Using three-dimensional distributed feedback to enhance selectivity of Bragg structures for free-electron lasers operating at sub-THz to THz frequencies

Bragg resonators forming a three-dimensional distributed feedback have been proposed. To study the electrodynamic properties of resonators of this type, a quasioptical model has been developed within the framework of the coupled-wave approach and their high selective properties have been demonstrated by the three-mode indices under conditions of substantial oversize. Analytical solutions for the eigenmodes' spectrum of the resonator were found within the framework of the geometric-optics approximation. Results of the theoretical analysis were corroborated by the CST simulations performed in the sub-THz range. The prospects of using such resonators in the project of a superpower free-electron laser, which is currently being constructed in collaboration between BINP RAS (Novosibirsk) and IAP RAS (Nizhny Novgorod) for operation at sub-THz to THz frequencies, are discussed.