Increase of the efficiency of bunching in free-electron lasers with the aid of induced betatron oscillations

A new variant is proposed of a free-electron laser in which induced betatron oscillations of the particles are used to increase the gain or efficiency. These induced oscillations are created by an additional spatially periodic magnetic field and the period is close to that of betatron electron oscillations in the magnetic field of an undulator. The amplitude of induced oscillations is sensitive to changes in the electron energy. This gives rise to an additional electron bunching in the field of a ponderomotive force, which may increase the small-signal gain or ensure amplification conditions close to a self-resonance, which increases significantly the efficiency of a free-electron laser.