Study On Transient X-ray Diffraction Under Laser Irradiation

Transient x-ray diffraction is one novel diagnostic technique which is used todiagnose the dynamic response of micro-structure under shock loading, such as elasticdeformation, elastic-plastic deformation and phase transition, et al. It can providedirect information about microscopic mechanisms governing shock-induceddeformation and structural changes at atomistic scales because of the ultra-short x-rayplumbing pulse. So this method is very important for researchers to understand thedynamic response of materials under extreme temperature and strain rate, and is a newmeans to know the material on macro-scale, as well as the new window of condensedphysics research.Transient x-ray diffraction we cared aimed to realize diagnosis of micro dynamicresponse with single crystal by laser irradiation. The process is that the single crystalwas irradiated by one laser beam, then at different delay time, x-ray pulse which wasgenerated by high-intensity laser beams incident on a thin metal foil, examines thestructure changes by laser induced.Firstly, the theory of transient x-ray diffraction was described. There are twotheories in x-ray diffraction domain: kinematic theory and dynamical theory. Inkinematic theory, only the interference of dispersion in different lattice plane isconsidered, while is taken no account of re-dispersion with diffraction line. That is tosay extinction effect which x-ray transmits in crystal is taken no account. It canapproximately confirm the position of diffraction peak with Bragg diffraction or Lauediffraction. Whereas, in dynamical theory, the action of all wave in crystal isconsidered, as well as extinction effect, and refractive index is corrected. In the theoryof transient x-ray diffraction, photoelectric absorption and extinction effect are takenaccount, which is a full-wave theory.Secondly, the diffraction data processing was discussed. The analysis principle ofdiffraction signals for transient x-ray diffraction is that the diffraction signals ofdifferent lattice planes are cones of x rays diffracted from a specific lattice plane in thecrystal, as we think the x-ray source is a divergent point source. This cone isdetermined by the Bragg angle and lattice normal vector. The appearance of the line onthe film is the intersection of this cone with the film detector. There are two steps indata processing, one is that make sure the lattice plane (hkl), another is simulation calculation by Matlab procedure to realize typical crystal structure.Thirdly, the experiments with elastic deformation of single crystal lithium fluorideand phase transition of single crystal iron by laser irradiation were implemented inSHENGUANG Ⅱ. In the experiment of LiF: the helium-like x rays were producedfrom north four laser beams acted on a thin Cu foil as probing pulse, the ninth laserbeam produced planar wave to load the single crystal LiF. The image plate recordedthe shocked diffraction signal of the lattice plane (002) as well as the unshocked signal.The experimental results show that the lattice plane is compressed along the shockloading direction [001] by11%. In the experiment of iron: we used shock compressedand ramp pulse compressed to study phase transition of iron.In the end, a summary and analysis of shortage have been made, as well as thedevelopment.