Possibilities for increasing the gain and efficiency in free-electron lasers with a double undulator

A new design of a free-electron laser is suggested in which an additional space-periodic magnetic field (second undulator) is used in addition to the basic undulator. Electron oscillations in the field of the second undulator are in resonance with the cyclotron rotation of particles in a longitudinal magnetic field. This gives rise to a mechanism, which governs the bunching of electrons in the field of a ponderomotive wave. This mechanism can prevail causing significant enhancement of the gain. Under different initial conditions an amplification regime can arise, which is close to a self-resonance when the efficiency can be high.