The Influence of Triple Junction Design on High Voltage Holdoff for Insulators in Vacuum

Abstract

Surface flashover of solid insulators in vacuum is a type of high voltage breakdown which typically occurs before any other type of breakdown in vacuum. As a result, surface flashover is quite often a limiting factor for the performance of high voltage systems. Most theories of surface flashover emphasize the importance of the triple junction (the insulator-electrode-vacuum junction) as the point of initial electron emission, an event which begins the surface flashover process. For this project, three unique insulator designs were developed to retard the emission of electrons from the triple junction, and thus, ultimately increase the holdoff voltage. Each of the three designs, in addition to the standard 45-degree frustum, were first modeled using FLUX-2D, a finite element analysis electromagnetic field code. Each sample was then tested using the Bushing Test Facility at Los Alamos National Laboratory. The first special design achieved a 7% higher average holdoff voltage than the standard 45-degree frustum, while the second special design achieved a 38% higher average holdoff voltage. The third special design, using an embedded conductor, did not show substantial improvement, but its failure modes suggest a few alterations that should lead to an insulator with relatively high holdoff voltages.