Influence of Field Profile of Surface Wave on Terahertz Backward-Wave Oscillator

A backward-wave oscillator (BWO) is an intense electromagnetic source based on an interaction between an electron beam and a backward wave. In this study, a surface wave is excited as a backward wave on a cylindrical corrugated waveguide by an annular electron beam. We examine BWO operations using two corrugated waveguides with different depths of corrugations. The depth changes the spatial extent of the surface wave in the radial direction. In the BWO experiment with shallow corrugation, a terahertz radiation with a total power of 1 kW is obtained by the electron beam with an energy of 52.4 keV and beam current of 17.3 A. The frequency of the radiation is confirmed by high pass filters (HPFs) and is in the range from 0.3 to 0.35 THz. In the case of deep corrugation, the radiation with a total power of 433.8 W is obtained with 51.3 keV and 28.1 A. The radiation powers relate to the field profile of the surface wave when the beam thickness is comparable to the width of the radial field spread. The total powers increase as the width. A figure of merit of terahertz BWO is evaluated taking the field profile into account.