Analysis of high order dispersion effects on a beat frequency-based THz free-electron laser

Experiments for coherent, wide-tunable frequency and high intensity terahertz (THz) free-electron laser (FEL) generation is under preparation at the Shanghai Soft X-ray FEL facility. In this paper, the effect of high order dispersion on a beat frequency-based THz FEL is presented. Theoretical and simulated analysis are carried out, and the results show that the third order dispersion will produce a cubic modulation on the phase of the seed laser, resulting in a degradation of the FEL performance. During the modulation processes, the nonlinear frequency chirp in the seed laser can lead to heterogeneous bunching structure in the electron beam, which will broaden the spectral bandwidth and reduce the pulse energy of THz radiation. To preserve the properties of the THz radiation, limitations on the phase errors of the seed laser are given.