Single-mode selection and axial mode control in a free-electron maser oscillator using a prebunched electron beam.

The effects of electron-beam prebunching on the radiation buildup process in a free-electron maser oscillator operating in the frequency range of 4–5 GHz were studied. An electron beam of 10 keV, 1 A was prebunched by means of a microwave tube section, accelerated to 70 keV, transported through a drift region by a solenoidal magnetic field and injected into a linearly polarized wiggler in which a free-electron laser interaction takes place. Single-frequency selection and locking due to electron-beam prebunching was demonstrated; we show that by tuning the electron-beam prebunching frequency we can set the oscillator frequency at any of the resonant frequencies of the cavity within the net gain bandwidth. An appreciable speedup of the oscillation buildup due to prebunching was also observed; the buildup time of radiation in the presence of prebunching is shortened compared to the case of the free-running oscillator. @S1063-651X~96!00609-5#