An investigation into the cumulative breakdown process of polymethylmethacrylate in quasi-uniform electric field under nanosecond pulses.

A group of complete images on the discharge channel developed in PMMA in quasi-uniform electric field under nanosecond pulses are observed with an on-line transmission microscope. The characteristics of the cumulative breakdown process are also generalized, which include initiating from the vicinity of the cathode, developing to the anode with a branch-like shape, and taking on a wormhole appearance when final breakdown occurs. The concluded characteristics are explained by referring to the conceptions of 'low density domain' and 'free radical' and considering the initial discharge channel as a virtual needle. The characteristics are helpful for designers to enhance the lifetime of insulators employed on a nanosecond time scale. [ABSTRACT FROM AUTHOR]