SPECTRAL NATURE OF LUMINOSITY ASSOCIATED WITH THE SURFACE FLASHOVER PROCESS

Investigations of the luminous events associated with the break-down along the surfaces of large band gap insulators such as polycrystalline alumina and monocrystalline quartz in vacuum were made by employing a phototube and a very sensitive photomultiplier tube (PMT). The onset of luminous signals, in general, was found to start much before the onset of voltage collapse during breakdown. The early onset of luminosity may be due to the presence of various defect or trapping centers within the forbidden gap of the insulator. 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 the 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 approximately 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. Besides, the samples were found to emit light when no breakdown occurs. This no-breakdown light emission was found to be 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.