Co-axial 2D Bragg structures for a high-power free electron maser

A coaxial 2D Bragg structure can be obtained by machining the corrugation on the surfaces of either inner or outer conductors of a coaxial waveguide which can be presented as r(z,φ) = R_in,out+a₁cos(k_zz)cos(mφ), where R_in,out are the radii of inner ("in") and outer ("out") conductors, a₁ is the amplitude of the corrugation, k_z=2π/d_z and d_z is the period of the corrugation along the longitudinal coordinate z and m is the corrugation variation number along the azimuthal coordinate φ. Taking into account the small perturbations of the waveguide surfaces, the RF field inside the 2D Bragg structure can be considered as a superposition of four partial waves. Using the code MAGIC, the RF field scattering on the corrugation has been studied and indirect coupling between incident and reflected waves has been demonstrated. In recent experiments, an annular electron beam of diameter 7 cm with electron energy of up to 450 keV and beam current up to 1.6 kA has been measured. An efficiency of ∼15% has been predicted for the FEM with a 2D Bragg cavity. The immediate goal of the experiment is to generate ∼100 MW of power at an operating frequency of 37.5 GHz.