Pohang Light Source-II Research Articles

In-vacuum undulators for the first phase beamlines of Korea-4GSR

Korea-4GSR, a new synchrotron radiation facility currently under construction in Ochang, Chungbuk, South Korea, introduces three types of in-vacuum undulators (IVUs) for its first phase hard X-ray beamlines: IVU20, IVU22, and IVU24. These IVU types share a common 3-m-long framework capable of adjusting the magnetic gap size between 5 and 18 mm, but they differ in the undulator period length (λu). This study characterizes their photon beams in terms of brightness, spectral coverage, source size, angular divergence, coherent fraction, coherent flux, and total and central cone radiation powers, using undulator calculations. The three IVU types are comparable in brightness. IVU20 is the most coherent, although lacking spectral continuity at around 7.5 keV. IVU22 and IVU24 ensure spectral continuity, but their coherent flux is moderately compromised. The performance of the undulators is assessed in comparison to the Pohang Light Source-II (PLS-II) undulator and the U21 undulator at Advanced Photon Source Upgrade (APS-U).

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

Troubleshooting of the beam dump problem caused by helium vessel pressure fluctuation during the cryogenic adsorber regeneration process of a superconducting RF system for synchrotron user facilities

The Pohang Light Source-II (PLS-II) is a synchrotron user-service facility that should provide a user-service period of 190 days per year with stable operation. A series of beam dump problems related to the superconducting radio-frequency (SRF) system but of unknown cause occurred twenty-five times from 2016 to 2018, which severely degraded the stable operation of the PLS-II and the SRF system. Intensive investigation has shown that the relevant beam dumps occurred when the helium (He) flow control valves operated momentarily during the adsorber regeneration process of the cryogenic system. As the control valves operate momentarily, the He vessel pressure of the cryomodule instantaneously fluctuates, and the resonance frequency of the SRF cavity accordingly changes. If the instantaneous change in the resonance frequency is larger than the low-level RF (LLRF) control ability, then incorrect handling of LLRF control occurs, resulting in a beam dump. The instantaneous changes in the He vessel pressure occur via three different mechanisms. Solutions for each beam dump mechanism were proposed and implemented. After each solution was carried out, no beam dump occurred during the adsorber regeneration process. The relation between the He vessel pressure and the resonance frequency of the SRF cavity was also studied to see how the solution works.

Multimodal X-ray probe station at 9C beamline of Pohang Light Source-II.

In this study, the conceptual design and performance of a multimodal X-ray probe station recently installed at the 9C coherent X-ray scattering beamline of the Pohang Light Source-II are presented. The purpose of this apparatus is to measure coherent X-ray diffraction, X-ray fluorescence and electrical properties simultaneously. A miniature vacuum probe station equipped with a four-point probe was mounted on a six-axis motion hexapod. This can be used to study the structural and chemical evolution of thin films or nanostructures, as well as device performance including electronic transport properties. This probe station also provides the capability of varying sample environments such as gas atmosphere using a mass-flow-control system and sample temperatures up to 600°C using a pyrolytic boron nitride heater. The in situ annealing of ZnO thin films and the performance of ZnO nanostructure-based X-ray photodetectors are discussed. These results demonstrate that a multimodal X-ray probe station can be used for performing in situ and operando experiments to investigate structural phase transitions involving electrical resistivity switching.

Investigation of correlative parameters to evaluate EUV lithographic performance of PMMA.

Investigations to evaluate the extreme ultraviolet (EUV) lithographic performance of 160 nm thick poly(methyl methacrylate) with 13.5 nm wavelength EUV light were performed using a synchrotron radiation source at Pohang Light Source-II (PLS-II). The single system enabled the determination of the sensitivity, contrast, linear absorption coefficient, critical dimension, and line edge roughness of polymer thin films through tests and measurements. The experimental findings were also compared to theoretical results and those of previously reported studies. According to the results of the dose-to-clear test and transmission measurements, the critical dimension of a line and space pattern (>50 nm) via interference lithography with 250 nm pitch grating agreed well with the results calculated using the lumped parameter model. The experimental results demonstrated that the equipment and test protocol can be used for EUV material infrastructure evaluation in academia and in industry.

Transverse bunch-by-bunch feedback system for time-resolved experiments at PLS-II.

A description of the upgraded bunch-by-bunch feedback system for time-resolved experiments at Pohang Light Source II (PLS-II) is provided. The bunch-by-bunch feedback system has been upgraded to increase the single-bunch current in the hybrid fill pattern of the PLS-II facility. The project is part of the SPring-8 and PLS-II collaboration. The main features of the upgrade are to employ a single 500 MHz analog-to-digital converter (ADC) instead of the previous four 125 MHz interleaved ADCs for 500 MHz rate, to replace a single-loop two-dimensional feedback with two independent one-dimensional feedback loops, to implement the tune measurement function with a single bunch, and mainly to implement single-bunch and stretcher control. The realization of a400 mA hybrid fill pattern including a 10 mA single bunch demonstrates the precision of the upgraded bunch-by-bunch feedback system.

Comparative study of hard X-ray undulator beamline performance in the Korean 4GSR and the PLS-II

A new light source based on the electron storage ring, dubbed the “diffraction-limited storage ring” (DLSR) to keep the full intrinsic wave nature of X-rays had been proposed since the early stage of storage ring history and has finally been developed successfully, and an upgrade and a new construction programs have now chosen in the worldwide synchrotron facilities. The construction of the so-called “4th generation storage ring” (4GSR), which is a newly-coined term aiming in the same direction, was decided in Korea. The Korean 4GSR is expected to be 10–100 times brighter than the Pohang Light Source-II (PLS-II). Hard X-ray undulator beamlines will benefit from the 4GSR due to its low emittance approaching the diffraction limit. In the PLS-II, more than 10 hard X-ray undulator beamlines are currently in operation. We present a comparative study of the representative hard X-ray undulator beamlines by using the cutting-edge diffraction-spectroscopy techniques in the PLS-II and the 4GSR for better understanding the upcoming light source in Korea. The figures-of-merit of the two specific experimental techniques, resonant inelastic X-ray scattering (RIXS) and resonant X-ray emission spectroscopy (RXES), are discussed for comparison of the two light sources. Both RIXS and RXES are sometimes referred to as a “renaissance” in X-ray science and are, therefore, strongly expected to be adopted in the 4GSR beamlines.

Synchrotron serial crystallography at 11C beamline of Pohang Light Source-II

Synchrotron serial crystallography at 11C beamline of Pohang Light Source-II

High-Speed X-Ray Projection Microscopy at Beamline 9D of the Pohang Light Source-II

We constructed a high-speed X-ray projection microscope at beamline 9D of the Pohang Light Source-II (PLS-II). The microscope was able to take X-ray projection micrographs with a submil-lisecond exposure time by using whitebeam X-rays from the PLS-II bending-magnet synchrotron radiation. It also takes advantage of edge enhancement in phase-contrast imaging, which significantly improves the visibility of soft materials and microfluids. Using the microscope, we captured the spreading of a water droplet on a hydrophilic and oleophilic nonwoven membrane at 1,264 frames per second. The computed tomography (CT), which visualized a three-dimensional wooden structure at a spatial resolution of 2.7 µm, took 2.5 s for scanning.

New Injection Schemes for Pohang Light Source II and Pohang Accelerator Laboratory 4th Generation Storage Ring

In this paper, we describe four different injection schemes for the Pohang Light Source II (PLS-II) and the Pohang Accelerator Laboratory 4th Generation Storage Ring (PAL-4GSR). Starting from the present conventional four-kicker injection scheme which is being operated in PLS-II, we propose a non-linear single-kicker injection scheme for the PLS-II and two on-axis injection schemes for the PAL-4GSR. All three proposed injection schemes are well demonstrated to accumulate the beam in the storage rings without beam loss. In this paper, we present the simulation results for various new injection schemes after introducing the lattices of the PLS-II and the PAL-4GSR.

Development of XANES nanoscopy on BL7C at PLS-II.

X-ray absorption near-edge structure (XANES) imaging is a powerful tool to visualize the chemical state distribution of transition-metal-based materials at synchrotron radiation facilities. In recent years, the electrochemical working rechargeable battery has been the most studied material in XANES imaging owing to the large increase of portable electronics and electric vehicles. This work acknowledges the importance of battery analysis and has developed the XANES imaging system on BL7C at Pohang Light Source-II (PLS-II). BL7C employs an undulator taper configuration to obtain an energy band >130 eV near the K-absorption edge of the target element with a minimum energy interval >0.2 eV. While measuring energy-dependent images, the zone plate translation maintains the best focus, and then various data processes such as background correction, image registration and clustering allow single XANES spectrum extraction and chemical distribution mapping. Here, the XANES imaging process is described, the XANES spectrum quality is identified and the chemical states of the partially charged cathode material used in lithium-ion batteries as an application example are examined.

Fast Interlock System for Machine Protection in PLS-II

We overview the system design of the Pohang Light Source-II (PLS-II), and demonstrate that synchrotron radiation is the cause of a recent incident that damaged the vacuum components. To preclude further incidents, we designed a system that shuts off the beam if such damage is imminent. We used an ANSYS simulation to estimate the appropriate time allowed for this shut-off then. A Fast Interlock System is implemented to accomplish beam shut-off within this time. In bench test, this system shut down the beam within the acceptable time. The installation and the operation of the Fast Interlock System in PLS-II is planned.

Observation of the Talbot Effect at Beamline 6C Bio Medical Imaging of the Pohang Light Source-II

The Talbot effect is used in novel contrast imaging techniques that measure X-ray refractions and scattering. We examined formation of periodic fringes by the Talbot effect at beamline 6C Bio Medical Imaging of the Pohang Light Source-II, a facility for synchrotron radiation X-ray imaging. Talbot fringes were not observable under a normal illumination condition at the beamline, but appeared when we made a narrow opening of a slit next to the synchrotron radiation source that should have improved the transverse coherency of the beam. The Talbot fringes were identified by comparison with a numerical simulation, and also measuring the fringe visibility at fractional Talbot distances using grating interferometry.

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.

Development and performance test of a new high power RF window in S-band PLS-II LINAC

A prototype of RF window was developed in collaboration with the Pohang Accelerator Laboratory (PAL) and domestic companies. High power performance tests of the single RF window were conducted at PAL to verify the operational characteristics for its application in the Pohang Light Source-II (PLS-II) linear accelerator (Linac). The tests were performed in the in-situ facility consisting of a modulator, klystron, waveguide network, vacuum system, cooling system, and RF analyzing equipment. The test results with Stanford linear accelerator energy doubler (SLED) have shown no breakdown up to 75 MW peak power with 4.5 μs RF pulse width at a repetition rate of 10 Hz. The test results with the current operation level of PLS-II Linac confirm that the RF window well satisfies the criteria for PLS-II Linac operation.

Development of new S-band RF window for stable high-power operation in linear accelerator RF system

For stable high-power operation, a new RF window is developed in the S-band linear accelerator (Linac) RF systems of the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL). The new RF window is designed to mitigate the strength of the electric field at the ceramic disk and also at the waveguide-cavity coupling structure of the conventional RF window. By replacing the pill-box type cavity in the conventional RF window with an overmoded cavity, the electric field component perpendicular to the ceramic disk that caused most of the multipacting breakdowns in the ceramic disk was reduced by an order of magnitude. The reduced electric field at the ceramic disk eliminated the Ti–N coating process on the ceramic surface in the fabrication procedure of the new RF window, preventing the incomplete coating from spoiling the RF transmission and lowering the fabrication cost. The overmoded cavity was coupled with input and output waveguides through dual side-wall coupling irises to reduce the electric field strength at the waveguide-cavity coupling structure and the possibility of mode competitions in the overmoded cavity. A prototype of the new RF window was fabricated and fully tested with the Klystron peak input power, pulse duration and pulse repetition rate of 75 MW, 4.5 μs and 10 Hz, respectively, at the high-power test stand. The first mass-produced new RF window installed in the PLS-II Linac is running in normal operation mode. No fault is reported to date. Plans are being made to install the new RF window to all S-band accelerator RF modules of the PLS-II and PAL-XFEL Linacs. This new RF window may be applied to the output windows of S-band power sources like Klystron as wells as the waveguide windows of accelerator facilities which operate in S-band.

Reduction of RF accelerating voltage of Pohang Light Source-II superconducting RF cavity for stable top-up mode operation

The Pohang Light Source-II (PLS-II) is currently providing a top-up mode user-service operation with maximum available beam current of 400 mA and a beam emittance of below 10 nm-rad. The dimension of the beam bunch shortened to accomplish a low beam emittance of below 10 nm-rad from a high beam current of 400 mA increases the bunch charge density. As a result, the electron beam lifetime is significantly degraded and a high gradient of power is lost in the vacuum components of the storage ring. A study on how to reduce the bunch charge density without degrading beam emittance found that reducing the RF accelerating voltage (Vacc) can lower the bunch charge density by lengthening the bunch in the longitudinal direction. In addition, the Vacc required for stable operation with beam current of 400 mA can be reduced by lowering the external cavity quality factors (Qext values) of the superconducting cavities (SCs). To control the Qext values of SCs gradually without accessing the accelerator tunnel, a remote control motorized three-probe-tuner was installed in the transmission line of each SC. The optimum installation position of the three-probe-tuner was determined by using a finite-difference time-domain (FDTD) simulation and by experimenting on various installation positions of the three-probe-tuner. The Qext values of all the SCs were lowered to 1.40 × 105, and then, the Vacc required to store the beam current of 400 mA was decreased from 4.8 MV to 4.2 MV, which corresponds to 10% lengthening of the beam bunches. The stable operation with the reduced Vacc was confirmed during a 400 mA ten-day top-up mode user-service. Currently, the RF system of the PLS-II storage ring delivers the user-service operation with lowered Qext values to reduce the power loss at the vacuum components as well as the cryogenic heat load of SCs, and no significant problems have been found. This method of reducing the Vacc may also be applied in other synchrotron facilities.

Fast global orbit feedback system in PLS-II

The transverse position of the electron beam in the Pohang Light Source-II is stabilized by the global orbit feedback system. A slow orbit feedback system has been operating at 2 Hz, and a fast orbit feedback (FOFB) system at 813 Hz was installed recently. This FOFB system consists of 96 electron-beam-position monitors, 48 horizontal fast correctors, 48 vertical fast correctors and Versa Module Europa bus control system. We present the design and implementation of the FOFB system and its test result. Simulation analysis is presented and future improvements are suggested.

Development of new S-band SLED for PAL-XFEL Linac

In order to achieve beam acceleration to the beam energy of 10GeV at the end of its 716m-long linear accelerator (Linac), the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is going to operate the Stanford Linear Accelerator Energy Doubler (SLED) at the maximum klystron output peak power of 80MW, with a pulse length of 4μs, and at a repetition rate of 60Hz. The original SLED that had been used in Pohang Light Source-II (PLS-II) can no longer sustain such a high-power operation because excessive radiation caused by RF breakdown has been frequently detected even at the lower klystron peak power during the PLS-II operation. Therefore, a new SLED is designed by modifying both the 3-dB power hybrid and the waveguide-cavity coupling structure of the original SLED where the excessive radiation has been mainly detected. The finite-difference time-domain (FDTD) simulation in the CST Microwave Studio shows that the new SLED has a peak electric field and a surface current lower than those of the original SLED at the same level of the RF input peak power, which would secure stable high-power operation. All of the 42 SLEDs in the PAL-XFEL Linac are newly fabricated and installed. During the RF conditioning of the PAL-XFEL Linac, no significant vacuum and radiation issue was found in the new SLEDs. Finally, the accelerated electron beam energy of 10GeV obtained at the end of the PAL-XFEL Linac verified that the RF performance of the new SLED is stable.

Effects of an elliptically polarizing undulator on the beam dynamics in PLS-II

The non-linear effects caused by the intrinsic field transverse roll-off in an Elliptically Polarizing Undulator with 72-mm period and maximum peak field of 0.72 T (EPU72) on the dynamics aperture of the Pohang Light Source II (PLS-II) were investigated. A kick map and Frequency map analysis both demonstrated that EPU72 will not reduce the lifetime or cause injection problems in PLS-II.