Predischarge process investigations of solid semitransparent dielectrics

Present results from the investigation of predischarge and discharge processes obtained on polymethyl methacrylate samples by using an image-converter camera, a schlieren system, and other diagnostic systems. The investigation was carried out within a needle-plane electrode system, with short-duration pulses of both polarities being applied to the electrodes. PMMC sample thickness ranged from 0.14 mm to 10.3 mm. It is shown that, as pulses having a certain amplitude are applied, the dielectric appears to crack. In some of the numerous cracks discharges can be observed which result in breakdown with increased voltage. The maximum velocity at which cracks appear at negative and positive polarities is higher than that at which a crack spreads under the critical mechanical stress applied. The velocity of discharge propagation in a crack is substantially higher than that of crack formation, corresponding approximately to streamer velocity in the air