Optical studies of surface discharge under dc voltage in vacuum

Utilizing a high-sensitivity photomultiplier tube (PMT) and an intensified charge coupled device (ICCD) camera, the light emission from the surface of polytetrafluoroethylene (PTFE) and alumina ceramic samples was investigated in vacuum under a stepped dc voltage in order to study the initiation and development of surface discharge. Different from the classical electrode contact by butting electrodes on a sample directly, to achieve a fine contact between electrodes and the surface of a sample, the metalized electrodes were employed by sputtering two additional gold contacts on each sample in advance. The two types of materials showed quite different optical characteristics. It was suggested that there were several processes possibly related to the initiation mechanism of the surface discharge. Besides the field electron emission from the cathode triple junction (CTJ) for the classical electrode contact, there were charge injection and transport in the surface states of dielectrics, and charge trapping and detrapping phenomena. Electroluminescence (EL) emission was emitted due to the radiative recombination of electrons and holes injected. These processes were strongly dependent on the electrode contact manner and the surface states of insulating materials.