Free-electron masers based on two-dimensional distributed feedback

The results of experimental studies of a coaxial free-electron masers (FEM) based on two-dimensional distributed feedback are presented. In the first set of experiments the FEM´s interaction region was formed by a two-mirror cavity with 2D Bragg structures as input and output mirrors. In the second set of experiments a cavity with 2D and 1D mirrors was used. In both cases the input mirror provided two-dimensional (2D) distributed feedback and ensured mode selection over the wave azimuthal index, while the output mirror was used to close the feedback loop inside the two-mirror cavity. It was demonstrated that substituting the 2D output mirror with a 1D Bragg structure increased the output power while still maintaining spectral purity and spatial and temporal coherence. The FEM has been driven by an oversized high-current (1.5 kA) thin annular electron beam of 200 ns pulse duration. An maximum output power of ~60 MW corresponding to an efficiency of 10% was measured for an FEM based on 2D/1D cavity. The directional mode pattern of the microwave radiation from the output horn was also measured and compared with theoretical prediction. Analysing the spectrum of the RF signal the location of the operating frequency was found.