The increase in efficiency of free electron lasers by using cyclotron resonances of electrons with a combination wave

A nonlinear theory of FEL's based on stimulated undulatory radiation (ubitrons) and stimulated wave scattering (scattrons) for the cyclotron resonance of electrons with a combination wave is developed. Analogy of such types of ubitrons and scattrons with cyclotron resonance masers (CRM's) operating under the normal and anomalous Doppler conditions is traced. In particular, when the phase velocity of a combination wave is close to the velocity of light, the electron motion is of autoresonance character. The latter provides high efficiency up to ≈20%.