Breakdown in ceramic window

A capacitive type coaxial ceramic window is designed for the SC linac. The coaxial power coupler has inner conductor diameter 34.8 mm and outer conductor diameter 80 mm. An alumina disk of diameter 150 mm and thickness 3.6 mm is used as ceramic. The diameter of the inner conductor of the coupler near the ceramic is increased to 39.2 mm to match the impedance at 1050 MHz. A copper prototype of the window is fabricated and characterized. A performance of the window is often degraded because of the multipacting. A CST particle studio is used to simulate the multipacting trajectories. The particle trajectories are observed at different locations and different power levels up to 1 MW. The results are dependent on the initial position of the primary electron and power level. A few soft multipacting barriers are observed at the operating power level.