Surface flashover and surface deterioration of polymer insulations in vacuum

Results are presented of investigations carried out to record the various factors affecting the surface flashover strength across perspex (PMMA) and Teflon (TFP) insulators in a vacuum. The factors studied include the dependence of flashover AC and DC voltage stresses on sample length and field conditions. The influence of the number of sequential flashovers on the magnitude of flashover voltage and on the surface deterioration of the polymer samples in vacuum is also examined. Under an ultimate pressure of 0.1 Pa the flashover voltage for perspex is always higher than that of Teflon. Thus the DC flashover voltage values are higher than those of AC values, whereby the negative polarity flashover voltage is higher than that of the positive polarity. The flashover voltage increases in general with increased sample length, so the flashover voltage values under nonuniform fields are higher than those under uniform field conditions. The flashover voltage decreases after about 35 flashovers; such a number of flashovers causes treelike eroded paths on the insulating surface, thus leading to a marked decrease of flashover voltage value