Breakdown voltages of dielectric-coated sphere-plane electrode configurations in high vacuum

Stainless-steel sphere-plane electrode configurations with a gap distance of 1 mm were investigated. The sphere electrodes were coated with different kinds of dielectric material, such as polyurethane and epoxy resin, with a thickness of 5 mu m. The measurements were performed with AC voltage and both polarities of DC voltage. Compared to uncoated sphere electrodes, lower breakdown voltages of the dielectric-coated electrodes were obtained when applying AC voltage. When the coated sphere electrode was stressed with negative DC voltage the breakdown voltage increased slightly, contrary to positive DC voltages for which lower breakdown voltages were measured. Coated electrodes prestressed with discharges had a lowered AC and DC breakdown voltage.<>