STUDIES ON INSULATION DESIGN OF HIGH VOLTAGE DIELECTRIC SPACERS USED IN VACUUM

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The charging of a dielectric insulator is considered to be a critical phenomenon leading bridged vacuum gaps to flashover. The goal of this study is to obtain an engineering aspect for designing the shape of spacer taking the charging phenomena into account. 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 excitatin. We have also employed a simulation technique that analyzes the charge distribution on the insulator based on the secondary emission electron avalanche (SEEA) model.First, the influence of surface roughness on the charging of an insulator in vacuum has been investigated. A cylindrical insulating spacer made of acryl resin (PMMA), Teflon (PTFE) or alumina ceramics (92% Al_2O_3) is exarnined under rarnped, or stepped DC voltage application. Each sample has a roughness ranging … More from 0.15 to 50 μm. The results exhibit that the surface roughness decisively affects the charging, which is more pronounced for more highly polished insulators. For instance, the smooth surface of a cylindrical specimen made of PMMA or alumina acquires charge several times higher than that with rough surface. The specimen made of PIFE is hard to acquire the surface charge irrespective of its surface roughness. When an insulator is subjected to a stepwise DC voltage, the probe signal shows that the charging occurs with considerable delay, ranging from 10^<-5>s up to 10 s depending on the material, the voltage height and also the surface roughness of the insulator. Two-dimensional Monte Carlo simulation based on the secondary emission electron avalanche model has been performed to analyze the progress of the charging. During the delay period, positive charge accumulates on the surface of the insulator near the cathode, which further accelerates the charging.Second, the specimen in the shape of a conical frustum has been investigated. The spacer with a slight positive or a negative cone angle (-25゜≦α≦+2゜ ) acquires a positive charge of which density increases almost linearly with the applied voltage. The more positively angled spacer acquires no charge. These results agree with the theoretical prediction based on the SEEA mechanism. On the contrary, the spacer with a cone angle α<-25゜ acquires a localized negative charge. This result differs from the theoretical prediction in which a considerably high-density negative charge accumulates over the surface. We have shown that the progress of negative charging ceases due to the relaxation of the electric field near the cathode TJ.These results clearly show the importance of surface treatments and the spacer shape in the insulation design. Overall results show that insulators that are hard to acquire the surface charge have very good insulation performance. Less