Production of powerful spatially coherent radiation in free electron lasers based on two-dimensional distributed feedback

For intense oversize relativistic electron beams with sheet and annular geometry the use of two-dimensional (2D) distributed feedback is beneficial for providing spatial coherence of the radiation and, thus, can be used to increase the total radiation power. Such 2D feedback can be realized in planar and co-axial 2D Bragg resonators having double-periodic corrugations of the metallic side walls. Modelling of the nonlinear dynamics of the FEL with 2D distributed feedback demonstrated the advantages of the novel feedback mechanism for production of spatial coherent radiation from large size electron beams. Simulation results are confirmed by recent experimental results where narrow frequency radiation was obtained at Ka-band co-axial and W-band planar 2D Bragg FELs which were realized at the Strathclyde University and Budker INP RAS.