Theoretical and Experimental Studies on the Asymmetric Mode Competition in an Overmoded Ka-Band Cerenkov Oscillator

In the development of a Ka-band overmoded Cerenkov oscillator, the issue of asymmetric modes competition should be paid attention to as they can be potentially excited in the overmoded cavity and then affect the performance of device seriously. Based on the 3-D models, the influences of the structural parameter such as “ldr” on the resonance characteristics for the symmetric modes and the asymmetric modes are analyzed. Then according to the start conditions for the modes, the roles of the Ezfield distribution and the guiding magnetic field in spurring the asymmetric modes are discussed and accordingly the ways to alleviate the asymmetric modes competition are suggested. The 3-D particle-in-cell (PIC) simulation results show that, with the proposed conditions, the optimized device is operating stably with a purified output mode of TM
₀₁
. Finally, the optimized structure is experimentally tested. In accordance with the theoretical prediction and the simulation results, the experimental results show that under specific conditions the asymmetric modes are very likely to be excited and they can even dominate the beam-wave interaction process. Nevertheless, using the proposed method, the output mode is pure of TM
₀₁
, demonstrating the feasibility of the proposed method in suppressing the asymmetric modes competition.