On-line monitoring of the spatial properties of hard X-ray free-electron lasers based on a grating splitter

Abstract

X‐ray free‐electron lasers (XFELs) play an increasingly important role in addressing the new scientific challenges relating to their high brightness, high coherence and femtosecond time structure. As a result of pulse‐by‐pulse fluctuations, the pulses of an XFEL beam may demonstrate subtle differences in intensity, energy spectrum, coherence, wavefront, etc., and thus on‐line monitoring and diagnosis of a single pulse are required for many XFEL experiments. Here a new method is presented, based on a grating splitter and bending‐crystal analyser, for single‐pulse on‐line monitoring of the spatial characteristics including the intensity profile, coherence and wavefront, which was suggested and applied experimentally to the temporal diagnosis of an XFEL single pulse. This simulation testifies that the intensity distribution, coherence and wavefront of the first‐order diffracted beam of a grating preserve the properties of the incident beam, by using the coherent mode decomposition of the Gaussian–Schell model and Fourier optics. Indicatively, the first‐order diffraction of appropriate gratings can be used as an alternative for on‐line monitoring of the spatial properties of a single pulse without any characteristic deformation of the principal diffracted beam. However, an interesting simulation result suggests that the surface roughness of gratings will degrade the spatial characteristics in the case of a partially coherent incident beam. So, there exists a suitable roughness value for non‐destructive monitoring of the spatial properties of the downstream beam, which depends on the specific optical path. Here, experiments based on synchrotron radiation X‐rays are carried out in order to verify this method in principle. The experimental results are consistent with the theoretical calculations.

Real time high accuracy phase contrast imaging with parallel acquisition speckle tracking*

“…According to Ref. [19], the intensity distribution, coherence, and wavefront of the first-order diffracted beam of a grating preserves the properties of the incident beam. This needs to be achieved in the far-field corresponding to the grating to separate the diffracted light.…”

“…Samples will be put into the zeroth order beam. Due to the property of the grating splitter, [19] we can record the zeroth and first order beams as reference and sample image, respectively. In this way, errors caused by time-varying fluctuations will be recorded in both images.…”

“…So, a parallel acquisition speckle tracking (PAST) method is proposed in this paper. It combines near field speckle and property of grating splitter [19] to collect sample and reference images simultaneously. In this case, errors introduced by external fluctuations will be recorded in all images and can be eliminated naturally by our algorithm, leading to more robust and accurate results.…”

Single-pulse characterization of the focal spot size of X-ray free-electron lasers using coherent diffraction imaging

“…As the peak intensity of each XFEL pulse is very high, detectors can be damaged, and most spot-size measurement methods used in synchrotron facilities are no longer suitable. There are already a few methods to characterize the XFEL spot size, such as knife-edge scanning, damage detection on the substrate and Hartmann wavefront sensors (Koyama et al, 2013;Pikuz et al, 2015;Keitel et al, 2016;Hua et al, 2019). For example, the knife-edge scan method is widely used to measure the spot size at the focus position.…”

Synthetic single crystal diamonds for X-ray optics

“…They were made of single-crystal diamond, which is considered as an ideal material because it does not cause any undesired diffuse scattering, and at the same time withstands high heat-load induced by synchrotron radiation. Moreover, diamond beamsplitters would give much impact on high repetition rate X-ray free electron lasers (XFELs) facilities with providing beam sharing, in-situ beam monitoring and outcoupling in cavity-based XFEL schemes [7,8] EUV Diamond crystal plates with (111) surface orientation are of primary importance for diffracting X-ray optics owing to a large intrinsic energy bandwidth of the {111} Bragg reflection (∆E/E=5. 7 10 -5 ) and the resulting flux of the reflected Xrays compared with those of silicon [9,10].…”