Higher Harmonics in Free Electron Laser with a Two-Wavenumber Plane Wiggler Magnetic Field

The higher harmonics of the free electron laser (FEL) with a two-wavenumber plane wiggler magnetic field, which may induce a lot of higher harmonics, are investigated. Using a two-wavenumber wiggler comprised of two separate periodicities, k_ω and k_ω + Δk, numerical simulations show that the higher harmonic may be amplified and the first harmonic may be suppressed in this kind of FEL with an optimized parameter Δk. The coupling coefficient and gain coefficient in low-gain regime of the harmonic are also obtained.