Optical investigation of ac preflashover of alumina ceramic in vacuum

Optical phenomena accompanying the preflashover along a planar metal-alumina-metal structure were investigated under stepped ac voltage in vacuum. Two kinds of electrode contacts with and without sputtered gold films were employed. For sputtered alumina, the luminescence became observable at ∼ 1 kV_peak, and revealed two stages depending on the amplitude of applied voltage. For non-sputtered alumina the optical emission appeared at a much higher voltage and presented irregular and discrete light pulses. The energy band at the metal-alumina interface for the two kinds of electrode contacts is responsible for the relevant optical mechanisms. For non-sputtered contact, the light emission was initiated by field electron emission from the triple junction. While for the sputtered contact, prior to the electron emission, electrons/holes could be injected from electrodes into the surface layer of alumina and electroluminescence phenomena occur due to the radiative electron-hole recombination. Injected electrons form a long-term negative space charge region away from each electrode and hence at a critical applied voltage, the trapped electrons are detrapped resulting in intense light emission. These processes play a significant role in the development of flashover.