Pohang Accelerator Laboratory X-ray Free Electron Laser Research Articles

Development of an X-ray ionization beam position monitor for PAL-XFEL soft X-rays.

The Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) operates hard X-ray and soft X-ray beamlines for conducting scientific experiments providing intense ultrashort X-ray pulses based on the self-amplified spontaneous emission (SASE) process. The X-ray free-electron laser is characterized by strong pulse-to-pulse fluctuations resulting from the SASE process. Therefore, online photon diagnostics are very important for rigorous measurements. The concept of photo-absorption and emission using solid materials is seldom considered in soft X-ray beamline diagnostics. Instead, gas monitoring detectors, which utilize the photo-ionization of noble gas, are employed for monitoring the beam intensity. To track the beam position at the soft X-ray beamline in addition to those intensity monitors, an X-ray ionization beam position monitor (XIBPM) has been developed and characterized at the soft X-ray beamline of PAL-XFEL. The XIBPM utilizes ionization of either the residual gas in an ultra-high-vacuum environment or injected krypton gas, along with a microchannel plate with phosphor. The XIBPM was tested separately for monitoring horizontal and vertical beam positions, confirming the feasibility of tracking relative changes in beam position both on average and down to single-shot measurements. This paper presents the basic structure and test results of the newly developed non-invasive XIBPM.

Construction and Operation Plan for the Second Hard X-ray Beamline at PAL-XFEL

The second hard X-ray beamline (HX2) at PAL-XFEL (Pohang Accelerator Laboratory X-ray Free Electron Laser) is currently in progress for construction to meet the increasing demands of researchers in various fields. With higher electron beam energy and a larger undulator parameter K, HX2 can deliver over twice the FEL intensity of the existing hard X-ray beamline (HX1) in the photon energy range of 2 to 7 keV. Consequently, HX1 will operate within the electron beam energy range of 7 to 10.5 GeV, providing FELs from 6.5 to 20 keV when both HX1 and HX2 are operated simultaneously. Most of the components have already been designed and prepared, enabling us to start construction as scheduled. Following this construction, we have also the upgrade plan for advanced operation of new extreme X-ray sources such as attosecond X-ray pulses and TW (1012 W, terawatt) scale X-ray pulses. In this report, we will introduce about construction and future upgrade plan of HX2 at PAL-XFEL.

X-ray beam test of fabricated photo-diodes for Pohang-Accelerator-Laboratory X-ray free-electron laser

The Pohang-Accelerator-Laboratory X-ray free-electron laser (PAL-XFEL) emits intense, ultra-short X-ray pulses, enabling studies on ultra-small and -fast dynamics in various fields. We designed and fabricated silicon p-intrinsic-n photo-diodes (PDs) to detect strong X-ray pulses. Four types of PDs were fabricated with different metal shapes on the P- and N-sides. The fabrication process was split into two parts, based on the thickness of the Al metal and anti-reflective coating (ARC) layers, to evaluate the fabrication process feasibility and the detection performance. A beam test was performed with X-ray pulses at the soft X-ray beamline of the PAL-XFEL using the proposed PDs and a commercial PD. The test was designed to evaluate the effects of the metal and ARC layers and compare the detection efficiency depending on the X-ray entrance side and signal readout side. The waveforms were saved during the test and integrated to obtain the charges. Thereafter, they were analyzed and normalized, and compared. This paper presents the preparation, setup, and procedure of the beam test, and the test results are described.

Observing ice structure of micron-sized vapor-deposited ice with an x-ray free-electron laser.

The direct observation of the structure of micrometer-sized vapor-deposited ice is performed at Pohang Accelerator Laboratory x-ray free electron laser (PAL-XFEL). The formation of micrometer-sized ice crystals and their structure is important in various fields, including atmospheric science, cryobiology, and astrophysics, but understanding the structure of micrometer-sized ice crystals remains challenging due to the lack of direct observation. Using intense x-ray diffraction from PAL-XFEL, we could observe the structure of micrometer-sized vapor-deposited ice below 150 K with a thickness of 2-50 μm grown in an ultrahigh vacuum chamber. The structure of the ice grown comprises cubic and hexagonal sequences that are randomly arranged to produce a stacking-disordered ice. We observed that ice with a high cubicity of more than 80% was transformed to partially oriented hexagonal ice when the thickness of the ice deposition grew beyond 5 μm. This suggests that precise temperature control and clean deposition conditions allow μm-thick ice films with high cubicity to be grown on hydrophilic Si3N4 membranes. The low influence of impurities could enable in situ diffraction experiments of ice nucleation and growth from interfacial layers to bulk ice.

Sample Delivery Systems for Serial Femtosecond Crystallography at the PAL-XFEL

Serial femtosecond crystallography (SFX) using an X-ray free electron laser (XFEL) enables the determination of room-temperature structures without causing radiation damage. Using an optical pump-probe or mix-and-injection, SFX enables the intermediate state visualization of a molecular reaction. In SFX experiments, serial and stable microcrystal delivery to the X-ray interaction point is vital for reasonable data collection and efficient beam time. The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) facility established SFX instruments at a nanocrystallography and coherent imaging (NCI) experimental station. Various sample delivery methods, including injection, fixed-target scanning, and hybrid methods, have been developed and applied to collect XFEL diffraction data. Herein, we report the currently available sample delivery methods for SFX at the NCI experimental station at the PAL-XFEL. This article will help PAL-XFEL users access the SFX system for their experiments.

Development of photo-diodes for Pohang-Accelerator-Laboratory X-ray free-electron laser

The Pohang-Accelerator-Laboratory X-ray free-electron laser (PAL-XFEL) provides intense ultra-short X-ray pulses via a self-amplified spontaneous emission (SASE) process. Since fluctuations are induced by the intrinsic characteristics of the SASE process, we designed and fabricated photo-diodes (PDs) to be used in quadrant beam position and pop-in monitoring systems for measuring intensity and position, and beam size, respectively. Six 500μm-thick high-resistivity silicon wafers were used for different fabrication processes to vary the thicknesses of bias electrode and anti-reflective coating layers optimized for the quantum efficiency (QE) at typical wavelengths of 266 and 400 nm used for pump–probe experiments at the PAL-XFEL. In this paper, we present the electrical characteristics, QE, signal-to-noise ratio, energy resolution of the fabricated PDs, and compared them with those of commercial PD.

Structural basis for receptor selectivity and inverse agonism in S1P5 receptors

The bioactive lysophospholipid sphingosine-1-phosphate (S1P) acts via five different subtypes of S1P receptors (S1PRs) - S1P1-5. S1P5 is predominantly expressed in nervous and immune systems, regulating the egress of natural killer cells from lymph nodes and playing a role in immune and neurodegenerative disorders, as well as carcinogenesis. Several S1PR therapeutic drugs have been developed to treat these diseases; however, they lack receptor subtype selectivity, which leads to side effects. In this article, we describe a 2.2 Å resolution room temperature crystal structure of the human S1P5 receptor in complex with a selective inverse agonist determined by serial femtosecond crystallography (SFX) at the Pohang Accelerator Laboratory X-Ray Free Electron Laser (PAL-XFEL) and analyze its structure-activity relationship data. The structure demonstrates a unique ligand-binding mode, involving an allosteric sub-pocket, which clarifies the receptor subtype selectivity and provides a template for structure-based drug design. Together with previously published S1PR structures in complex with antagonists and agonists, our structure with S1P5-inverse agonist sheds light on the activation mechanism and reveals structural determinants of the inverse agonism in the S1PR family.

Preliminary XFEL data from spontaneously grown endo-1,4-β-xylanase crystals from Hypocrea virens.

The enzymatic degradation of semi-cellulosic substrates has recently received immense attention. The enzyme endo-1,4-β-xylanase is essential for the complete digestion of complex and heterogeneous hemicellulose. Here, the purification, crystallization and preliminary X-ray free-electron laser (XFEL) diffraction analysis of endo-1,4-β-xylanase from the fungus Hypocrea virens (HviGH11) are reported. Codon-optimized HviGH11 was overexpressed in Escherichia coli and spontaneously crystallized after His-tag purification and concentration. Preliminary XFEL diffraction data were collected at the Pohang Accelerator Laboratory XFEL (PAL-XFEL). A total of 1021 images containing Bragg peaks were obtained and indexed. The HviGH11 crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 43.80, b = 51.90, c = 94.90 Å. Using 956 diffraction patterns, the phasing problem was solved and an initial model structure of HviGH11 was obtained.

Single-Shot Coherent X-ray Imaging Instrument at PAL-XFEL

We developed a single-shot coherent X-ray imaging instrument at the hard X-ray beamline of the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This experimental platform was established to conduct a variety of XFEL experiments, including coherent diffraction imaging (CDI), X-ray photon correlation spectroscopy (XPCS), and coherent X-ray scattering (CXS). Based on the forward-scattering geometry, this instrument utilizes a fixed-target method for sample delivery. It is well optimized for single-shot-based experiments in which one expects to observe the ultrafast phenomena of nanoparticles at picosecond temporal and nanometer spatial resolutions. In this paper, we introduce a single-shot coherent X-ray imaging instrument and report pump–probe coherent diffraction imaging (PPCDI) of Ag nanoparticles as an example of its applications.

Numerical study of the second harmonic generation in FELs

A numerical study of the effect of betatron oscillations on the second harmonic generation in free-electron lasers (FELs) is presented. Analytical expressions for the effective coupling strength factors are derived that clearly distinguish all contributions in subharmonics and each polarization of the radiation. A three-dimensional time-dependent numerical FEL code that takes into account the main FEL effects and the individual contribution of each electron to the second harmonic generation is presented. Also, the X- and Y-polarizations of the second harmonic are analyzed. The second harmonic was detected in experiments at the Advanced Photon Source (APS) Low Energy Undulator Test Line (LEUTL) and Linac Coherent Light Source (LCLS) in the soft X-ray regime. The approach presented in the article can be useful for a comprehensive study and diagnostics of XFELs. In the paper, the LCLS and Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) experiments are modeled. The simulation results are in a good agreement with the experimental data.

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

Intensity optimization of x-ray free-electron laser by using phase shifters

This paper presents a method that uses a gap scan of the phase shifter to optimize the intensity of a free-electron laser (FEL) by matching its phase with that of the electron beam. Phase shifters are essential instruments, especially for a long undulator line, which is segmented by drift sections. The phase-matched (in-phase) and the 180\ifmmode^\circ\else\textdegree\fi{} offset (out-of-phase) conditions are investigated using linear theory, FEL simulations, and experiments to understand how the phase shifter affects FEL amplification. We show that the FEL intensity is dominantly reduced by phase mismatch in the saturation region, where the microbunched electron beam is sufficiently developed, and that the difference of FEL intensity between the in-phase and out-of-phase conditions is an effect of evolution of the bunching factor. At the Pohang Accelerator Laboratory x-ray Free-Electron Laser (PAL-XFEL), the gap scan of the phase shifter at 9.7 keV increased FEL intensity by 4 times compared to the calculated gap of the phase shifter. This intensity increase was obtained dominantly in the saturation region.

FEL Simulation of New Hard X-ray Undulator Line at PAL-XFEL

The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) has been successfully operating as a remarkably-stable XFEL facility in the world. The hard X-ray beamline, however, has only one undulator line (HX1) with a 26-mm undulator period for which the maximum undulator parameter K is 1.87. The lowest photon energy that can be generated from the HX1 with a maximum electron beam energy at PAL-XFEL (10.5 GeV) is about 14.65 keV. When a lower photon energy than that is required by the beamline users, the electron beam energy has to be decreased, which results in a decreased accessible FEL pulse energy. Therefore, a new hard X-ray undulator line (HX2) with a higher undulator parameter K is needed to make full use of the PAL-XFEL performance in the lower photon energies by increasing the resonant electron beam energy. The undulator period of the HX2 is decided as 35 mm by using Ming Xie’s fitting formula to estimate the performance of the HX2. FEL simulations with the GENESIS code are carried out to evaluate the performance of the HX2, including the effect of the post-saturation region. The undulator tapering configuration is optimized by maximizing the FEL intensity for each case. We show that the radiation power at the end of the HX2 can be increased up to 2.5 times higher than that of the HX1 over the entire target photon-energy range of the HX2 (2 ∼ 10 keV) by utilizing an undulator with a longer period and a higher undulator parameter K.

Time-resolved resonant elastic soft x-ray scattering at Pohang Accelerator Laboratory X-ray Free Electron Laser.

Resonant elastic x-ray scattering has been widely employed for exploring complex electronic ordering phenomena, such as charge, spin, and orbital order, in particular, in strongly correlated electronic systems. In addition, recent developments in pump-probe x-ray scattering allow us to expand the investigation of the temporal dynamics of such orders. Here, we introduce a new time-resolved Resonant Soft X-ray Scattering (tr-RSXS) endstation developed at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This endstation has an optical laser (wavelength of 800 nm plus harmonics) as the pump source. Based on the commissioning results, the tr-RSXS at PAL-XFEL can deliver a soft x-ray probe (400 eV-1300 eV) with a time resolution of ∼100 fs without jitter correction. As an example, the temporal dynamics of a charge density wave on a high-temperature cuprate superconductor is demonstrated.

Development of a Fast Sample-exchange System for Fixed Target Single-shot Imaging at PAL-XFEL

This paper describes the development of a fast exchange system to exchange thin membrane containing samples for single-shot experiments using micron focusing X-ray pulses at Pohang Accelerator Laboratory X-ray free electron laser (PAL-XFEL). The mechanism uses a compact gripper that can remove used samples and mount new ones in a vacuum chamber. The lead time of sample exchange is reduced more than half and increases data-collection time during limited beamtime. We believe that this

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.

Design of a High-Efficiency S-band Klystron with a Multi-cell Output Cavity

As large-scale scientific facilities develop, the klystron efficiency becomes an important issue to reduce high operating costs. We are developing high-efficiency klystrons for the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) at the Pohang Accelerator Laboratory (PAL). The newly developed klystrons should have the same specifications, including perveance and overall length, to replace existing ones. Under these limited conditions, we designed the klystrons based on a multi-cell output cavity to improve the power-conversion efficiency significatly. We simulated the beam-dynamics using the Field Charge Interaction (FCI) code with a MATLAB script to find the optimum design parameters for high efficiency. We have determined the design parameters, such as cell tuning frequencies, inter-cell distances, couplings between cells and the Qe of the last cell, of the multi-cell output cavity.

Undulator operation for PAL-XFEL large bandwidth modes

Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is operational for photon users since 2017. The hard X-ray beamline provides FEL with wavelengths from 0.1 and 0.7 nm, and the soft X-ray line provides 1 to 3 nm. The nominal operation of PAL-XFEL is based on the self-amplified spontaneous emission (SASE) process, where the energy spread of an electron beam is 10−3 to 10−4 to reach saturation. The bandwidth of XFEL radiation is naturally a similar order as of the energy spread of the electron beam. For certain types of XFEL user experiments, a large bandwidth of a few percent is required for the soft X-ray beamline. A large XFEL bandwidth may be obtained by increasing the energy spread of an electron beam. In such a case, the SASE process becomes however inefficient. Methods to achieve a large bandwidth X-ray free electron laser were proposed recently. In those methods, an electron beam is deflected transversely and sent to undulators with a vertical offset. The undulators have transverse field gradients. In this paper, we study the undulator operation parameters to achieve a large bandwidth by using the hardware setup of the PAL-XFEL soft X-ray beamline.

Isolated terawatt attosecond hard X-ray pulse generated from single current spike

Isolated terawatt (TW) attosecond (as) hard X-ray pulse is greatly desired for four-dimensional investigations of natural phenomena with picometer spatial and attosecond temporal resolutions. Since the demand for such sources is continuously increasing, the possibility of generating such pulse by a single current spike without the use of optical or electron delay units in an undulator line is addressed. The conditions of a current spike (width and height) and a modulation laser pulse (wavelength and power) is also discussed. We demonstrate that an isolated TW-level as a hard X-ray can be produced by a properly chosen single current spike in an electron bunch with simulation results. By using realistic specifications of an electron bunch of the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), we show that an isolated, >1.0 TW and ~36 as X-ray pulse at 12.4 keV can be generated in an optimized-tapered undulator line. This result opens a new vista for current XFEL operation: the attosecond XFEL.

Parameter Optimization of PAL-XFEL Injector

A photoinjector is used as the electron source to generate a high peak current and low emittance beam for an X-ray free electron laser (FEL). The beam emittance is one of the critical parameters to determine the FEL performance together with the slice energy spread and the peak current. The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) was constructed in 2015, and the beam commissioning was carried out in spring 2016. The injector is running routinely for PAL-XFEL user operation. The operational parameters of the injector have been optimized experimentally, and these are somewhat different from the originally designed ones. Therefore, we study numerically the injector parameters based on the empirically optimized parameters and review the present operating condition.