Improving withstand voltage by roughening the surface of an insulating spacer used in vacuum

This paper describes a simple and reliable method to improve the surface insulation strength of a spacer used in vacuum. This approach roughens the surface of a cylindrical spacer to an average roughness higher than 2 μm. The spacer materials examined are SiO₂, PMMA, PTFE and Al₂O₃. The spacer is subjected to a ramped DC voltage and its surface charging is observed by using an electrostatic probe embedded into the cathode. It has been found that the surface roughness R_a decisively affects the charging, which decreases as R_a increases. By increasing R_a larger than about 2 μm, the charging does not occur until a higher voltage is applied, thus improving the insulation property.