Masing and single-mode locking in a free-electron maser employing prebunched electron beam

We report a first demonstration of single-mode selection in a free-electron maser (FEM) using electron-beam prebunching at or near the natural oscillation frequencies of the resonator. The FEM oscillation frequency can be selectively locked to each eigenfrequency of the resonant waveguide cavity within the frequency band of the FEM net gain. When the electron beam is prebunched at a frequency close to an eigenfrequency of the cavity, the oscillation buildup process is sped up and the radiation buildup time is shortened significantly. Measurements are in good agreement with collective (Raman) free-electron laser theory.