Analysis and Suppression of Mode Competition in a Relativistic Klystron Oscillator

The suppression of asymmetric mode competition is one of the basic conditions for proper operation of high-power microwave (HPM) sources. This article presents experimental results on mode competition in a relativistic klystron oscillator (RKO) operating under low magnetic field conditions. A competing mode with a frequency of 6.27 GHz is observed in the experiments and is in good agreement with the particle-in-cell (PIC) simulation results. It is determined that the competing mode was the TM211 mode. The asymmetric mode competition mechanism is found by theoretical and simulation studies to be that the asymmetric mode TM211 mode and the symmetric mode TM011 mode of the bunching cavity have the similar external quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q} _{\text {e}}$ </tex-math></inline-formula>. Upon modifying the bunching cavity of the RKO, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q} _{\text {e}}$ </tex-math></inline-formula> increases from 1036 to 3421 for the symmetric mode and decreases from 1041 to 611 for the asymmetric mode, while other physical properties are less changed. The efficacy of this technique in suppressing asymmetric mode competition has been substantiated through both PIC simulations and experimental results.