Abstract
This paper proposes and demonstrates a simple method using the intensity correlation of X-ray fluorescence to evaluate the focused beam size of an X-ray free-electron laser (XFEL). This method was applied to the sub-micrometre focused XFEL beam at the SPring-8 Angstrom Compact Free Electron Laser, and the beam size evaluated using the proposed method was consistent with that measured using the knife-edge scan method. The proposed method is readily applicable to extremely small X-ray spots and can be applied for the precise diagnostics of sub-10 nm focused X-ray beams which have recently emerged.
Highlights
The recent advent of X-ray free-electron lasers (XFELs) (Emma et al, 2010, Ishikawa et al, 2012) has had a significant impact in various fields of X-ray science
The most advanced focusing optic system at SPring-8 Angstrom Compact Free Electron Laser (SACLA) is a sub-10 nm focusing system based on a multilayer KB mirror, which was developed by combining sophisticated mirror fabrication and wavefront sensing techniques (Matsuyama et al, 2018)
A simple and direct method was developed to evaluate the beam size of tightly focused XFEL beams using the X-ray fluorescence generated by irradiating XFEL pulses onto metal foils. This method was applied to the sub-micrometre focused XFEL beam at SACLA, and the results were compared with focus characterization by the knife-edge scan method
Summary
The recent advent of X-ray free-electron lasers (XFELs) (Emma et al, 2010, Ishikawa et al, 2012) has had a significant impact in various fields of X-ray science. In combination with other advantageous characteristics such as ultrashort pulse duration (Inubushi et al, 2012; Behrens et al, 2014) and nearly full transverse coherence (Gutt et al, 2012; Inoue et al, 2015), XFELs enable the realization of several groundbreaking experiments, such as damage-free structure determination of protein molecules (Chapman et al, 2011), generation of exotic states of matter via X-ray irradiation (Vinko et al, 2012) and the capturing of ultrafast transient states in chemical reactions (Kim et al, 2015) To enhance this unique capability of XFELs further, focusing optics have been intensively developed and applied to various types of experiments. This method was applied to the sub-micrometre focused XFEL beam at SACLA, and the results were compared with focus characterization by the knife-edge scan method
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