The diffraction of x-rays in quasi-perfect thin crystals of elements with high Z can generate multiple diffracted beams at the exiting surface of a crystal. These beams propagate in the free space parallel to each other and share the same divergence and monochromatic properties. These beams have a spatial separation that varies between few nm and couples of μm. Due to the different path that the photons follow, these x-ray beams present a temporal delay between each other in the order of the fs. It is for these that the x-ray beams generated by this ultrafast diffraction process are so-called x-ray echoes. The x-ray echoes can only be described using the dynamical diffraction theory formalism. Here, we present simulations with the expected diffracted wave-fronts both in the diffracted and forward direction in several compounds, such as Ni, GaAs, InSb and in particular Au. This work presents also the dependence with energy and thickness of the x-ray echoes. The properties of these x-ray beams can produce ambiguous results while performing temporal studies, that became obvious with fs and sub-fs pulse facilities. Moreover, the spatial overlapping of the echoes can mislead the scientist to think that speckles from a small crystal could be generated from different centers of diffraction, while in reality are a dynamical diffraction effect.
The European X-Ray Free Electron Laser (EuXFEL) is a unique facility that provides femtosecond x-ray pulses of high pulse energy at MHz repetition rate. However, the high peak power results in a high dynamical heat load in the optical components, like monochromators, which reduces the intensity of the transmitted pulses significantly as compared to the full capacity of the EuXFEL source. To address these challenges at the high photon energy instruments of EuXFEL, we propose a diamond channel cut monochromator as an alternative to the standard Si monochromators. Diamond has a lower absorption cross-section at high photon energies and a higher thermal conductivity compared to Si, making diamond a promising candidate for x-ray optics applications under high heat load conditions. Here, we present a finite element model (FEM) of the temperature increase in diamond and the resulting thermal expansion to estimate the changes in the diffraction profile and the expected monochromator transmission depending on the number of pulses.
Single crystal based optics is widely used at X-ray Free-Electron Laser (XFEL) facilities for beam tailoring. Here, we present recent developments in simulations of coherent X-ray wavefront propagation through crystals. In particular, the Laue case of dynamical diffraction has been implemented in Synchrotron Radiation Work- shop (SRW). Various effects due to wavefront transformation of short XFEL pulses have been simulated in the WavePropaGator (WPG) framework. Possible experimental observations of the effects are discussed.
The emergence of new high brilliance and high coherence facilities such as X-ray Free Electron Lasers (XFELs) and 4th generation synchrotrons open a new era in X-ray optics. Dynamical diffraction effects before disregarded are starting to play a role in the beam control of large scale facilities. In the case of XFEL facilities the temporal characteristics of the dynamical diffraction by thin perfect crystals can be used as a tool to generate femtosecond monochromatic pulses, in the case of self-seeding in the hard X-ray regime, but could even be used as method to characterize materials in this temporal range. In this contribution we present the first steps in the understanding of the spatial-displacement dependence of forward beams diffracted by thin crystals. The data collected by this technique is compared with crystal models based in dynamical diffraction theory. This type of study could open a new field to understand low strain materials in the femtosecond regime.
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