Nonlinear harmonic generation in free-electron lasers with helical wigglers

It is widely believed that harmonics are suppressed in helical wigglers. However, linear harmonic generation (LHG) occurs by an azimuthal resonance that excites circularly polarized, off-axis waves, where the hth harmonic varies as exp(ihtheta). Nonlinear harmonic generation (NHG) is driven by bunching at the fundamental and has different properites from LHG. While NHG has been studied in planar wigglers, there has been no analysis of NHG in helical wigglers. The 3D simulation code MEDUSA has been modified for this purpose, and it is shown that NHG is substantial in helical wigglers and that the even and odd harmonics have comparable intensities.