Impulse Breakdown in PMMA under Miegavolt, Nanosecond Excitation

Single impulse dielectric breakdown tests were performed on thick (0.16 to 1.91 cm) polymethylmethacrylate (PMMA) specimens using a uniform field geometry with 7.5 cm diameter Bruce contour electrodes. The applied voltage rose within 20 ns to 2 MV, remained there about 30 ns and decayed in about 65 ns. In some cases, specimens were deliberately altered by surface scribing, pin pricking, drilling small dimples, drilling holes in portions of laminated specimens, and combinations of these. Light emission during the breakdown pulse and damage morphology were studied. Results of these experiments include the following: Each specimen contained many partial channels as well as a main breakdown path. The number varied greatly with overvoltage. Specimens 0.16 cm thick (peak field 12 MV/cm) contained about 2000 partial tree-like channels in various stages of growth, while 1.91 cm thick specimens (peak field 1.1 MV/cm) contained about 20 partial channels. In all unaltered specimens, the channels originated at the anode surface. However, when deep cavities were intentionally introduced into the cathode surface, channels originated at these artificially produced defects as well as on the defect-free anode surface. Intense light emission occurred from both the partial channels and the main discharge channel. The main discharge path is formed only after one of the partial discharge channels has grown completely across the specimen.