Accelerator Laboratory X-ray Free Electron 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.

Generation of time-synchronized two-color X-ray free-electron laser pulses using phase shifters

To optimize the intensity of X-ray free-electron lasers (XFELs), phase shifters, oriented in phase with the phases of the XFEL pulse and electron beam, are typically installed at undulator lines. Although a π-offset between the phases (i.e., an “out-of-phase” configuration) can suppress the XFEL intensity at resonant frequencies, it can also generate a side-band spectrum, which results in a two-color XFEL pulse; the dynamics of such a pulse can be described using the spontaneous radiation or low gain theory. This attributes of this two-color XFEL pulse can be amplified (log-scale amplification) through an undulator line with out-of-phase phase shifters. In this study, the features of two-color XFEL pulses were evaluated through theory, simulations and experiments performed at Pohang Accelerator Laboratory X-ray Free Electron Laser. The XFEL gain slope and energy separation between the two-color spectral peaks were consistent through theoretical expectation, and the results of simulation and experiment. The experimentally determined two-color XFEL pulse energy was 250 μJ at a photon energy of 12.38 keV with a separation of 60 eV.

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.

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.

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.

Observation of harmonic lasing in the Angstrom regime at European X-ray Free Electron Laser

Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5--15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

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

Effect of high slice energy spread of an electron beam on the generation of isolated, terawatt, attosecond X-ray free-electron laser pulse

Attosecond (asec) X-ray free-electron laser (XFEL) has attracted considerable interest over the past years. Nowadays typical XFEL application experiments demand 1010–1011 photons per pulse, which corresponds to a peak power of terawatts (TW) in case of asec hard X-ray pulse. To the realization of such TW asec-XFEL pulse, however, the unavoidable increase of slice energy spread (SES) due to laser heater, which is commonly used to mitigate the micro-bunching instability (MBI), would be a major obstacle. To deal with this problem, the effect of such a SES is investigated in this work. The results reveal that (1) SES of a current spike is linearly proportional to the peak current of a current spike in an electron beam, (2) surprisingly, this linearity is independent of the wavelength of an energy modulation driving laser which is used to make a current spike and (3) the gain length of current spike in the undulator is sensitive to the initial SES, so there is an optimal peak current of the current spike for successful FEL lasing process. Utilizing these characteristics, a series of simulations with parameters for Pohang Accelerator Laboratory X-ray Free Electron Laser was carried out to demonstrate that an isolated, TW asec-XFEL pulse can be generated even when the SES is increased due to the usage of laser heater to prevent the MBI in the XFEL. We show that an isolated X-ray pulse with >1 TW and a pulse duration of 73 as (~3 × 1010 photons/pulse at 12.4 keV or 0.1 nm) can be generated by using ten current spikes with optimal peak current. It becomes clear for the first time that the disadvantage from the increased SES can be indeed overcome.

ClickX: a visualization-based program for preprocessing of serial crystallography data.

Serial crystallography is a powerful technique in structure determination using many small crystals at X-ray free-electron laser or synchrotron radiation facilities. The large diffraction data volumes require high-throughput software to preprocess the raw images for subsequent analysis. ClickX is a program designated for serial crystallography data preprocessing, capable of rapid data sorting for online feedback and peak-finding refinement by parameter optimization. The graphical user interface (GUI) provides convenient access to various operations such as pattern visualization, statistics plotting and parameter tuning. A batch job module is implemented to facilitate large-data-volume processing. A two-step geometry calibration for single-panel detectors is also integrated into the GUI, where the beam center and detector tilting angles are optimized using an ellipse center shifting method first, then all six parameters, including the photon energy and detector distance, are refined together using a residual minimization method. Implemented in Python, ClickX has good portability and extensibility, so that it can be installed, configured and used on any computing platform that provides a Python interface or common data file format. ClickX has been tested in online analysis at the Pohang Accelerator Laboratory X-ray Free-Electron Laser, Korea, and the Linac Coherent Light Source, USA. It has also been applied in post-experimental data analysis. The source code is available via https://github.com/LiuLab-CSRC/ClickX under a GNU General Public License.

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.

Coherence and pulse duration characterization of the PAL-XFEL in the hard X-ray regime

We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.

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.

Soft X-ray harmonic lasing self-seeded free electron laser at Pohang Accelerator Laboratory X-ray free electron laser

The demonstration of a harmonic lasing self-seeded free-electron laser (HLSS FEL) scheme in the soft X-ray range at the Pohang Accelerator Laboratory X-ray Free Electron Laser is presented. We report the experimental results of HLSS FEL radiation with the shortest wavelength of 1 nm by using the optimized phase shift of 2/3π. The key feature of the HLSS scheme is that the mode number is decreased (the longitudinal coherence length is enhanced) which is directly observed using a single-shot spectrometer. The spectral brightness is enhanced by a factor of 1.7 compared to the self-amplified spontaneous emission FEL because of the narrowed bandwidth. Our results show a good agreement with the theoretical expectation and simulations. The HLSS mode is a promising standard operation mode to generate a stable and high-brightness X-ray FEL that will provide more benefits to users for various applications.