High speed photographic investigation of the AC breakdown of an air/solid composite

High speed photography has been used for many decades in the study of gas breakdown but only recently has the technology become adequate to permit similar work with solids. Using a simple camera we previously investigated
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the impulse wavefront breakdown of a composite consisting of a column of air in series with a layer of polymethyl methacrylate (PMMA). The hypothesis in vogue
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is that such insulation breaks down because a discharge in the weaker gaseous section acts like a thin, conducting extension of the metal electrode closing the air column. This extension conveys the applied voltage to the dielectric layer which breaks down if the intrinsic electric strength is reached in the solid at the point of contact with the discharge channel. Our previous investigation demonstrated the inadequacy of this hypothesis. It was shown that the spark was initiated in the solid well before a luminous discharge channel had formed connecting the dielectric surface and the metal electrode. It was proposed that the establishment of a spark depended on the prior establishment of appreciable conductivity in the dielectric. This was attributed
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to the build-up of great stress by charges deposited onto its surface from discharges in the air column, which were shown to occur at intervals on the wavefront as the voltage increased to breakdown. It was of interest to determine whether or not similar conclusions could be drawn about the technically-more-important breakdown by alternating voltage, but the mode of camera shutter operation prevented its application to this problem. The investigation had to wait for the development of the camera described below.