INSULATOR-SURFACE FLASHOVER CONDITIONING - A STREAK PHOTOGRAPHIC INVESTIGATION

Conditioning behavior of surface flashover strength of a large band ga p insulator (quartz) is investigated with the aid of streak photograph y in conjunction with electrical (voltage and current) and optical UV (phototube) diagnostics. The breakdown strength of the sample is found to be conditioned during the initial test as well as the subsequent t est, performed after an overnight rest period. A critical analysis of the results, obtained through streak photography and UV emission, sugg ests that the light emission during the initial test is dominated by v isible light whereas the light emission during the subsequent test (i. e., after overnight rest period) is dominated by UV. The initial condi tioning is proposed to be due primarily to the desorption of absorbed molecules and rearrangement of the same by migration kinetics. The pro cesses such as i) negative charge formation on the insulator surface ( by the plasma interaction) and ii) trapping of electrons (detrapped fr om shallow traps) into the deep traps are proposed to be responsible f or the observed conditioning after the overnight rest period. The brea kdown strength of the sample was found to degrade after experiencing b reakdown at higher fields (i.e., after second day test). This decondit ioning behavior is discussed in terms of evaporation of electrode mate rial and deposition of the same on the insulator near the cathode end.