Harmonic contents of and detailed study on a high-gain harmonic generation free electron laser

In this paper, we calculate the third harmonic of a High Gain Harmonic Generation (HGHG) Free Electron Laser (FEL) at saturation. In the HGHG FEL scheme, there is an external dispersion section, which provides an efficient microbunching. Study on the emittance effect in such an external dispersion section suggests a new optimization for the HGHG FEL. We finally discuss how to reduce the incoherent undulator radiation which is a noise with respect to the seed laser.