Spectroscopic observations of surface flashover across an insulator in vacuum under pulsed excitation

Spectroscopic investigations of surface flashover of insulators are performed with materials such as alumina and PMMA (polymethyl methacrylate) in vacuum under the application of a high-voltage pulse. The spectra of emitted light from the flashover plasma reveal the roles played by the background gases in the system (H atomic line and N₂ molecular band), desorbed gases such as CO from the insulator, material defects (AlO molecular bands), and excited species of the electrode material (as seen from the atomic lines Al I, Al II, and Al III). In contrast, the spectra of the preflashover phenomenon shows only the AlO bands and a weak molecular CO band. This observation suggest that not only desorbed gases, but also material defects from the specimen insulator, can be a dominant cause for the initiation of the surface flashover plasma in vacuum