不同温度下复杂介质结构内带电规律仿真分析

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

卫星上某些介质结构会遭遇较大范围的温度变化,其电导率会随之出现数量级的变化,这将显著影响内带电结果.受限于电导率-温度模型和内带电三维仿真工具,该温度效应远没有得到深入研究.为此,在真空变温(253—353 K)和强电场(MV/m量级)条件下测试了某种星用改性聚酰亚胺介质的电导率,借鉴Arrhenius电导率-温度模型并考虑强电场下电导率的增强效应,发现电导活化能取值为0.40 e V时,可得到良好的拟合结果.在此基础上,同时考虑辐射诱导电导率,采用地球同步轨道恶劣电子辐射能谱,对该类介质盘环结构进行内带电三维仿真,发现其内带电程度随温度降低而显著增加,带电最严重的区域位于靠近辐射源的接地面边线.温度低于250 K时,2 mm屏蔽铝板下该区域的场强可达到107V/m量级,发生介质击穿放电的可能性较大.所讨论的电导率-温度模型与内带电三维建模方法对进一步评估卫星介质结构内带电程度和做好防护设计具有重要参考意义.

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 1.0×10^9 m-2·s^-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.