Spectral nature of luminosity associated with the surface flashover process

The luminous events associated with the breakdown along the surfaces of large band gap insulators such as polycrystalline alumina and monocrystalline quartz in vacuum were investigated using a phototube and a very sensitive photomultiplier tube (PMT). The spectral nature of the light emitted during breakdown was resolved (in terms of the wavelength) by interfacing a monochromator with the PMT. In the case of the monocrystalline quartz specimen, the breakdown luminosity was observed to be associated with defects located at 2.76 and 1.91 eV below the conduction band edge. The breakdown luminosity of polycrystalline alumina was found to be associated with the defects corresponding to energy levels at ~1.91, 2.25, 2.45 and 2.76 eV. The formation of these defects is discussed in terms of the nonstoichiometric nature of the lattice structure at the surface. The samples were found to emit light when no breakdown occurs. This emission is attributed to the deep level defects. The observed results suggest that the surface flashover process is primarily controlled by the defect or trapping centers located within the forbidden gap of the insulators