Delay characteristics of surface charging on a cylindrical insulator in vacuum

The charging of an insulator is considered as a critical phenomenon leading a bridged gap to flashover in vacuum. Based on the theory which Harmisch and Boersch (1963) proposed as secondary emission electron avalanche (SEEA) mechanism, Anderson (1980) explained the V-t characteristic of a bridged vacuum gap exposed to a short voltage pulse in connection with space charge effects due to ionization of desorbed gases in vacuum. In this theory, however, it is assumed that the charging takes place immediately after the voltage application, thereby neglecting the time necessary for the surface. charging. We have developed a probe method which enables us to do a real-time observation of the surface charging in vacuum. The probe measures the electric field change associated with the surface charging on the cathode very close to the triple junction, where the insulator, cathode and vacuum meet, without disturbing the electric field in the gap nor the charge distribution on the insulator. This paper reports the results of probe measurement which characterize temporal behavior of charging on a cylindrical insulator when it is exposed to a trapezoidal or a stepped DC voltage