## 4 Diagnostics  
X-ray diffraction (XRD) measurement is widely used in high-pressure sciences to observe structural changes of a compressed matter. In our experimental platform, instruments for XRD measurement were developed as the main diagnostic. In order to avoid a decrease in signal intensity caused by a polarization factor of the horizontally polarized XFEL pulses, instruments are arranged in the vertical  
Figure 2: Schematic of the experimental chamber. In the chamber, high-power nanosecond laser, which is focused by a lens with a focal length of 500 mm, irradiates a sample from the bottom side. The laser focusing lens, sample system and a flat panel detector are placed in the chamber. A dual multiport charged coupled device (Dual-MPCCD) detector locates outside the chamber.  
dispersion plane. There are two options in geometries of XRD measurements: a reflection geometry and a transmission geometry. Figure 3a shows the arrangement of the instruments in the reflection geometry. The XRD signal are detected with a flat panel detector (FPD), which is a two-dimensional X-ray image detector. The specification of the FPD is summarized in Table 1. The typical distance from a sample to the FPD detector is 130 mm. In this case, the detectable diffraction angle is from 20 to 80 degrees, so that the range of observation angle is 60 degrees. The angular resolution of a single pixel at the center of the FPD is geometrically estimated to be \(\sim\)0.04 degrees. The transmission geometry is shown in Figure 3b. In this geometry, the FPD detects diffracted X-rays after passing through a sample. The typical distance from a sample to the FPD is 130 mm, which is the same as the reflection geometry. Thus, the observation range and the angular resolution are also identical. As an optional X-ray detector, a dual multiport charged coupled device (Dual-MPCCD) detector [23]--of which specification is also shown in Table 1--is available. The Dual-MPCCD detector is mounted outside the chamber and can observe the diffracted X-rays through a Kapton window. An angular resolution is much higher than that of FPD due to the small pixels and a long distance from a sample, although the observation range is limited. As complementary techniques, small-angle X-ray scattering measurement (SAXS) and X-ray imaging with absorption or phase contrasts are applicable in this platform.  
Figure 3: Experimental configurations of X-ray diffraction (XRD) measurements for **(a)** reflection geometry and **(b)** transmission geometry in the experimental chamber. To avoid a decrease of the XRD signal caused by a polarization factor, the dispersion plane of XRD measurement is vertical. Optics for VISAR can keep the normal direction of a sample surface, even when an angle of a sample is changed.  
Instruments for optical measurements were also prepared. A typical optical diagnostic is a Velocity Interferometer System for Any Reflector (VISAR) [24; 25] to diagnoses a velocity of a shock wave from a fringe pattern using a probe laser (VISAR laser) reflected by a sample. The final optics of the VISAR system should be mounted on a direction normal to the sample surface for keeping the same axis to both the incident and the reflected paths of the VISAR laser. In the developed chamber, the sample stages and the VISAR final optics are both mounted on the motorized stages on a circle. Therefore, the angle between the sample surface and the VISAR optics can be maintained by adjusting the positions of the stages independently on the circle even when the sample is rotated in \(\theta\)x. Here, it should be noted that all optics for the VISAR in the chamber are also mounted on the circle. Particularly, a mirror placed at the center of the circle to change the beam direction to/from the sample but maintains the direction to/from the window on the chamber wall. This configuration allows keeping the optical path of the VISAR laser independently on the final optics position. A dove prism is used to compensate for the image rotation on the cameras due to the angle adjustment.