First operation of free-electron maser based on a two-mirror cavity defined by 2D and 1D bragg structures

Abstract

Summary form only given. First experimental study of operation of a co-axial free-electron maser (FEM) based on a two-mirror cavity formed by a 2D Bragg input and 1D Bragg output reflector is presented. The input mirror provides two-dimensional (2D) distributed feedback and ensures mode selection over the wave azimuthal index. The use of a 1D Bragg structure as an output mirror reduces the cavity Q-factor, improves the RF field profiles inside the cavity and increases the output power as compared to FEMs based solely on 2D Bragg structures. The FEM has been driven by an oversized high-current (1.4 kA), mildly relativistic (440 kV) thin (0.2 cm wall thickness) annular (7 cm diameter) electron beam of 200 ns pulse duration. The electron beam was guided through a coaxial transmission line of length ~2 m with the diameters of inner and outer conductors of 6 and 8 cm respectively. An azimuthally symmetric undulator of period 4 cm located inside the uniform magnetic guide field was used. The undulator field was slowly up-tapered over the initial 6 periods. A solenoid of length 2.55 m and diameter 30 cm surrounding the co-axial electron gun, undulator and co-axial transmission line interaction region was used to guide the electron beam. The amplitude of the undulator could be varied up to 0.06 T while the amplitude of the guide magnetic field could be changed up to 1 T. The two-mirror cavity of length 81 cm (input 2D Bragg mirror 10.4 cm, output 1D Bragg mirror 10 cm) was located inside the transmission line along the uniform pan of the undulator. An output power of ~50 MW corresponding to an efficiency of ~8% was measured. The directional mode pattern of the microwave radiation launched from the output horn was also measured. Using cut-off filters the location of the operating frequency was found to lie between 35 GHz and 39 GHz. Future work will involve detailed study of the output radiation spectrum and further optimisation of the cavity configuration to achieve higher efficiency and higher output power