Parasitic wave excitation by multimode coupling in a raman-regime free-electron laser

Mechanisms of parasitic wave excitation in a high-current Raman-regime free-electron laser (FEL) are investigated by use of a one-dimensional multimode FEL amplification code. It is found that multimode coupling among sideband waves and the primary wave plays an important role in the parasitic wave excitation in addition to the sideband instability. Such a multimode coupling process is observed in both the linear amplification stage and the nonlinear trapping stage, and modulates the output signal to a level comparable with that solely due to the sideband instability.