Dielectric surface flashover in vacuum at 100 K

Cryogenic components in high power electrical systems and in power electronics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielectric test sample and electrodes in vacuum are cooled to less than 100 K and the flashover development process is characterized using fast electrical and optical diagnostics. Three consecutive development stages for flashover in self-breakdown mode with a gap distance of 0.5 cm can be distinguished: (1) A fast current rise to mA-amplitudes within several nanoseconds which is probably associated with field emission, followed by (2) a slow current rise to ampere-amplitudes with a duration of several 10's to 100's of nanoseconds associated with secondary emission avalanche saturation, and (3) a transition to a rapid gaseous ionization above the sample caused by electron induced outgassing, leading to impedance-limited current amplitudes of several 100 A. The temperature dependence of these three stages is investigated for different materials. Phase (1) shows a higher final current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreasing function of breakdown voltage and only slightly different for low and room temperature, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature, in general.