Initiation mechanism of electrical tree under alternating stress-electron impact or UV photo-degradation?

The formation process of the microcavity is important for understanding the initiation phase of an electrical tree. Two models have been proposed for the mechanism of microcavity formation and electrical tree initiation. One is the electron impact model. In this model, electrons injected from a metal electrode or conducting particle are accelerated in the polymer free volume by high electric field. These hot electrons collide with polymer chains to break them. The other is the photo-degradation model. Electroluminescence (EL) is a photoemission phenomenon observed before tree initiation. In this model, ultraviolet (UV) components of electroluminescence cause polymer degradation. In this paper, we compare these two models