/m under the condition of 2 mm aluminum board with temperature 250 K. So the dielectric breakdown discharge is most likely to occur within this domain. Above all, under the condition of the material intrinsic conductivity (mainly depending on temperature) comparable to the radiation induced conductivity, temperature will play an important role in internal charging. This model for temperature-dependent conductivity and the method of 3D simulation of internal charging have great significance in both further evaluating spacecraft internal charging and implementing well protective designs.