Basic Studies on Insulation Design of Dielectric Spacers in Vacuum

点击翻译按钮获取中文摘要

The charging of an dielectric insulator is considered to be a critical phenomenon leading bridged vacuum gaps by dielectric spacers to flashover. The goal of this study is to obtain an engineering aspects for designing the shape of spacer taking the charging phenomena into account. For this purpose we have developed two simulation techniques to analyze the charge distribution on the spacer surface and also a measuring technique for clarifying the temporal behavior of the surface charge. One of the simulation technique analyzes the charge distribution in an equilibrium state of charging based on the secondary emission electron avalanche (SEEA) model. This technique has been applied to a cylindrical spacer, fundamental shape, made of acryl resin (PMMA), teflon (PTFE) or alumina ceramics (92% A12O3), and we have confirmed that the simulated and measured distributions well agree with each other. The other simulation technique analyzes the charge distribution during developing phase of the charging. Both techniques have been proved to be a powerful tool to study charging phenomena not only for a simple cylindrical insulator but also for spacers actually utilized in high voltage devices under vacuum condition. To investigate the charging process, we have employed an electrostatic method which uses a small isolated part of the cathode as a probe. This probe technique enables us a real-time observation of surface charging under a ramped or stepwise DC voltage excitation. Influence of surface roughness on the charging of an insulator in vacuum has been investigated. The results exhibit that the surface roughness decisively affects charging characteristics, which is more pronounced for more highly polished insulators.