Streak photography of the dynamic-electrical discharge behavior on insulator surfaces in vacuum

High-speed streak photography of surface breakdown or flashover events along the surface of monocrystalline quartz and polycrystalline alumina specimens is reported. The initiation of visible luminosity was observed to be a function of surface properties of the samples. It has been shown that light could originate independently at the anode as well as at the cathode, indicating that the initiation of surface flashover could start in any region. The propagation rate of luminosity, bridging the electrodes completely, was found to vary depending upon the location at which the light originated. The initiation of light at the cathode is discussed on the basis of an electron induced collision-ionization process and of radiative recombination of electrons in the conduction band with the empty or positively charged traps or defect centers. The emission of light at the anode end is discussed along the lines of a recombination process involving field ionized traps or defect centers and conduction electrons and of the hole-induced excitation of activator or defect centers by inelastic collision. >