Electric current in dc surface flashover in vacuum

Dielectric surface flashover in vacuum is characterized by a three-phase de velopment, 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 compar ison of the flashover dynamics at 77 and 300 K. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche r eaching 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 cu rrents in the ampere level. The final phase three is characterized again by a fast (nanoseconds) current rise up to the impedance-limited current on t he order of 100 A in this specific apparatus. The development during phase two and three is described by a zero-dimensional model, where electron-indu ced outgassing leads to a Townsend-like gas discharge above the surface. Th e feedback mechanism towards a self-sustained discharge is due to space cha rges leading to an enhanced field emission from the cathode. A priori unkno wn model parameters, such as field enhancement factors, outgassing rate, an d the buildup of the gas density above the surface, are determined by fitti ng calculated results to experimental data. (C) 1999 American Institute of Physics. [S0021-8979(99)02906-0].