Characterization of grating monochromator performance based on start-to-end FEL pulse propagation method.

Grating monochromators are crucial optical elements in soft X-ray free-electron laser (XFEL) beamlines. Accurately evaluating the properties of grating monochromators with near-realistic XFEL pulse is of paramount importance. In this study, we utilize the start-to-end pulse propagation method to conduct a characterization of grating monochromator performance at the FEL-1 beamline of S³FEL. The primary focuses include evaluating the monochromator's resolving power, assessing the impact of longitudinal source jitter on resolving power, analyzing diffraction effects due to grating's limited aperture and investigating the influence of thermal deformation. The novelty of this research lies in the direct use of FEL pulse propagation, providing simulation results that are more reliable than those obtained using Gaussian sources. The simulations reveal that the resolving power of the monochromator is significantly influenced by the aforementioned factors, highlighting the importance of considering real FEL beam characteristics in the design and evaluation of grating monochromators for FEL applications.