Theoretical and experimental comparison of FEL-oscillators with conventional and reversed guide field

Results of theoretical and experimental studies of free-electron laser (FEL)-oscillators with guide magnetic field of both conventional and reversed orientation are presented. It is shown that in regimes far from the cyclotron resonance (including reversed guide field regimes) the quality of the electron beam formed in slowly up-tapered wiggler entrance is better than in regimes close to the cyclotron resonance. As a result, a FEL operating in the reversed guide field regime possesses a rather small sensitivity to the initial spread of beam parameters. A capability of high-efficiency FEL operation in reversed guide field regime is corroborated experimentally, and a record for millimeter-wavelength FEL-oscillators efficiency of 26% is obtained. In accordance with the simulations in conventional guide field regimes, under a magnetic field smaller than the resonance value, the process of RF-generation was accompanied with beam current losses caused by electron fall-out on the wall of the transporting channel.