SINGLE-MODE SELECTION AND AXIAL MODE CONTROL IN A FREE-ELECTRON MASEROSCILLATOR USING A PREBUNCHED ELECTRON-BEAM

The effects of electron-beam prebunching on the radiation buildup proc ess in a free-electron maser oscillator operating in the frequency ran ge of 4-5 GHz were studied. An electron beam of 10 keV, 1 A was prebun ched by means of a microwave tube section, accelerated to 70 keV, tran sported through a drift region by a solenoidal magnetic field and inje cted 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 th e electron-beam prebunching frequency we can set the oscillator freque ncy at any of the resonant frequencies of the cavity within the net ga in bandwidth. An appreciable speedup of the oscillation buildup due to prebunching was also observed; the buildup time of radiation in the p resence of prebunching is shortened compared to the case of the free-r unning oscillator.