First operation of a powerful FEL with two-dimensional distributed feedback

A W-band (75 GHz) FEL of planar geometry driven by a sheet electron beam was realised using the pulse accelerator ELMI (0.8 MeV/3 kA/5 μs). To provide the spatial coherence of radiation from different parts of the electron beam with a cross-section of 0.4×12 cm two-dimensional distributed feedback systems have been employed using a 2-D Bragg resonator of planar geometry. The resonator consisted of two 2-D Bragg reflectors separated by a regular waveguide section. The total energy in the microwave pulse of microsecond duration was 100 J corresponding to a power of ∼100 MW. The main component of the FEL radiation spectrum was at 75 GHz that corresponded to the zone of effective Bragg reflection found from “cold” microwave testing of the resonator. The experimental data compared well with the results of theoretical analysis.