Pulsed flashover of solid dielectrics in vacuum and gases
Large increases in the flashover potential of three out of four organic polymer materials were observed after the materials were exposed to the byproducts of a pulsed arc in vacuum. Solid cylinders of Lucite, Lexan, Delrin and Blue Nylon with a height of one centimeter and a diameter of 6.35 centimeters were placed between brass electrodes in a vacuum chamber. Measurements of the flashover potential of each material were made in vacuum (1 X 10 Torr), in pure N2 gas, and in a gas mixture composed of 20% SFg + 80% N2 at three pressures (635 Torr, 317.5 Torr, and 12.7 Torr). The voltage applied to the electrodes was a 5/135 microsecond double exponential pulse. In vacuum, samples of Lucite, Delrin, and Lexan that had been exposed to the pulsed arc source exhibited a flashover potential at least 68% higher than identical, but unexposed samples. In N^ gas and in the gas mixture, samples of the same three materials that had been exposed to the arc exhibited a flashover potential at least 300% higher than unexposed samples when tested at a pressure of 12.7 Torr. At the two higher pressures the flashover potential for exposed and unexposed samples was approximately the same. These results, interpreted using the best available flashover models, show that it is a change in the secondary electron emission coefficient of the insulator surface which is responsible for the higher flashover potential. This change is caused by exposure to the vacuum arc. A second result of this investigation is that surface damage of the insulators, caused by flashover events, is strongly dependent on the properties of the gas surrounding the insulator.