Hard X-ray Self-seeded Free Electron Laser Related Physics Research

X-ray free electron lasers(FELs)can generate incomparable high brightness,ultra-fast,and excellent transverse coherent X-ray pulses,which have shown remark-able scientific capabilities in many fields such as biology,chemistry,and physics.Most of the existing and under construction short-wavelength FEL facilities,such as the Linac Coherent Light Source(LCLS),SPring-8 Angstrom Compact Free Electron Laser(SACLA),European X-ray Free-Electron Laser(European XFEL),Pohang Accelerator Laboratory X-ray Free Electron Laser(PAL-XFEL),Swiss X-ray Free Electron Laser(Swiss FEL),and Shanghai High-Repetition-Rate XFEL and Extreme Light Facility(SHINE),are based on self-amplified spontaneous emission(SASE).The SASE X-ray pulse is characterized by outstanding transverse coherent but par-tial temporal coherence due to arising from shot noise.The temporal coherence can be dramatically improved by self-seeding scheme in X-ray regime.Especially,the single-crystal hard X-ray self-seeding(HXRSS)scheme has been demonstrated at LCLS.The fully coherent X-ray FELs can enable the Coherent Diffraction Imag-ing(CDI)from single molecule and probe ultra-fast phenomena and fundamental physics under extreme conditions.In addition,these experiments call for terawatt(TW)output power,fully coherent,ultra-fast,and stable X-ray FEL pulses.Fur-thermore,as the increases of repetition rate,the heat-load may damage the crystal monochromator.Therefore,this thesis first focuses the studies on the approaches of seed gen-eration and seed spectrum properties which determine the output characteristics of HXRSS.Then the efficiency enhancement and stability of HXRSS FEL are in-vestigated.Moreover,for LCLS-II operating at a high repetition rate,an optimal one-stage HXRSS scheme is proposed.This scheme can cover the operation modes of superconducting radio frequency(SCRF)linac based 4 GeV and the high energy upgrade(LCLS-II-HE)8 GeV electron beam,and copper(CuRF)linac based elec-tron beam.(1)Background and theory:The history of FELs development,the current status of FELs,and the potential research directions of FELs are reviewed in Chapter I.In order to further discuss the relevant physics of HXRSS FEL,the basic theory of FEL and the dynamical theory of X-ray diffraction are reviewed in Chapter 2 and Chapter 3,respectively.(2)Basics of HXRSS FEL:The numerical simulations of seed generation,in-cluding 1-D,quasi-3D,and 3D approaches are investigated in Chapter 4.Meanwhile,the short bunch and long bunch operation modes of HXRSS are studied.Moreover,the spectral properties of the monochromatic seed are investigated.The basics pave the way of further investigation of HXRSS FEL.(3)High efficiency HXRSS:In Chapter 5,for the purpose of high efficiency HXRSS investigation,an useful approach is proposed to enhance the seed energy by tuning the SASE incident angle off the Bragg condition which the Bragg photon energy is corresponding to the central photon energy of SASE spectrum.The in-vestigations conclude that,for a given diffraction geometry,the seed energy always can be maximized by finding an optimal crystal detuning.Furthermore,the crystal detuning effect in X-ray FEL oscillator(XFELO)is also investigated.(4)Stability investigation of HXRSS:The experimental results at LCLS show that the fluctuation of HXRSS FEL is large which limits its applications.The stability of HXRSS are investigated in Chapter 6.The reasons leading to the fluc-tuation of HXRSS are very complex,including the seed energy fluctuation related to the bandwidth of crystal monochromator,the intrinsic fluctuation of SASE,the energy jitter of electron bunch,the jitter of the angle between the SASE propagation direction and the crystal surface,the relative time jitter between the electron bunch and seed,the transverse overlap between the electron bunch and seed,and the crys-tal detuning effect.In practice,most of the reasons will essentially contribute to the seed energy fluctuation,and eventually result in large fluctuation of HXRSS FEL.(5)HXRSS scheme of LCLS-II:For high repetition rate machine like LCLS-II,the heat-load issue on the X-ray optics is a challenge to the design,such as the configuration of HXRSS scheme.For the damage consideration,LCLS-? initially has a two-stage self-seeding scheme.However,the work in this thesis finds that the two-stage scheme has no advantage over one-stage self-seeding scheme.For the two-stage scheme,the seed energy after the first crystal monochromator is too small to overcome the shot noise power,and the output pulse energy is smaller than that of one stage scheme.An optimal one-stage HXRSS scheme is proposed,and can cover all the operation modes,including SCRF-Linac based electron beam(4 GeV and 8 GeV)and CuRF-Linac based electron beam.