Observations from the High-Speed Imaging of Dielectric Breakdown in Space-Charged Polymer

In this experiment, the dynamics of high-speed electrical tree growth in electron-charged bulk polymethyl methacrylate were studied using a gigahertz frame rate camera. In the first of two experimental runs, samples were charged using a 4.5 MeV electron Dynamitron accelerator and transported to Los Alamos National Laboratory for imaging. Dielectric breakdown was induced in each sample using an exploding bridge wire detonator and the formation of the subsequent electrical trees were imaged using a SIMX16 framing camera. Through this first imaging campaign and a subsequent campaign undertaken a year later, the propagation of the electrical tree was found to exhibit several distinct sequences: initiation, continued propagation, and cessation. It was found that the electron detrapping occurs simultaneously with the electrical tree damage propagation, with the electron detrapping occurring in a narrow band at the propagation front edge. Further, backlit images of the discharge revealed shock propagation off the already formed channels, proving that the damage trails that form the electrical tree are forming faster than the speed of sound in the material.