A broadband gyrotron backward-wave oscillator with tapered interaction structure and magnetic field

Abstract

The gyro-monotron and gyrotron backward-wave oscillator (gyro-BWO) are the two oscillator versions of gyrotrons. While serving different functions, they are also radically different in the RF field formation mechanisms. The gyro-monotron RF field profile is essentially fixed by the resonant interaction structure, while the gyro-BWO possesses an extra degree of freedom in that the axial RF field profile is self-determined by the beam-wave interaction in a waveguide structure. The present study examines ways to utilize the latter feature for bandwidth broadening with a tapered magnetic field, while also employing a tapered waveguide to enhance the interaction efficiency. We begin with a mode competition analysis, which suggests the theoretical feasibility of broadband frequency tuning in single-mode operation. It is then shown in theory that, by controlling the RF field profile with an up- or down-tapered magnetic field, the gyro-BWO is capable of efficient operation with a much improved tunable bandwidth.