A traveling-wave ring resonator with Bragg deflectors in a two-stage terahertz free-electron laser

We propose a scheme of a two-stage terahertz free-electron laser (FEL) based on a Bragg traveling-wave ring resonator and parallel ribbon relativistic electron beams. The first beam moves in the field of a planar undulator and generates a pumping wave in the millimeter range. At the second stage, this wave transforms into terahertz radiation in the course of intraresonator backscattering on the second parallel electron beam. The ring resonator consists of four Bragg deflectors, each deflecting radiation by 90$^\circ$ in the vicinity of Bragg resonance, thus closing the feedback ring of the low-frequency pumping generator and simultaneously providing effective transfer of the pumping wave to the scattering stage.