## 3 Evaluations at SPring-8 Angstrom Compact Free-Electron Laser (SACLA)  
### Evaluation of Focusing Optics  
The designed mirrors were fabricated using an ultra-precise computer-controlled process [18] with a surface figure precision (figure error) of 0.30 to 0.52 nm root mean square (RMS) and surface roughness of 0.11 to 0.14 nm RMS. The properties of the focused beam were evaluated at EH5, BL3 of SACLA. The focusing mirrors were aligned using a Foucault knife-edge test [19], with a 200 \(\upmu\)m diameter gold wire. For focusing a beam size of 100 nm (FWHM), the alignment tolerances of the horizontal mirror (which are more severe than those of the vertical mirror) were estimated to be \(\pm\)1.6  
Figure 2: Schematic drawing of the mirror and the sample chamber. The wall on the near side of the sample chamber is presented as transparent to show the inside of the chamber. A beryllium window separates the mirror and sample chambers. An optical microscope is used to observe the sample at the focus from outside the vacuum chamber. Various sample environments, stage arrangements and sample supply systems can be operated utilizing a 100-mm space upstream of the focus. Both the downstream side and the upper side and one lateral side of the sample chamber can be altered to have a large exit port allowing the data collection for forward scattering geometry, X-ray diffraction and X-ray emission spectroscopy. Furthermore, the sample chamber can be altered based on experimental requests while keeping the mirror chamber unchanged.  
\(\upmu\)rad in the incident angles, \(\pm 10\)\(\upmu\)rad in perpendicularity between the mirrors and \(\pm 2\) mm in the focal length at a relatively high photon energy of 12 keV.  
Intensity distributions of the focus were measured via the knife-edge scanning method using the wire. The typical focused beam profiles obtained at a photon energy of 10 keV are presented in Figure 3. To prevent ablation of the wire by the intense focused beam, the incident pulse energy for the mirrors was adequately attenuated with silicon attenuation plates. We applied single-crystal silicon attenuators with a total thickness of 1.3 mm and mirror polishing surfaces to avoid influences on focuses beam sizes. The shot-to-shot fluctuations of the incident pulse energy to the mirrors were measured with an intensity monitor [20] and were normalized to eliminate their influence on the profile measurements. The typical beam sizes observed, \(210\) nm \(\times\) 120 nm (FWHM), in the vertical and horizontal directions, show most of the reflected pulse energy concentrated at the main peak and low-intensity distributions around the main peak. The effect of vibration from the vacuum pump on the focused beam size is negligibly small. We estimated that 50% beam power width are 135 nm and 85 nm in the vertical and horizontal directions, respectively. The width of the beam is defined as the distance between the points of the knife-edge scan profiles that are 25% and 75% of the total beam power. The focused beam had a pulse energy of 150 \(\upmu\)J, which is 25% of the pulse energy generated at the XFEL light source. From these measured values, the fluence and intensity of the focused pulses reached \(8\times 10^{5}\) J/cm\({}^{2}\) and \(1\times 10^{20}\) W/cm\({}^{2}\), assuming a pulse duration of 7 fs [21; 22].  
Figure 3: Typical intensity distributions of focused beam at a photon energy of 10 keV measured using the knife-edge scanning method. The focused beam size of 210 nm \(\times\) 120 nm (full width half maximum (FWHM)) was evaluated in the (**a**) vertical direction and (**b**) horizontal direction. 30 shot intensities were averaged for each data point at an X-ray free-electron laser (XFEL) repetition rate of 30 Hz.