Dielectric surface flashover in vacuum at 100 K

Cryogenic components in high power electrical systems and in power electron ics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielec tric test sample and electrodes in vacuum are cooled to <100 K and flashove r is characterized using fast electrical and optical diagnostics. Three con secutive 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 amp litudes within similar to 2 ns, probably associated with field emission, fo llowed by (2) a slow current rise to similar to 5 to 10 A amplitude with du ration of 40 ns to 1 mu s , associated with secondary emission avalanche sa turation, and (3) a transition to a rapid gaseous ionization above the samp le caused by electron induced outgassing, leading to impedance-limited curr ent amplitudes of less than or equal to 300 A. Phase (1) shows a higher fin al current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreas ing function of breakdown voltage and only slightly dependent on temperatur e, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature.