Tuning of the emission frequency and generation of higher harmonics in free-electron lasers with intense pumping field

The operation of free-electron lasers with high strengths of the periodic magnetostatic pumping fields, when the emission is nondipolar in character, is studied on the basis of the nonlinear theory of stimulated undulator emission. The generation of the higher harmonics of electron bounce frequencies is investigated. It is shown that the efficiency decreases as the harmonic number increases due to the narrowing of the negative-reabsorption band. Attention is given to the possibility of tuning the laser emission by changing the pumping field strength.