The effect of wiggler imperfections on nonlinear harmonic generation in free-electron lasers

The generation of harmonics through a nonlinear mechanism driven by bunchin g at the fundamental has sparked interest in using this process as a path t oward an X-ray free-electron laser (FEL). An important issue in this regard is the sensitivity of the nonlinear harmonic generation to wiggler imperfe ctions. Typically, linear instabilities in FELs are characterized by increa sing sensitivity to both electron beam and wiggler quality with increasing harmonic number. However, since the nonlinear harmonic generation mechanism is driven by the growth of the fundamental, the sensitivity of the nonline ar harmonic mechanism is not severely greater than that of the fundamental. in this paper, we study the effects of wiggler imperfections on the nonlin ear harmonics in a 1.5-Angstrom FEL, and show that the decline in the third harmonic emission with increasing levels of niggler imperfections roughly tracks that of the fundamental.