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In this talk,the mechanism of nanosecond breakdown at metal/vacuum interface in the high-power microwave waveguides is reported.The high-gradient breakdown seriously limits the power capacity of microwave equipments[1-2].The electrons under the intense E-field 1 MV/cm have a high energy at several hundred keV,leading to a little true secondary yield and small backscattered yield as well as low ionization cross-section.Thus,in order to realize the ionization avalanche in nanosecond HPM discharge at metal surface,the ambient desorbed or evaporated gas pressure above the metal surface is calculated as high as several Torr.The local high pressure may come from the evaporated microscopic protrusions due to Joule heating and gas desorption.