Operation of Free-Electron Maser Based on a Two-Dimensional Distributed Feedback

Summary form only given. Intensive studies of operation of a coaxial high-power Free-Electron Maser (FEM) based on a two-dimensional distributed feedback are currently taking place at the University of Strathclyde. It has been demonstrated that highest output power and efficiency was achieved if the FEM two-mirror cavity was formed by a 2D Bragg (input mirror) and 1D Bragg structure (output mirror) (2D-1D cavity) in comparison with the case when 2D-2D two-mirror cavity was installed. 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 decreases the power losses inside the output mirror as compared to a 2D Bragg output mirror. The FEM has been driven by an oversized high-current, mildly relativistic thin (0.2 cm wall thickness) annular (7 cm diameter) electron beam of 200 ns pulse duration. The electron beam was guided thorough a coaxial transmission line of length ~2m with the diameters of inner and outer conductors of 6 and 8 cm respectively. The directional mode pattern of the microwave radiation launched from the output horn was measured. The output power measured at the detector was integrated over the radiation pattern resulting in an FEM output power of ~60MW corresponding to an efficiency of ~10%. To study the frequency of the output radiation first the cut-off filters were used and it has been established that the 2D Bragg FEM operated within the 36 GHz and 39 GHz. The frequency was also measured using a heterodyne frequency diagnostic. Microwave radiation from the FEM was mixed in a non-linear waveguide balanced mixer and the resultant intermediate signal captured on a LeCroy digitizing oscilloscope. By taking the FFT of the recorded signal and knowing accurately the local oscillator frequency the radiation spectrum was measured and the main maximum coincided with 37.3 GHz which is in good agreement with theoretical predictions.