Novel Operations Of High-repetition-rate X-ray Free-electron Lasers

Due to their short wavelength,high peak brightness,coherence,and short pulse duration,x-ray free-electron lasers(XFELs)play an essential role in many disciplines such as biology,chemistry,materials science,and condensed matter physics.In recent years,high-repetition-rate XFELs based on superconducting linac have been proposed and are rapidly becoming a frontier in the FEL field.High-repetition-rate XFELs will significantly broaden the FEL application ranges,but they also pose many challenges for the machine design and operation.Continuous-wave XFELs are challenging to realize electron-beam energy control by conventional methods such as changing the trigger frequency of the accelerating structures,limiting the radiation wavelength tuning range of each undulator line.In this dissertation,we propose and systematically design a beam energy control unit to achieve bunch-by-bunch beam energy control in a continuous-wave XFEL.Simulation results based on SHINE parameters show that the energy control unit can tune the beam energy from 1.5 to 8.7 Ge V.The large bandwidth XFEL is important for spectroscopy experiments and x-ray crystallography.The critical challenge for high-repetition-rate XFELs is how to obtain XFEL pulses with the largest possible bandwidth without changing the layout and components of an existing facility.In this dissertation,to obtain large-bandwidth XFEL pulses,we propose to use the many-objective evolutionary algorithm(NSGA-III)to optimize the working point of the overcompression mode.Due to the lack of a seed laser with high peak power and high repetition rate,it is difficult to operate the seeded XFELs at a high repetition rate.In this dissertation,we propose and experimentally demonstrate the self-amplification of coherent energy modulation.Based on the existing experimental conditions of the SXFEL,we realized FEL lasing at the 7th harmonic of the seed laser in a single-stage HGHG setup and the30 th harmonic in a two-stage cascaded HGHG setup with a laser-induced energy modulation as small as 1.8 times the slice energy spread.The results pave the way for the construction of a high-repetition-rate seeded XFEL.The continuous interaction between the laser and the relativistic electron beam in the undulator is the most fundamental principle behind XFELs.In this dissertation,we experimentally validate and measure the interaction between the laser and the electron beam in a single dipole magnet,revealing the most fundamental FEL process.In addition,based on the self-modulation mechanism,we experimentally demonstrate that the energy modulation obtained in the dipole magnet can be used for the 6th harmonic lasing in a single-stage HGHG setup.This experiment illustrates that dipole magnets can substitute for undulators to achieve more compact laser heaters or modulators for plasma accelerators.The results also open a new path for the design and operation of future coherent light sources.