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.