The Theoretical Study Of X-ray Free-electron Laser Oscillator

Due to the wide range of applications of X-rays in biology,physics,chemistry and medical research,the last century witnessed great breakthroughs of X-ray light sources.The generation of hard X-ray free-electron laser(XFEL)at LCLS in 2009 is a mark for the beginning of X-ray laser era.Motivated by the success of LCLS,there are a great number of XFEL facilities are now completed or under construction around the world.Almost all of them employs self-amplified spontaneous emission(SASE)as the lasing scheme,which starts from electron beam initial density shot noise.Although SASE is capable of producing X-ray with highly transverse coherence,it has poorly longitudinal coherence and power stability.Thus solutions of improving performance of SASE have attracted great attention ever since the first light of LCLS.In this thesis,we explored one of the most potential scheme of generating fully coherent,high brightness,stable hard X-ray by X-ray free-electron laser(XFELO).On the one hand,the principles of XFELO has not been demonstrated exponentially,since the realization of relatively high level of requirements is difficult.On the other hand,XFELO involves X-ray Bragg diffraction,X-ray propagation and nonlinear FEL physics at saturation regime,which are hard to analysis theoretically.Thus the XFELO is normally investigated by numerical simulation.This thesis presents a thoroughly investigation of XFELO both in theory and simulation.For the theoretical study,we have established a simplified one-dimensional model for XFELO fast simulation.By introducing the electron phase space distribution function in high gain FELs to the low gain XFELO,we are able to calculate the FEL single pass gain by solving partial differential equation.Comparing to the traditional method of tracking macro-particles,the new method is capable of reducing simulation time from several weeks to a few minutes.For the numerical simulation,we have introduced the first three-dimensional X-ray Bragg diffraction code-Bright by making some approximations in X-ray diffraction dynamic theories.Bright is able to cooperate closely with Genesis and OPC,which enables us to conduct fully three-dimensional XFELO simulations.The results is crucial for predicting the cavity stability conditions of XFELO.Employing this simulation method,we conducted a systematical design of XFELO using the parameters of Shanghai coherent light source(SCLF).The study provides optimum XFELO parameters as well as predicts the intensity of output X-ray pulse.It is might be valuable for the design and construction of XFELO facilities around the world.In order to further improve the output performance of XFELO,we explored the possibility of introducing some concepts in high gain FELs to low gain XFELO.Since XFELO works at FEL low gain regime,it offers small single pass gain.For further enhancement of its energy extraction efficiency and output peak power,we made a proposal of gain cascaded XFELO and XFELO seeded FEL.The gain cascaded XFELO scheme uses delay elements between stages of undulators to let electron beam lasing at various parts inside different undulators,and achieves cascaded FEL gain.It is able to significantly improve the energy extraction efficiency as well as the output peak power.By matching chirped electron beam energy with the XFELO bandwidth,the bunch tail is able to work inside XFELO,while the bunch head lasing inside the following FEL amplifier.The XFELO seed FEL is expected to generate TW level output peak power.