First lasing of a Cherenkov free-electron laser with annular electron beam

A Cherenkov FEL with an annular electron beam has been succesfully operated. During the first lasing experiments output was found for accelerating voltages ranging from 85 to 150 kV and currents from 1 to 10 A. At a particular accelerating voltage lasing was observed for several distinct settings of the magnetic field (0.7–1.6 T). This field is used to compress and guide the electron beam along the dielectric liner. Depending on the magnetic field setting, the output was reproducible or showed large fluctuations within a single laser pulse as well as from pulse to pulse. Due to a misalignment of the cathode the total output power is only of the order of 100 W. From theoretical considerations the spectrum is expected to center around 19 GHz.