Electric current in dc surface flashover in vacuum

Dielectric surface flashover in vacuum is characterized by a three-phase development, as shown by current measurements covering the range from 10(-4) to 100 A, assisted by x-ray emission measurements, high speed photography, and time-resolved spectroscopy. Further information is gained from a comparison of the flashover dynamics at 77 and 300 K. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification, with a duration on the order of 100 ns, reaching currents in the ampere level. The final phase three is characterized again by a fast (nanoseconds) current rise up to the impedance-limited current on the order of 100 A in this specific apparatus. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. The feedback mechanism towards a self-sustained discharge is due to space charges leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as field enhancement factors, outgassing rate, and the buildup of the gas density above the surface, are determined by fitting calculated results to experimental data. (C) 1999 American Institute of Physics. [S0021-8979(99)02906-0].