Microwave surface flashover of dielectrics in vacuum

Threshold characteristics of pulsed dielectric breakdown in vacuum, obtained at various experimental conditions, are considered in detail. As would be expected, the breakdown time delay τ_d (supposing τ_d<τ_p, the microwave pulse duration) depends on many initial parameters, such as the microwave intensity J, the nature of the irradiated material and its surface quality and the vacuum characteristics, At the same time experimental results, for microwave intensities above the continuous wave threshold of J=10⁷ W/cm², showed that the energy density W=J/τ_d is approximately constant for a given material and its operating conditions. An increase of pulsed microwave surface flashover thresholds after preliminary surface conditioning by a discharge series or under the application of a constant electrical potential from an external source to the dielectric sample was observed and measured. Experimental data allowed us to clarify the physical model of the microwave surface flashover phenomenon. It is based on the dominating role of the saturated secondary electron emission avalanche, electron stimulated gas desorption, and the resultant microwave breakdown of the surface gas cloud. Experimental and theoretical results are in satisfactory agreement