Optical observation of preflashover phenomena from polytetrafluoroethylene in a planar concentric structure

The optical emission from polytetrafluoroethylene (PTFE) surfaces in a planar concentric metal-polymer-metal structure was investigated under ac electric field in vacuum. Two kinds of optical pattern were observed before the flashover; they were strongly dependent on the magnitude of applied voltage. At a lower applied voltage, the light emission was weak and uniform and was observed close to the edges of both electrodes. It may be considered as electroluminescence phenomena due to the radiative recombination of electrons and holes injected from the electrode into the polymer surface layer. With increase of the applied voltage, the optical emission became significantly strong and irregular, and its activity area gradually expanded from the inner electrode to the outer electrode. The accumulation of electrons injected is supposed to form a long-term negative space charge region away from the inner electrode. A strong modification of the local electric field near its border would result in the detrapping of trapped electrons at a critical applied voltage and hence cause forward/backward secondary electron emission on the surface of PTFE, accompanied by intense light emission and a wide luminescent area, and probably lead to the final surface flashover. It is concluded that the negative charge region is likely located around similar to2.6 mm from the inner electrode edge. (C) 2003 American Institute of Physics.