Co-Axial Free-Electron Maser Based on Two-Dimensional Distributed Feedback

Abstract

Summary form only given. First measurements of microwave radiation from a co-axial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback are presented. To drive the FEM a high current accelerator (HCA) based on a magnetically insulated explosive emission carbon cathode was used. A driving electron beam voltage (Vhsa) of 450 kV and pulse duration of ~200 ns was applied to the HCA resulting in the generation of a thin (0.2 cm) annular electron beam of current 500 A and mean diameter of 7.0 cm. The electron beam was guided through a co-axial transmission line of length ~2 m with the diameters of inner and outer conductors of 6 and 8 cm respectively. A guide solenoid of length 2.55 m and diameter 30 cm surrounded the co-axial electron gun, undulator and co-axial transmission line interaction region. A two-mirror cavity of length 81 cm (input 2D Bragg mirror 10.4 cm, output 2D Bragg mirror 5.6 cm both defined by 'chessboard' corrugations on the inner conductor) was located inside the transmission line along the uniform part of the undulator. The frequency of the output radiation of the FEM was measured to be 37.3 GHz using a heterodyne frequency diagnostic. The mode pattern measured in the 'hot' experiments (when an electron beam was present) was compared with the radiation pattern measured in cold microwave experiments when the horn was excited by a TEM wave of frequency 37.3 GHz. The output power of the FEM was measured by integrating the power densities over space and was calculated to be (14plusmn0.15) MW (efficiency of 6%). Additional evidence of the high peak power was obtained from the observation of the illumination of a neon bulb panel located at a distance of 20 cm from the output window. To reduce amplitude of the RF field of the TEM wave on the surface of inner conductor a new 2D Bragg reflector has been constructed using copper electroforming technology, which has smooth sinusoidal corrugations on the inner surface of the outer conductor. Good agreement has been obtained between theory and cold microwave measurements. To investigate an increase FEM output power and efficiency a new cavity configuration based on the copper 2D Bragg reflector will be studied.