Monochromatic K$\alpha $ imaging for beam monitoring of an XFEL and a high-power femtosecond laser

The spatial overlap of an X-ray Free Electron Laser (XFEL) and a high-power femtosecond laser must be ensured in order to study the plasma condition of a laser-irradiated region in time-resolved pump-probe experiments. In an experiment at the Matter in Extreme Conditions (MEC) end-station of the Linac Coherent Light Source, we applied monochromatic x-ray imaging for determining positions of the beam-target interaction by measuring XFEL- and laser-induced K$\alpha $ x rays with a spherical crystal imager (SCI). A thin titanium foil was irradiated by a MEC's 25-TW femtosecond laser, while a 7.0 keV XFEL beam was used to probe an isochorically heated plasma. Measured 4.51 keV Ti K$\alpha $ x rays produced by various sizes of the XFEL pulses penetrating through the foil were ranged from $\sim $80 $\mu $m in diameter down to 20x40 $\mu $m$^{\mathrm{2}}$. The laser-induced K$\alpha $ spots were measured to be between 40 and 80 $\mu $m FWHM in diameter. Successful beam overlapping was observed on \textasciitilde 58{\%} of all two-beam shots for 10 $\mu $m thick samples. Results reveal that imprecise target positioning is a major cause of large beam offsets. Details of the experiment and results including a correlation between SCI and x-ray Thomson Scattering signals will be discussed. This material is based upon work supported by the National Science Foundation under Grant No. 1707357.