Relationship between vacuum surface flashover and charging characteristics for various kinds of alumina ceramics

Alumina ceramics is widely used as an insulating material for high voltage devices, because of low outgassing, hardness, low dielectric loss and high bulk breakdown voltage. However, there is serious problem to solve that the flashover voltage along the surface in vacuum is lower than that of a vacuum gap at the same length between electrodes. Many intensive researches have been carried out to improve surface flashover voltage in vacuum. Even so the mechanism of surface flashover is not always clarified yet. One of reason is expected to charging characteristics which depend on the material properties of insulator. In this paper, we focused on the relationship between vacuum flashover characteristics and surface charge caused by high voltage application and surface flashover occurrence for three kinds of alumina ceramics.