Experimental and theoretical study of 2D Bragg structures for a coaxial free electron maser

Abstract

Experimental results are presented of the first observation of two-dimensional distributed feedback in a coaxial 2D-Bragg structure. The use of two-dimensional distributed feedback has been proposed as a method of producing spatially coherent radiation from extremely powerful large sized relativistic electron beams. To obtain coherent high-power (GW) microwave and millimetre wave radiation, 2D Bragg resonators are needed to overcome the problems of mode selection and synchronisation of radiation from different parts of an oversized beam. The design and cold microwave measurements of a 2D Bragg coaxial structure for use in a Free Electron Maser (FEM) driven by an annular electron beam of circumference 25 times larger than the radiation wavelength is presented. The cavity for the 2D Bragg FEM consists of two 2D Bragg coaxial reflectors separated by a regular coaxial waveguide. The formation of pure two-dimensional distributed feedback without a mixture of one-dimensional parasitic feedback was demonstrated for the first time in this series of experiments. The first experimental comparison of 2D and 1D Bragg structures was also conducted and good agreement between experimental results and theoretical predictions was observed. The eigenmodes of the two-mirror cavity were calculated and it was shown that a single-mode steady-state operation regime could be obtained in a FEM based on such a novel cavity.