Nonlinear harmonic generation in free-electron laser oscillators

Nonlinear harmonic generation has been described theoretically, and observed, in free-electron laser (FEL) amplifiers. The mechanism relies on the nonlinear, higher-order bunching when the fundamental reaches high intensities, and can also be expected in high-power FEL oscillators. We use the 3D polychromatic simulation code MEDUSA to study the nonlinear harmonic generation in FEL oscillators. Strong harmonic growth is, indeed, found when an oscillator is near saturation, and the third (fifth) harmonic can reach power levels approaching 1% (0.1%) that of the fundamental.