Development process of surface flashover along alumina ceramics under AC voltage in vacuum

Using optical methods, the light emission from alumina ceramic surface during the developing process of flashover was investigated under ac voltage in vacuum. A new planar surface model was designed in this study. In order to keep a fine contact between electrodes and alumina, gold films were sputtered onto the surface of alumina samples. Two types of optical patterns, i.e., surface electroluminescence (EL) under lower voltage application (stage A) and light emission due to surface partial discharges (PD) under higher voltage (stage B), were observed, respectively. Two stages of light emission showed distinct optical spectra, phase-resolved and space-resolved distribution. The EL is concluded to occur prior to the field electron emission and it is due to the recombination of charge carriers injected from electrodes into the surface layer of alumina. Moreover, it is supposed that the space charge accumulation from injected charges would affect the initiation and development of surface flashover greatly.