Generation of large-bandwidth x-ray free electron laser with evolutionary many-objective optimization algorithm

X-ray free-electron lasers (XFELs) are cutting-edge scientific instruments for a wide range of disciplines. Conventionally, the narrow bandwidth is pursued in an XFEL. However, in recent years, the large-bandwidth XFEL operation schemes are proposed for x-ray spectroscopy and x-ray crystallography, in which overcompression is a promising scheme to produce broad-bandwidth XFEL pulses through increasing the electron beam energy chirp. In this paper, combining with the beam yaw correction to overcome the transverse slice misalignment caused by the coherent synchrotron radiation, finding out the overcompression working point of the linac is treated as a many-objective (having four or more objectives) optimization problem, thus the nondominated sorting genetic algorithm III (NSGA-III) is applied to the beam dynamic optimization for the first time. Start-to-end simulations demonstrate a full bandwidth of 4.79% for Shanghai soft x-ray free-electron laser user facility. Furthermore, the NSGA-III is found to be very efficient by providing a set of reference points with preferences. This paves the way for applying this algorithm to online optimization of a real machine