Analysis of multimode radiation from 0.4 THz backward wave oscillator

A terahertz backward wave oscillator (BWO) is a high-power terahertz wave source based on the Cherenkov interaction between an electron beam and a slow backward wave. Excitation of slow wave modes with different mode numbers degrades the performance of BWO due to mode competition. Analyzing the mode spectrum is a difficult challenge to confirm single mode operation. In this study, the excitation of the slow waves in a 0.4 THz cylindrical corrugated waveguide is examined. In the corrugated waveguide, azimuthal modes become slow backward waves with evanescent fields. We found the superposition of the modes with different azimuthal indices has an asymmetric field intensity and the excitation of the superposed modes produces the terahertz radiation with the asymmetric pattern. We conduct a BWO experiment using a 0.4 THz corrugated waveguide. The experimental radiation pattern is obtained and corresponds well to the calculated asymmetric pattern. The symmetry of the radiation can be regarded as an indicator of the mode control.