Investigation of the Failure Mechanism for an S-Band Pillbox Output Window Applied in High-Average-Power Klystrons

On the basis of the results of X-ray photoelectron spectroscopy, theoretical analyses, and numerical simulations, the failure mechanism of an S-band pillbox output window applied in a high-average-power klystron is discussed. The influence of the high-order cylindrical-guide modes on the microwave power loss, the direction of the power flow and the window disk cracking are investigated. The high-order cylindrical-guide TM₁₁ mode existing on the surface of the window ceramic disk may cause the carbon film deposition and excessive radio frequency losses on the disk and lead to the breakdown of the window.