Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.