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 fr om the cyclotron resonance (including reversed guide field regimes) th e quality of the electron beam formed in slowly up-tapered wiggler ent rance is better than in regimes close to the cyclotron resonance; As a result, FEL operating in the reversed guide field regime possesses a rather small sensitivity to the initial spread of beam parameters. A c apability of high-efficiency FEL operation in reversed guide field reg ime is corroborated experimentally, and a record for millimeter-wavele ngth 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 o n the wall of the transporting channel.