The Physical Study Of X-ray Free Electron Laser Oscillator

X-ray free electron lasers(XFELs)are new generation of accelerated-based light source with ultra-high peak brightness,extremely short pulses,and continuously tunable output wavelengths,enabling unprecedented discoveries in structural biology,chemistry,materials science,and fundamental research in condensed matter.The major operation mode of XFEL is the self-amplified spontaneous emission(SASE)mode,whose radiation amplification starts from the noise of the electron beam,resulting in its poor longitudinal coherence and spectral stability.Therefore,how to generate stable,fully coherent X-ray pulses has become an essential research topic in the XFEL community in the last decade.As one of the popular candidates to solve this problem,the X-ray free-electron laser oscillator(XFELO)studied in this thesis can achieve a fully coherent X-ray radiation output with true laser-like properties.Due to the lack of high-repetition-rate XFEL and experimental conditions,the current study of XFELO is still dominated by numerical simulations.In order to make up for the deficiency of existing FEL simulation software in Bragg diffraction simulation,we have written a 3D diffraction calculation program,named BRIGHT,based on X-ray diffraction dynamics with certain approximation for XFEL pulses.It can work closely with other FEL related codes,such as GENESIS and OPC,and is an important tool for the subsequent study of XFELO.Another fundamental challenge in XFELO operation is that the Bragg reflecting crystal exposed to X-ray pulses is subject to thermal deformation,which shifts and distorts the Bragg reflectivity curve,thus affecting the stability of the optical cavity.In this thesis,a new method is developed to analyze the thermal loading of the crystal.The method implements the physical process of Bragg reflection in GEANT4 to obtain accurate information about the absorption of XFELO pulses by the crystal.The energy absorption information extracted from the GEANT4 simulations is then used to analyze the transient thermal behavior including single and multi-pulse inputs,using finite element analysis software.The results show that for a typical XFELO pulse,about 10 J of energy is deposited in an area with a radius of several tens of microns.Based on the simulation results of the finite element software,a simplified model is developed in this thesis to couple the crystal heat load in the XFELO.The results show that when a large amount of heat is accumulated in the crystal,the pulse energy drops significantly and has large oscillations due to the negative feedback of the temperature change on the pulse energy.The control of the radiative light field based on XFELO is also an important research topic in this thesis.First,a tapered crossed-planar undulator scheme is proposed to generate arbitrarily polarized X-ray pulses in XFELO.Based on the Shanghai High Repetition Rate X-ray FEL and Extreme Light Facility(SHINE)simulation results show that it can produce polarization-controllable,fully coherent hard X-ray pulses with99.9% polarization degree and a switching rate close to 20 k Hz.Secondly,this thesis proposes a new method that allows the XFELO to directly produce a high-intensity OAM beam.The method utilizes Bragg mirrors and longitudinal-transverse mode coupling to achieve mode selection in a typical XFELO configuration,thereby enabling naturally generation of fully coherent hard X-ray beams carrying OAM.Simulations show that a fully coherent hard X-ray OAM beam of 1 MHz can be generated without optical mode converters and with pulse energies up to 120 J.We believe that this simple approach can significantly advance the generation of X-ray OAM while stimulating the development of experimental methods.Finally,we analyze the feasibility of the XFELO mode based on SHINE and give the corresponding physical design.XFELO is expected to produce fully coherent x-rays at 6-15 ke V photon energies,we give the output performance at different energies,and then complete the start-to-end simulations.The basic results show that the output of XFELO is close to the Fourier transition limit,and the generated spectral brightness is about two orders of magnitude higher than that of the SASE mode.It is hoped that the theoretical study in this thesis can provide reference and guidance for the construction of XFELO devices in the future.