Report on first masing and single-mode locking in a prebunched beam FEM oscillator

The effects of electron beam prebunching on the radiation buildup process in a free electron maser oscillator operating in a collective-Raman regime and oscillating in the frequency range of 4 to 5 GHz are studied. An electron beam is prebunched, accelerated to 70 keV, transported through a drift region, and injected into a linearly polarized wiggler in which FEL interaction takes place. Single-frequency selection and locking due to electron beam prebunching is demonstrated; we show that the electron beam prebunching can be used to force the oscillator to achieve steady-state single-mode operation and to select the oscillator frequency to be any of the resonator natural frequencies of the cavity in net gain bandwidth. An appreciable speed-up of the oscillation buildup due to prebunching was also observed.