High-speed photography of surface flashover of solid insulators under impulse voltages in vacuum

Results are presented of an investigation into the flashover of glass and high-density alumina cylinders 1.25 cm in length between uniform field electrodes in vacuum. Photographs taken with an image-converter camera with a framing rate of 2×107 frames/sec are shown. At approximately 80% of the voltage required to produce a flashover, partial discharges occur. When the voltage applied to the gap is increased to breakdown level, the partial discharges persist as the first stage of a two-stage arc formation. The development of the discharge is shown to be consistent with a flashover mechanism involving charging of the solid surface by electrons emitted at the cathode.