Optical measurement of breakdown processes in polymethylmethacrylate under nanosecond pulse voltage application

High-speed framing photography has revealed the propagation velocity of breakdown channel in polymethylate (PMMA) to be between 30 and 120 kmf/s for positive 50-kV point voltage. The waveforms of the prebreakdown current in PMMA are essentially the same as those in liquid dielectrics. The breakdown time lag was precisely measured from the current. A linear relation between formative time lag and gap spacing is observed for gap length greater than a critical value, indicating another constant propagation velocity in this region. The velocity is deduced from the relation to be 2.2 km/s. This value is fairly close to that of longitudinal waves in the solid.<
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