Analysis and Suppression of High-Order Mode Oscillation in an S-Band Klystron

High-order mode oscillation occurred in an S-band experimental klystron with 300 MHz instantaneous bandwidth. The cause of the oscillation was analyzed and simulated. A method was proposed to suppress the oscillation through adjusting the structure and size of a resonant cavity in the tube, in which the high-order mode frequency can obviously be changed and the variation of the fundamental mode frequency is very small. Improved designs for the resonant cavity with Ansoft HFSS code and corresponding experiment have been performed, which shows that the simulation results are in good agreement with the experimental measurements. The testing result of an experimental klystron with this improved resonant cavity indicates that the high-order mode oscillation can effectively be suppressed.