Peculiarities of the Mode Spectrum in Free-Electron Masers Based on Oversized Bragg Resonators with a Corrugation Phase Step

Abstract

We study the operating mode splitting caused by interaction of the neighboring Bragg scattering zones in an oversized Bragg resonator with a corrugation phase step, which is operated at the coupled forward and backward waveguide modes with different transverse structures. This effect is described within the framework of the coupled-wave approach using an advanced four-wave model. It is shown that this effect deteriorates the selective properties of the resonator and, finally, restricts the output power and reduces stability of the narrow-band operating regime in the free-electron masers (FEMs) based on such resonators. The results of the theoretical analysis were corroborated by 3D simulations and “cold” electrodynamic tests. Experimental studies of 30-GHz FEMs with the Bragg resonators having different corrugation depths demonstrated the onset of both narrow-band single-mode and multifrequency multimode oscillation regimes in such resonators. The possibility of power enhancement by using passive compression of the FEM output pulse in a double-frequency oscillation regime is discussed.