Novel Electrical Tree Type Observed using Gigahertz Frame Rate Camera to Image Dielectric Breakdown in Space-Charged Polymer

We have successfully imaged dielectric breakdown occurring in electron irradiated space-charged polymethyl methacrylate (PMMA) blocks at a nanosecond time resolution. These images, which are the first of their kind, reveal two distinct dielectric breakdown treeing effects, one of which is absent from the present literature on dielectric breakdown. This new mode of electrical tree formed during dielectric breakdown of a space-charged polymer demonstrates unique behavior from its better-documented and widely known “branch-type” channels, including structural and formation dynamics differences. Due to its threadlike, smooth appearance, this new type of electrical tree has been coined “vine-type,” in contrast to its “branch-type” counterpart. Both vine-type and branch-type electrical trees were imaged as they formed in the electron irradiated samples, allowing for close study of the channel formation dynamics. The speeds at which the channels form for both types were directly measured from these images and found to exceed 10⁷ m/s and 10⁶ m/s for vine-type and branch-type, respectively—speeds approaching 4% the speed of light in the material. These speeds are faster than previously predicted by models and imply the channel growth is the fastest physical phenomenon in a solid material. Further, the propagation of the electrical tree formation was imaged and three regimes of breakdown channel formation were identified and characterized for the two modes of electrical trees, solving a decades-long mystery on the plasma channel formation process in space-charge dielectric breakdown.