Circular Free-Electron Laser Using Velvet-Covered Cathode

Generation of microwave radiation from a rotating electron beam in a circular wiggler magnetic field (circular free-electron laser) has been studied. The electron beam is emitted by a velvet-covered cathode; its energy is typically 300-500 keV and its beam current is 160 A. Measured frequency, power and pulse width of the radiated microwave are 12.7 GHz, 11 kW and 500 ns, respectively. Measured frequency agrees with the theoretical prediction. The interaction between the beam mode and the traveling magnetic waveguide mode is discussed based upon the dispersion relation.