# Single-Shot Coherent X-Ray Imaging Instrument at PAL-XFEL
## 2 Materials and Methods
### Overview of the Whole System
#### 2.1.1 Sample Chamber and Loadlock Chamber  
Figure 2 shows a schematic view of the sample chamber. This chamber is optimized for single-shot-based XFEL experiments and designed to operate in vacuum and He gas ambient environments. It includes a beam position monitor, two four-jaw slits, a right-angle mirror, and a sample stage. As shown in Figure 2a, the sample chamber has five open ports: an incoming X-ray entrance port, a pump laser entrance port, and others for observing the interaction region. The port for the pump laser allows pump laser pulses to pass with an incident angle of 15 degrees with respect to the X-ray. A clear aperture from this port to the X-ray interaction region is secured to deliver the pump laser. The sample chamber stays  
Figure 1: Single-shot coherent X-ray imaging instrument at NCI endstation, Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). X-ray free electron laser (XFEL) pulses are focused by Kirkpatrick–Baez mirrors (KB mirrors) with a ~5.96 m long focal length and delivered to the sample chamber. Fresh samples are supplied using a thin membrane to each X-ray pulse in the sample chamber. Diffracted signals from samples are collected by the multiport charge-coupled device (MPCCD). To avoid damage to the sensor, the beam stopper is located just upstream of the detector and blocks the direct X-ray beam. The detector parts marked by the blue box are mounted on the rail to easily change the distance between the sample and detector (the XFEL beam propagates following the z-axis in the figure). on the XY-motion stage for its alignment to the XFEL beam. The chamber alignment can be confirmed through a beam position monitor that is equipped with a Ce:YAG (yttrium aluminum garnet) crystal and is located at its most upstream part. Downstream of the beam position monitor, two four-jaw slits block parasitic scattering mostly from the KB mirrors. Each slit blade is made of 250-\(\upmu\)m-thick silicon with a 5-degree bevel cut and mounted on the linear stage to make a clean X-ray beam at the interaction region. Four motor stages are attached on the metal frame, which has a 5 mm by 5 mm clear square aperture for X-ray pass and a long hole on the right side for optical laser pass. The second slit is designed to place blades close to the interaction region using a cone (Figure 2b). A right-side blade has a 3-mm diameter hole for optical laser pass. The right-angle mirror with a 2-mm long hole is located between the second slit stage and the sample stage. The mirror is mounted on the XY linear motion stage to pass X-ray pulses and optical laser pulses. To watch a membrane through a long-distance microscope (UWZ100, Union Optical Co., Ltd., Tokyo, Japan), this mirror is used. The working distance of the microscope is 100 mm, with better than sub-10 \(\upmu\)m image resolution. For fixed-target sample delivery, the membranes must be watched for X-ray alignment. As the X-ray interaction plane can be identified with the focal plane of the microscope, sample collection, beam size measurement, and timing synchronization are conducted at the same plane using the microscope. The sample stage consists of four high-precision motor stages. These motion stages are stacked as shown in Figure 3. A 200-mm long-stroke motion stage is piled at the bottom. As a coarse component, the motion stage is used to align each component. A hexapod stage was established in the middle of the sample stage. The hexapod serves all components placing the X-ray interaction plane. Finally, two high-speed piezo stages are mounted on the hexapod. These are used to supply fresh samples to each X-ray pulse at the XFEL repetition rate of 60 Hz. The maximum speed of these stages is 200 mm/s. Figure 2: (**a**) The overall schematics of the sample chamber. (**b**) Internal components of the sample chamber. Major components are marked with numbers. (1) X-ray pulse entrance port. (2) Beam position monitor, Ce:YAG (yttrium aluminum garnet), with a linear stage.