Abstract
We present some preliminary results from a study aimed at the characterization of the wavefront of X-ray free electron laser (XFEL) beams in the same operation conditions as for single particle imaging (or flash X-ray imaging) experiments. The varying illumination produced by wavefront fluctuations between several pulses leads to a partially coherent average beam which can be decomposed into several coherent modes using ptychographic reconstruction algorithms. Such a decomposition can give insight into pulse-to-pulse variations of the wavefront. We discuss data collected at the Linac Coherent Light Source (LCLS) and FERMI.
Highlights
Many X-ray experiments require precise knowledge of the beam
Wavefront sensing is instrumental in producing an accurate initial estimate of the complex-valued illumination function, useful to some phase retrieval algorithms for imaging experiments as it is the case of flash X-ray imaging [1] which has been the target application for this study
Several diffraction patterns have been collected at every scanning position in order to gather enough statistical significance and to make possible the analysis of the average beam before moving on to the single shot analysis
Summary
One example is the characterization of the focusing optics in operation conditions aimed at selecting a plane where optics-induced aberrations are minimized Such aberrations may have a strongly detrimental effect on any experiment performed at high power density and with a small focal spot, which is often the case at X-ray free-electron lasers (XFELs). Many approaches are available for beam diagnostics at XFELs such as Shack-Hartmann sensors, ablative imprints [2], Young’s double-slit experiments [3], grating interferometry [4], aerosol spheres diffraction [5], coherent scattering speckles analysis [6] and ptychography [7] This latter technique has been chosen to carry out this study as it has been already successfully applied to recover the full wavefront at high-brilliance X-ray sources [8, 9].
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