FREE-ELECTRON MASER OSCILLATIONS NEAR WAVE-GUIDE CUTOFF

In waveguide-based FEMs there are two possible frequency radiation ban ds corresponding to the two intersections of the beam line with the wa veguide dispersion curve. The low-frequency intersection point occurri ng near the waveguide cutoff frequency was studied by us experimentall y. Special modifications of the existing TAU FEM facility were made in order to investigate FEM radiation near cutoff. Some rectangular wave guide components, used for the radiation coupling from the resonator t o the detection system, were replaced by components based on a double- ridged waveguide having a lower cutoff frequency and a wider frequency band. FEM operation in the vicinity of the waveguide cutoff frequency was studied in free-running oscillator configuration using a unique e xperimental setup enabling time-dependent spectral measurements within a single radiation pulse. The observed FEM radiation was of good spec tral purity without any significant mode competition. (C) 1998 Elsevie r Science B.V. All rights reserved.