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

The characterization of X‐ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X‐ray free‐electron lasers (XFELs) are the latest generation of X‐ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X‐ray Free Electron Laser Facility was characterized using the method. The full width at half‐maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method. A new spot characterization method is proposed, based on coherent diffraction imaging that can accurately determine the focal spot size of a single X‐ray free‐electron laser pulse. This method was successfully applied to characterize the focal spot size at the Coherent Scattering and Imaging endstation of the Shanghai Soft X‐ray Free Electron Laser Facility.