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 /spl mu/m. The spacer materials examined are SiO/sub 2/, PMMA, PTFE and Al/sub 2/O/sub 3/. 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/sub a/ decisively affects the charging, which decreases as R/sub a/ increases. By increasing R/sub a/ larger than about 2 /spl mu/m, the charging does not occur until a higher voltage is applied, thus improving the insulation property.