Linear and saturated characteristics of a coaxial-waveguide gyrotron backward-wave oscillator

Abstract

A coaxial waveguide interaction structure may be suitable for a gyrotron backward-wave oscillator (gyro-BWO) operating in millimeter and submillimeter waves with good mode selectivity, frequency tunability, and high power. This study analyzes the linear and nonlinear behaviors of a coaxial-waveguide gyro-BWO by using a single-mode, self-consistent nonlinear code. Simulation results indicate that the coaxial gyro-BWO exhibits features similar to those of a cylindrical-waveguide gyro-BWO, such as nonlinear field contraction, the relation of start-oscillation current to interaction length, and the difference value of the transit angle between adjacent axial modes. Additionally, the coaxial gyro-BWO has distinctive characteristics due to its transverse geometrical parameter C, i.e., the ratio of the outer radius to the inner radius. The beam-wave coupling strength of the coaxial gyro-BWO is a function of parameter C. As a result, the start-oscillation current of the coaxial gyro-BWO varies as the C value selected varies. The coaxial gyro-BWOs with different C values require different interaction lengths to reach the saturated state for the same beam current. Parameter C also impacts the magnetic tuning bandwidth of the coaxial gyro-BWO. However, maximum efficiency at saturation, which was obtained by tuning the magnetic field, is not highly dependent on the value of parameter C.