Detailed studies of the spectral characteristics and spatial mode structure of a single-mode, high-power Raman free-electron maser (FEM) oscillator operating with a Bragg resonator are reported. The FEM oscillator generated approximately 1 MW of microwave power in a single axial mode at 27.47 GHz, with an electron beam energy of 320 keV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. These results indicate that a Raman free-electron maser can operate at high power and efficiency with stable single-mode output in the long pulse (equilibrium) regime.

• Received 28 September 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.2391