Research On Probing The Transverse Coherence Of The Self-amplified Spontaneous Emission Of A Free-electron Laser Using Near-field Heterodyne Speckle

X-ray Free Electron Lasers(XFEL)are capable of delivering x-ray pulse radiation with femtosecond time structures,unprecedented brilliance and,importantly,a high degree of coherence.By virtue of these superior properties,XFEL has been applying to a wide range of frontier research,including materials and environmental science,as well as structural biology and so on.Coincident with the wide applications of XFEL has been a lot of interest in characterization of coherence.At present,there are a variety of techniques for measuring the transverse coherence properties of XFEL,and these techniques can be divided into two classes: the first one being amplitude interferometry,for example,Young's double-pinhole experiment?grating interferometer.Such kind of measurements give access to the immediate availability of coherence,while,to the aim,this also requires necessity of varying experimental parameters,such as separation of two pinholes,distance of detection.The second is the intensity of correlation measurement,as demonstrated by Hanbury Brown and Twiss intensity interferometry.The setup of this method is very simple and it only needs recording the intensity distribution of incident radiation.But it is unavailable for transverse coherence of single pulse.Meanwhile,intensity distribution is sensitive to optical component surface roughness and its installation accuracy.To overcome these limitations,here we focus on the theoretical studies on the retrieval of transverse coherence of single pulse radiation from XFEL using near field heterodyne speckle method.At the same time,a feasible scheme,which we believe is suitable for XFEL case,has been firstly proposed to accurately extract the heterodyne signal and we obtained numerical evidence of the correction of this scheme.In this dissertation,referring to the design parameters of the Shanghai soft x-ray FEL user device,we perform the SASE-XFEL simulation and analyze the feature of the radiation.After that,the principle of recovering transverse coherence of XFEL using heterodyne speckles is firstly presented.A crucial step in this analysis is the subtraction of the background from the raw data,remaining the pure heterodyne signal.While,the existing method for this end is not straightforward with SASE-XFEL case.To circumvent this problem,here we propose a new novel scheme named “Twin measurement”,using a beam splitter to acquire the accurate heterodyne signal.It is found that this scheme is more robust and reasonable than original thought.Based on the available SASE-XFEL radiation data,the feasibility of this scheme has been proved by performing the numerical simulation of recovering heterodyne signal from single-particle or multi-particles.Looking forward,it is expected that this measurement scheme will be applied practically to Shanghai soft X-ray FEL and international XFELs equipment.