Recent Progress of the PAL-XFEL after User Service Start

The X-ray Free Electron Laser of the Pohang Accelerator Laboratory (PAL-XFEL) was open to the public in 2017 and started user service. Even after the user service had started, improvement of beam quality was continuously attempted, and several meaningful results were achieved. In this paper, we present recent progress in the PAL-XFEL, including the present status of its operation. The increase in the FEL pulse energy is explained, and the generation of a self-seeding FEL is described. In addition, the generation of a two-color FEL is mentioned.

Investigation of the damping wiggler effect and application on the PAL fourth-generation storage ring

An investigation of the damping wiggler effect to reduce the emittance in the Pohang Accelerator Laboratory (PAL), a fourth-generation storage ring (4GSR) that uses a multi-bend achromat, is presented. A 4GSR lattice which has reduced emittance and increased dynamic aperture to amplify the synergy with two existing light sources (PLS-II and PAL-XFEL) at PAL is described.

AP-XPS beamline, a platform for operando science at Pohang Accelerator Laboratory

Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. This beamline is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), soft X-ray absorption spectroscopy (soft-XAS) and scanning photoemission microscopy (SPEM) experiments. BL 8A has two branches, 8A1 SPEM and 8A2 AP-XPS, that share a plane undulator, the first mirror (M1) and the monochromator. The photon beam is switched between the two branches by changing the refocusing mirrors after the monochromator. The acceptance angle of M1 is kept glancing at 1.2°, and Pt is coated onto the mirrors to achieve high reflectance, which ensures a wide photon energy range (100–2000 eV) with high resolution at a photon flux of ∼1013 photons s−1. In this article, the main properties and performance of the beamline are reported, together with selected experiments performed on the new beamline and experimental system.

Hard X-ray self-seeding commissioning at PAL-XFEL

A wake monochromator based on a large-area diamond single crystal for hard X-ray self-seeding has been successfully installed and commissioned in the hard X-ray free-electron laser (FEL) at the Pohang Accelerator Laboratory with international collaboration. For this commissioning, the self-seeding was demonstrated with a low bunch charge (40 pC) and the nominal bunch charge (180 pC) of self-amplified spontaneous emission (SASE) operation. The FEL pulse lengths were estimated as 7 fs and 29.5 fs, respectively. In both cases, the average spectral brightness increased by more than three times compared with the SASE mode. The self-seeding experiment was demonstrated for the first time using a crystal with a thickness of 30 µm, and a narrow bandwidth of 0.22 eV (full width at half-maximum) was obtained at 8.3 keV, which confirmed the functionality of a crystal with such a small thickness. In the nominal bunch-charge self-seeding experiment, the histogram of the intensity integrated over a 1 eV bandwidth showed a well defined Gaussian profile, which is evidence of the saturated FEL and a minimal electron-energy jitter (∼1.2 × 10−4) effect. The corresponding low photon-energy jitter (∼2.4 × 10−4) of the SASE FEL pulse, which is two times lower than the Pierce parameter, enabled the seeding power to be maximized by maintaining the spectral overlap between SASE FEL gain and the monochromator.

Laser systems for time-resolved experiments at the Pohang Accelerator Laboratory X-ray Free-Electron Laser beamlines

Optical laser systems for ultrafast X-ray sciences have been established at the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) beamlines. Three Ti:sapphire regenerative amplifier systems are synchronized to the XFEL with femtosecond precision, and the low temporal jitter of the PAL-XFEL results in an experimental time resolution below 150 fs (full width at half-maximum). A fundamental wave and its harmonics are currently provided for all beamlines, and tunable sources from ultraviolet to near-infrared are available for one beamline. The position stability of the optical laser extracted from the intensity-based center of mass at the sample position is less than 3% (r.m.s.) of the spot size.

Carbonic anhydrase II microcrystals suitable for XFEL studies

Recent advances in X-ray free-electron laser (XFEL) sources have permitted the study of protein dynamics. Femtosecond X-ray pulses have allowed the visualization of intermediate states in enzyme catalysis. In this study, the growth of carbonic anhydrase II microcrystals (40–80 µm in length) suitable for the collection of XFEL diffraction data at the Pohang Accelerator Laboratory is demonstrated. The crystals diffracted to 1.7 Å resolution and were indexed in space group P21, with unit-cell parameters a = 42.2, b = 41.2, c = 72.0 Å, β = 104.2°. These preliminary results provide the necessary framework for time-resolved experiments to study carbonic anhydrase catalysis at XFEL beamlines.