## 2 Overview of PAL-XFEL
### Beamlines  
PAL-XFEL has successfully supported more than 200 user experiments as of 2021, since user operation began in June 2017. It has been continuously improved to provide extended experimental techniques and sample environments, and more than 10 endstations are currently operating for hard and soft X-ray experiments. The highest level of timing stability, 20 fs (root mean square, RMS) jitter between XFEL and the optical laser [3], ensures an experimental time resolution of 80 fs (FWHM) without jitter correction in typical diffraction measurements using an 800 nm, 40 fs pump. The X-ray photon flux in the beamlines is \(\geq\)10\({}^{11}\) photons/pulse for hard X-rays and \(\geq\)10\({}^{12}\) photons/pulse for soft X-rays. The excellent beam quality of the PAL-XFEL has been unveiled with respect to many ultrafast phenomena such as structural dynamics [9], chemical reaction dynamics [10; 11; 12], phase transition [13; 14; 15; 16], spin, charge, and magnetic dynamics [9; 17; 18; 19]. Figure 1 shows a schematic view of the PAL-XFEL beamlines, and the experimental techniques provided by each beamline/hutch and endstation are listed in Table 3. Two experimental hutches in the hard X-ray hall and two endstations in the soft X-ray hall are placed in tandem and share each FEL line. Accordingly, one user experiment can be conducted at once. Usually, the beamtime is supported on a shift basis, 12 h a day, and the beamtime can be exchanged day and night between HX and SX, or it can be supported continuously for 24 h for some experiments. Currently, a 60 Hz kicker at the branching point for the SX is under preparation. Therefore, the simultaneous operation of two FEL lines will be available shortly. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Experimental Hall** & **Beamline/Hutch** & **Endstation** & **Experimental Technique** \\ \hline \multirow{6}{*}{Hard X-ray} & \multirow{3}{*}{XSS} & FXS & X-ray scattering/diffraction \\ \cline{3-3}  & & & Solution scattering \\ \cline{3-3}  & & & X-ray absorption \\ \cline{2-3}  & & & Coherent X-ray imaging \\ \cline{3-3}  & & & Wide-angle scattering \\ \cline{3-3}  & & & X-ray absorption \\ \cline{3-3}  & & SFX & Crystallography \\ \hline \multirow{6}{*}{Soft X-ray} & \multirow{3}{*}{SSS} & RSXS & Resonant scattering \\ \cline{3-3}  & & XES/XAS & X-ray emission/absorption \\ \cline{1-1} \cline{3-3}  & & & Fourier-transform holography \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Classification of PAL-XFEL beamlines and endstations. The X-ray scattering and spectroscopy (XSS) beamline [20] aims to provide all fundamental time-resolved X-ray experiments, including scattering, diffraction, and spectroscopies. Currently, we support X-ray diffraction experiments on bulk or thin-film samples, as well as wide-angle X-ray scattering (WAXS) and X-ray absorption spectroscopy (XAS) on liquid-phase samples. New spectrometers for X-ray emission spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements will be introduced in 2022. In the XSS, the X-ray can be focused by a stack of Be compound refractive lenses (CRL), and the typical focus size at the sample position is about 10 \(\upmu\)m (FWHM) at 9.7 keV. Standard configurations utilize hexapod-based two-circle or four-circle diffractometers with an optical pump laser which has an incidence angle of about 10\({}^{\circ}\) in a horizontal or vertical direction. For the low-temperature experiments, cryostream (40-300 K) or displex (10-450 K) cryostats can be used together with diffractometers. Thanks to a platform on the hexapod and an auxiliary optical table combined with the four-circle diffractometer, we can flexibly modify the experimental setup around the sample position, or a user can integrate their own chamber. The nanocrystallography and coherent imaging (NCI) beamline provides small-angle X-ray scattering (SAXS) and WAXS measurements on thin-film, particle, and crystal samples [21; 22], in addition to XAS, mainly in the tender X-ray range from 2 keV to 5 keV.