In this paper, we present the design of a high-power, high-efficiency coaxial relativistic backward wave oscillator operating in the Q-band under a low magnetic field. The device employs a coaxial slow-wave structure with large transverse dimensions and a small gap between the inner and outer conductors. In high frequency devices, competition from asymmetric modes tends to be more severe. To suppress such mode competition, the electron beam is synchronized with the 9π/10 mode of the TEM wave for efficient interaction, while it asynchronously exchanges energy with the 7π/10 mode of the TM01 wave. This approach effectively inhibits the growth of asymmetric modes during the oscillation startup stage, and detailed theoretical validation is provided in the paper. A tapered waveguide is designed to ensure pure TEM-mode operation at the output. Furthermore, the lengths of two drift tube sections are adjusted to optimize the modulation depth of the electron beam and the energy extraction efficiency. Three-dimensional particle-in-cell simulation results demonstrate that, at a diode voltage of 330 kV and a beam current of 3 kA, the generation of microwaves at 46.1 GHz with an output power of 425 MW and a conversion efficiency of 42.9% is obtained.
Zhang et al. (Sun,) studied this question.