Abstract
An {X} -band superradiant relativistic backward-wave oscillator (RBWO) operating at a low magnetic field with power conversion efficiency exceeding unity is proposed. The device efficiency is improved from two aspects: beam transmission stability focused by a low guiding magnetic field and microwave transmission efficiency. First, a gradually decaying axial magnetic field is applied. It could not only suppress the initial radial oscillation amplitude of the electron beam envelope but also transform the high transverse momentum of electrons into the axial momentum, which is beneficial to the beam-wave interaction. Second, a novel structure with a front extractor is introduced. The backward wave is extracted into a coaxial channel outside the slow wave structure (SWS) and transmitted in TEM mode to avoid the power loss of the reflected microwave in the SWS caused by the dispersion effects. Particle-in-cell (PIC) simulation results reveal that when the intense relativistic electron beam (IREB) propagating in a 36-period superradiance-RBWO (SR-RBWO) is confined with a magnetic induction intensity that decreases from 0.81 T in cathode region to 0.43 T in interaction region, the output power of 1.04 GW corresponding to conversion factor {K} = 1.55 can be obtained. The merits of low-magnetic-field operation and high efficiency make this tube a promising device used for the miniaturization of high-power SR systems.