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

The creation of powerful free-electron lasers operating in the sub-THz and THz ranges, based on intense long-pulse relativistic electron beams, is associated with the need for extremely oversized electrodynamic systems with particular properties. To address this problem, the authors turn to innovative Bragg resonators implementing a distributed-feedback mechanism that can be termed ``three-dimensional''. The results indicate that using such Bragg resonators should make it possible to ensure selection over all three mode indices, with length scales reaching up to hundreds of radiation wavelengths in all spatial coordinates.