Influence of quality factor on operating mode of TM₀₂ mode relativistic backwave oscillator

The mode competition in an overmoded relativistic backward wave oscillator is studied through theoreticalanalysis and three-dimensional particle-in-cell simulation in this work. Based on the quality factor and couplingimpedance, the mode selection for a TM02 mode backward wave oscillator is achieved, and its output power andmagnetic field strength are optimized in the simulation. The quality factor is related to the group velocity and end reflection of each mode. The dispersion curves ofsome non-axisymmetric modes are very close, and the group velocities are basically equal. Therefore, the endreflection needs considering to distinguish between the quality factors of different modes. In frequency domainsimulation, analyzing the quality factor of each mode by using the S-11 parameter curve can avoid calculatingthe end reflection. The three-dimensional simulation results show that the coupling impedance and quality factor jointly affectthe operating mode. When the coupling impedance advantage of the working mode is not obvious, changing theresonant frequency of the high-frequency structure can affect the beam-wave interaction process, therebychanging the excitation mode. When the advantage is obvious, the beam-wave interaction of the excitationmode will not be destroyed by the resonant mode, and other modes of microwave output mainly come from theconversion of the same frequency modes. Due to the constant dispersion curve, the effect of resonance on themode is essentially the effect of the quality factor on the mode dominated by the end reflection. The insensitive parameters and the electron beam radius obtained from the simulation are used as theoptimal parameters, and the automatic optimization algorithm is used in combination with the two-dimensionalsimulation to perform multi-objective optimization design in the above device. The final output power of thebackward wave oscillator reaches 534 MW, with an efficiency of 23.64%, an increase of 221.7% compared withthe efficiency of the original device. The device operating mode remains stable, with a power ratio of TM(02)mode reaching 94.95%