The role of outgassing in surface flashover under vacuum

Results of high-speed electrical and optical diagnostics are used as a basi s to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flas hover development. Dielectric unipolar surface flashover under vacuum is ex perimentally characterized buy a three-phase development, which covers a cu rrent range from 10(-4) A to 100 A. Phase one comprises a fast (several nan oseconds) buildup of a saturated secondary electron avalanche reaching curr ent levels of 10 to 100 mA. Phase two is associated with a slow current amp lification reaching currents in the Ampere level within typically 100 ns, T he final phase is characterized by a fast current rise up to the impedance- limited current on the order of 100 A. The development during phase tno and three is described by a zero-dimensional model, where electron-induced out gassing leads to a Townsend-like gas discharge above the sur face. This is supported by time-resolved spectroscopy that reveals the existence of excit ed atomic hydrogen and ionic carbon before the final phase. The feedback me chanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parame ters, such as outgassing rate and gas density buildup above the surface, ar e determined by fitting calculated results to experimental data. The signif icance of outgassing is also discussed with a view to microwave surface fla shover.