Shanghai Soft X-ray Research Articles

Development and commissioning of a broadband online X-ray spectrometer for the SXFEL Facility.

A broadband online X-ray spectrometer has been designed and commissioned at the SUD beamline of the Shanghai Soft X-ray Free-Electron Laser Facility, which can deliver both SASE and seeded FEL pulses to user experiments, spanning the photon energy range of 50-620 eV. The resolving powers of the spectrometer calibrated via online measurement at 92 eV and 249 eV are ∼20000 and ∼15000, respectively, and the absolute photon energy is characterized by an electron time-of-flight spectrometer. The high energy resolution provided by the spectrometer can differentiate the fine structure in the FEL spectrum, to determine its pulse length.

Self-enhanced coherent harmonic amplification in seeded free-electron lasers

High-intensity, ultrashort, fully coherent X-ray pulses hold great potential for advancing spectroscopic techniques to unprecedented levels. Here, we propose a novel scheme for generating high-brightness and femtosecond-scale soft X-ray radiation within a seeded free-electron laser (FEL) operating at MHz repetition rates. This scheme relies on the principles of self-modulation and superradiance. A relatively weak energy modulation of the pre-bunched electron beam is significantly amplified by the coherent radiation emitted in the self-modulator. Consequently, a coherent signal at ultra-high harmonics of the seed is achieved, and this signal is further amplified in the subsequent radiator through the fresh bunch and superradiant processes. Based on the parameters of the Shanghai soft X-ray FEL facility, three-dimensional simulations have been performed. The simulation results demonstrate that an electron beam with a laser-induced energy modulation as small as 2.3 times the slice energy spread can generate ultrashort coherent radiation pulses of around 2 GW within the water window spectral range. Moreover, the experimental results demonstrate that self-enhanced coherent energy modulation enables the production of coherent signals up to the 15th harmonic of a 266-nm seed laser. These findings indicate that the proposed scheme promises to generate ultrashort and coherent soft X-ray pulses at MHz repetition rates.

Design of a Pulse Transformer for X-band Klystron

Future linear accelerators require klystrons with higher radio frequency (RF) to drive higher gradient accelerating structure. An x-band accelerator structure was used to accelerate electrons at the Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) in Shanghai Advanced Research Institute, Chinese Academy of Sciences (SARI-CAS). A pulse transformer is a crucial device in an RF system. This study presents a high-voltage pulse transformer used for a 50 MW x-band pulsed klystron in SXFEL. Typical specifications of the pulse transformer are peak pulse voltage 420 kV, peak pulse current 300 A, 50 Hz repetition rate and 1.5 μs flat-top pulse width. Design and optimization of pulse transformer are achieved by using equivalent circuit analytic methods and computational aided simulation. The relevant experiments show that this pulse transformer can meet the requirements of 50MW x-band klystron.

High-resolution multi-mode electron and ion imaging spectrometer at SXFEL

A new multi-mode electron and ion (MEI) imaging spectrometer with two arms of VMI and COLTRIMS/VMI (velocity map imaging/cold-target recoil-ion momentum spectroscopy) is designed to combine various photoelectron and ion detection modes for experiments at Shanghai soft x-ray free-electron laser (SXFEL) facility. The experiments can be optionally operated either with both ion and electron detection in a coincidence/covariance manner (VMI arm and COLTRIMS/VMI arm), or only photoelectron/photoion is detected with the high-resolution VMI arm. The simulated resolutions for 30–150 eV photoelectrons and 3.3 eV–18.0 eV N+ and N2+ photoions are up to 1.0% and 3.1%–1.0% according to our simulation, respectively. MEI spectrometer is expected to improve the experimental abilities significantly considering the low-repetition rate of the SXFEL and to enable the investigation of a diverse range of atomic and molecular phenomena triggered by soft x-ray free electron laser irradiation.

Design and first-round commissioning result of the SASE beamline at the Shanghai Soft X-ray FEL facility.

The Shanghai Soft X-ray Free-Electron Laser (SXFEL) is the first X-ray free-electron laser facility in China. The SASE beamline, which consists of a pink-beam branch and a mono-beam branch, is one of the two beamlines in the Phase-I construction. The pink-beam branch opened for users in 2023 after successful first-round beamline commissioning. In this paper, the design of the beamline is presented and the performance of the pink-beam branch is reported. The measured energy-resolving power of the online spectrometer is over 6000 @ 400 eV. The focusing spot size of the pink beam is less than 3 µm in both the horizontal and vertical at the endstation.

Study and simulation of cryogenic bi-periodic accelerating structure with TM02 mode

To further enhance the accelerating gradient of accelerators, we designed a cryogenic C-band standing wave bi-periodic accelerating structure for the Shanghai Soft X-ray Free Electron Laser Facility (SXFEL). According to the low-temperature environment, material characteristics and technological conditions, the design is completed and it is decided to design the accelerating structure into a bi-periodic magnetic coupling structure. It is a 17-cell structure consisting of 9 accelerating cavities and 8 coupling cavities. To guarantee the symmetry of the field, the structure is doubly-fed. Operating with the π/2 mode standing wave, it is much less sensitive than the standing-wave structure of π-mode. Additionally, the microwave mode is TM02 in coupling cavities that are larger and even less sensitive than the traditional bi-periodic structure. The shape of the coupling cavity can be redesigned to make it tunable.

Design and implementation of timing system for single-shot imaging at Shanghai soft X-ray free-electron laser

X-ray free-electron laser (XFEL), as a novel advanced X-ray light source, has excellent properties such as ultra-high brightness, ultra-shot pulse duration, and full coherence. The coherent X-ray diffraction imaging (CDI) has a lot of advantages at high resolution and quantitative imaging compared with the traditional lens based X-ray imaging methods. By combining the excellent properties of XFEL and advantages of CDI, the single-shot imaging has been realized, based on the concept of “diffraction before destruction”. Shanghai soft X-ray free-electron laser facility (SXFEL) is the first XFEL facility operated at the X-ray wavelength in China. The coherent scattering and imaging (CSI) endstation is the first commissioned endstation at SXFEL, focusing on the high spatiotemporal imaging for nano materials and micro materials by using a single-shot imaging method. To realize the single-shot experiment at XFEL, especially for single-shot imaging, the timing system plays a crucial role in ensuring the operation of the equipment in sequence. This paper introduces the design and implementation process of SXFEL single-shot imaging timing. The timing system is implemented with White Rabbit (WR) and digital delay and pulse generator (BNC505). Single-shot imaging is realized by synchronously moving the sample scanning stages and X-ray shutter to select a single pulse to illuminate the sample. At the same time, the X-ray detector is triggered with the timing system to record the single-shot diffraction pattern. During debugging, a gold nanodisks each with a side length of approximately 300 nm and a thickness of about 30 nm, as test samples, are imaged at the CSI endstation. The nanodisks are uniformly dispersed on Si<sub>3</sub>N<sub>4</sub> membranes for single-shot imaging. Because of the ultra-high peak intensity at the focus spot, the samples and membrane are ionized for each XFEL pulse shot. A raster scan is performed on the membranes at intervals of 50 μm to update the sample. With the timing system and X-ray shutter, single-shot diffraction patterns can be recorded by using an X-ray detector. From the image of the Si<sub>3</sub>N<sub>4</sub> membrane after raster scanning, the ionized holes with an interval of 50 μm can be recognized. Finally, phase retrieval is applied to the single-shot diffraction pattern to obtain a real-space image of the sample. The resolution of the reconstructed image is estimated by calculating the phase-retrieval transfer function (PRTF). With a citation of the PRTF curve dropping below <inline-formula><tex-math id="M3">\begin{document}$ 1/{\mathrm{e}} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240383_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240383_M3.png"/></alternatives></inline-formula>, the spatial frequency cutoff is determined to be 22.6 μm<sup>–1</sup>, corresponding to a half period resolution of 22.1 nm. The results show that the designed timing system can accurately control the time sequence of the imaging process, meeting the requirement for single-shot imaging within 50 Hz at SXFEL.

Feasibility verification of ultrafast FEL generation experimental scheme based on SXFEL

The photon energy in the soft X-ray range corresponds to the fundamental absorption edges of matter. Ultrashort X-ray pulses can be used to observe the breaking of chemical bonds in biochemical reactions and capture the transfer process of electrons in ultrafast physical phenomena. In this paper, the feasibility of ESASE experiments on Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) is theoretically verified. The results show that the ESASE scheme can produce ultrafast light pulses on the order of attosecond, with a peak power of 450 MW. At the same time, the simulation results in this paper verify the feasibility of chirped enhanced SASE schenme based on SXFEL. The results show that compared with the ESASE scheme, the power of the radiation pulse can be greatly improved by this scheme. A relatively low energy electron beam (1.5 GeV) was used to generate about 40 GW of radiation, and the length of the radiation pulse was significantly shortened.

Characterization of single-pulse photon energy and photon energy jitter at the Shanghai soft X-ray Free-Electron Laser

Characterization of single-pulse photon energy and photon energy jitter at the Shanghai soft X-ray Free-Electron Laser

Recent progress of THz source at the SXFEL

Coherent, wide-tunable frequency and high intensity terahertz (THz) source is under preparation at the Shanghai Soft X-ray free-electron laser facility (SXFEL). The electron bunches modulated by frequency beating light can generate coherent, wide-tunable, high intensity THz radiation from 0.1 to 30 THz through wigglers. The electromagnetic wiggler with peak magnetic field up to 1.75 T is adopted and the parameters of the wiggler are optimized to ensure the generation of strong field THz radiation. Due to the limitation of layout space of the SXFEL, the length of wiggler is limited within 5 meters. By properly increasing the charge of the electron beam, the THz pulse energy can be kept at sub mJ level under the proposed different parameters of the wiggler. In this article, we will present the possible layout of THz source on the SXFEL and the S2E simulation of THz radiation of sub mJ magnitude within the 5-meter wiggler.

Single-pulse characterization of the focal spot size of X-ray free-electron lasers using coherent diffraction imaging.

The characterization of X-ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X-ray free-electron lasers (XFELs) are the latest generation of X-ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X-ray Free Electron Laser Facility was characterized using the method. The full width at half-maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method.

Few-femtosecond X-ray pulse generation and pulse duration control in a seeded free-electron laser

With the development of ultrafast science, free-electron lasers (FELs) with ultrashort pulses have become a state-of-the-art tool in ultrafast phenomena studies. In an externally seeded FEL, the output pulse duration is usually determined both by the seed laser pulse duration and FEL amplification process, which can hardly reach the timescale of a few femtoseconds. In this study, through a simple method of changing the relative time delay and correspondingly the pulse energy of the two seed lasers employed in a seeded FEL, we demonstrated the possibility of generating few-femtosecond soft X-ray pulses and controlling the final FEL pulse durations. Based on theoretical calculations and practical experiments, we conducted a detailed study on the capabilities and limitations to this method with the parameters of the Shanghai Soft X-ray FEL Facility. Start-to-end simulations indicate that we can achieve ultrashort soft X-ray FEL pulses with the pulse duration down to 5.2 fs, and the final pulse durations can also be controlled in terms of relative time delays.

Inhibition of microbunching instability in the Shanghai soft X-ray Free-Electron Laser User Facility

A microbunching instability (MBI) driven by beam collective effects can deteriorate electron beam quality in advanced x-ray free electron lasers. In this paper, we demonstrate a scheme to suppress microbunching instability by adjusting the compression ratio of two-stage bunch compressor in linear accelerator of the Shanghai soft X-ray Free-Electron Laser User Facility (SXFEL-UF). The calculation of the microbunching instability gain and the mechanism of MBI suppression scheme can be revealed by the theoretical model in this paper. Numerical simulations are presented to verify the analytical results. Furthermore, benefit by the high temporal resolution of transverse deflecting structure (TDS), beam experiments for direct analysis of MBI were carried out at SXFEL-UF. Theoretical calculations as well as simulation and consistent experimental results illustrate that the suppression of microbunching instability can be achieved by increasing the first stage compression ratio of the linac in the SXFEL-UF. The feasible theme provides a promising beam optimization method for future X-FEL facilities.

Generating High-Power, Frequency Tunable Coherent THz Pulse in an X-ray Free-Electron Laser for THz Pump and X-ray Probe Experiments

Precisely synchronized X-ray and strong-field coherent terahertz (THz) enable the coherent THz excitation of many fundamental modes (THz pump) and the capturing of X-ray dynamic images of matter (X-ray probe), while the generation of such a light source is still a challenge for most existing techniques. In this paper, a novel X-ray free-electron laser based light source is proposed to produce a synchronized high-powered X-ray pulse and strong field, widely frequency tunable coherent THz pulse simultaneously. The technique adopts a frequency beating laser modulated electron bunch with a Giga-electron-volt beam energy to generate an X-ray pulse and a THz pulse sequentially by passing two individual undulator sections with different magnetic periods. Theoretical analysis and numerical simulations are carried out using the beam parameters of the Shanghai soft X-ray free-electron laser facility. The results show that the technique can generate synchronized 4 nm X-ray radiation with a peak power of 1.89 GW, and narrow-band THz radiation with a pulse energy of 1.62 mJ, and the frequency of THz radiation can be continuously tuned from 0.1 to 40 THz. The proposed technique can be used for THz pump and X-ray probe experiments for dynamic research on the interaction between THz pulse and matter at a femtosecond time scale.

Automatic online optimization on transverse emittance at SXFEL-UF injector

During the online tuning of a large-scale accelerator, the improvement of beam properties is usually conducted by operators manually, which is time-consuming and is highly dependent on their personal experience of it. In this work, we try to introduce the extremum seeking algorithm into the online beam transverse emittance optimization project in Shanghai soft X-ray Free-Electron Laser facility (SXFEL) Injector. The transverse emittance is essential to be well-controlled in the injector section, which facilitates the FEL lasing performance in the undulators. The experiment result demonstrates the efficiency of this automatic tuning approach and lays the foundation to further study and research on the achievement of automatic control in the free-electron laser facilities.

Optimization of the cavity beam-position monitor system for the Shanghai soft X-ray free-electron laser user facility

Optimization of the cavity beam-position monitor system for the Shanghai soft X-ray free-electron laser user facility

Optimized design method study for cavity BPM system

Beam-based alignment and feedback systems are essential for the operation of the Free Electron Lasers (FELs) and Linear colliders. They require a certain number of beam position monitors (BPMs) at selected locations and that their resolution can reach hundreds of nanometers or even the order of nanometers. Cavity BPMs adopting a resonant cavity structure having the advantage of high position resolution are widely used in the field of accelerators. A typical system includes a cavity BPM pickup, a radio frequency signal conditioning front-end and a digital signal processing electronic. How to systematically analyze the impact of the key parameters of each subsystem on the performance of the whole system, and combine the requirements of the system, operation mode of the facilities, the current level of equipment processing and the development of the electronics, so as to achieve the optimal and balanced allocation of the key technical indicators of each subsystem, is the primary issue to be considered when designing a CBPM system. In this paper, based on theoretical analysis, the relationship between the relative amplitude extraction uncertainty of the CBPM system and the key parameters of each subsystem is proposed, which could provide clear guidance for the design and optimization of a CBPM system. In addition, considering the engineering requirements of the Shanghai High repetition rate X-ray Free Electron Laser and Extreme Light facility (SHINE) for beam position monitor, the allocation of the key technical indicators of the CBPM system using the optimized design method is also introduced. Accordingly, the development of each subsystem was completed, and the beam test bench was also built at the Shanghai Soft X-ray FEL facility (SXFEL), based on the evaluation method of divided power, the beam experiment results show that the position measurement uncertainty of the CBPM system can reach 40 nm at the bunch charge of 100 pC, which is consistent with the theoretical analysis results and better than the requirements of the SHINE.

An application of a cavity-based beam arrival time measurement system: Beam energy measurement

A high-resolution cavity-based beam arrival/flight time measurement system has been established at SXFEL (Shanghai soft X-ray free-electron laser facility). During the development of the system, a correlation between the energy jitter and the beam flight time jitter was found which inspires and motivates us to further explore the possibility of using the beam arrival/flight time measurement system to measure the beam energy. On the one hand, beam energy is one of the most fundamental and critical parameters for accelerator facilities. Only precise handling of the beam energy enables precise control of the FEL (free-electron laser) radiation wavelength. On the other hand, there are only a few methods for beam energy measurement. The profile-based beam energy measurement method is commonly used but is interceptive. The BPM-based beam energy measurement scheme is non-interceptive but has a limited dynamic range and requires initial beam position calibration. This paper reports a promising application of a cavity-based high-resolution beam arrival/flight time measurement system: non-intercepting wide-range beam energy measurement. A brief introduction of the method principle and system implementation has been covered in this article. Moreover, a beam-based preliminary test has been performed and the test results show a linear correlation between the beam energy and the beam flight time under the current system configuration. The measured linear factor is 0.692 ps/MeV. In addition, the relative bunch energy measurement resolution is 0.0001.

First commissioning results of the coherent scattering and imaging endstation at the Shanghai soft X-ray free-electron laser facility

The Shanghai soft X-ray free-electron laser (SXFEL) user facility project started in 2016 and is expected to be open to users by 2022. It aims to deliver ultra-intense coherent femtosecond X-ray pulses to five endstations covering a range of 100–620 eV for ultrafast X-ray science. Two undulator lines are designed and constructed, based on different lasing modes: self-amplified spontaneous emission and echo-enabled harmonic generation. The coherent scattering and imaging (CSI) endstation is the first of five endstations to be commissioned online. It focuses on high-resolution single-shot imaging and the study of ultrafast dynamic processes using coherent forward scattering techniques. Both the single-shot holograms and coherent diffraction patterns were recorded and reconstructed for nanoscale imaging, indicating the excellent coherence and high peak power of the SXFEL and the possibility of “diffraction before destruction” experiments at the CSI endstation. In this study, we report the first commissioning results of the CSI endstation.

Kirkpatrick-Baez mirrors commissioning for coherent scattering and imaging endstation at SXFEL

Shanghai Soft X-ray Free-Electron Laser (SXFEL) is the first X-ray free-electron laser facility in China. The initial commissioning of the beamline was carried out in May 2021. Herein, we present a status report and the first experimental results obtained during the early commissioning of Kirkpatrick-Baez (KB) mirrors for the Coherent Scattering and Imaging (CSI) endstation, including three types of diagnostics. A bright X-ray focal spot of less than 3 μm was achieved by using edge-scan and silicon ablation imprint measurements. In order to confirm the spot size, the attenuated beam and full beam are used respectively for the two measurement methods.