Microwave studies of a tunable free-electron laser in combined axial and wiggler magnetic fields

Measurements of narrow-band (Δω/ω≤0.02) microwave emission from a tunable (7≤ω/2π≤21 GHz) Raman, free-electron laser operating in a single TE11 waveguide mode are reported. Approximately 100 kW of rf power has been observed at an electronic efficiency of 12%, and ∼1 MW of rf power has been generated at a reduced efficiency of 8%. Frequency, gain, and rf power measurements have been carried out for various values of the guide magnetic field, below, above, and near to the resonance between the cyclotron frequency of the guide magnetic field and the frequency associated with the periodic wiggler magnetic field. The results are in very good agreement with the predictions of three-dimensional free-electron laser theory.