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

This paper describes a simple and reliable method of improving the surface insulation strength of a spacer used in vacuum. The method is to roughen the spacer surface to an average roughness R_a higher than 1 or 2 μm. The material of the spacer examined is SiO₂, PMMA, PTFE or Al₂O₃ and their shape is a right cylinder with 10 mm in height and 54 mm in diameter. The spacer is subjected to a ramped DC voltage and its surface charging is observed by using an electrostatic probe embedded in the cathode. It has been found that R_a decisively affects the charging, which decreases as R_a increases. Increasing R_a larger than about 2 μm suppresses the charging until a higher applied voltage is reached, thus improving the insulation property.