Role of coulombic centers on insulator surface breakdown characteristics

Pulse surface breakdown (flashover) and conditioning characteristics of polycrystalline alumina and single crystal quartz in vacuum were investigated. An electrode system which greatly facilitates time-coordinated electrical and optical measurements of surface discharges along insulators was designed and validated. The surface flashover characteristics obtained using the electrode system are discussed and compared with those obtained using the conventional parallel plane electrode system. A procedure for the conditioning of insulators in vacuum is presented. The single crystal quartz specimens were found to exhibit higher conditioned breakdown strength ( approximately 350 kV/cm) compared to polycrystalline alumina ( approximately 230 kV/cm). The mechanisms associated with the breakdown characteristics are discussed in terms of negative and positive space charge formation involving coulombic centers (traps) and adsorbed layers. The pre-breakdown and breakdown X-ray activities and luminosity are also presented and discussed.<>