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.

A Review Study on Amplification of X-Ray Free Electron Laser Pulse in Plasma

In view of X-ray Free Electron Laser (XFEL) intensity prospects, we reviewed the past and recent work in view of amplification of powerful XFEL laser pulse to achieve intensity in the regime of high field science. We report here some of the relevant work investigated in this field and predicted further scalings and possibilities for XFEL pulse amplification.

Optical beat notes assisted attosecond soft X-ray pulse generation in high-gain free electron lasers

Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena.In this paper,a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating.Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation,which helps to generate a gradually varied spacing electron pulse train.Subsequently,the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation.Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique.

硬X射线自由电子激光装置中低温工厂工艺冷却水系统设计

硬X射线自由电子激光装置项目建设了国内最大的超流氦低温系统,为了让系统中冷箱、压缩机等热耗较大的低温设备实现恒温调节,保证低温系统的正常稳定运行,需设置供水水温恒定的冷却水系统。文章从工艺要求、总体布局、方案设计、系统组成、运行模式及温度控制等方面细介绍了一号井低温工厂工艺冷却水系统设计,并在试运行期间对其进行能量回收和备用冷源流量验证试验,以验证设计效果,为类似项目的设计提供经验和借鉴。

Development of readout electronics for bunch arrival-time monitor system at SXFEL

A bunch arrival-time monitor(BAM) system,based on electro-optical intensity modulation scheme, is under study at Shanghai Soft X-ray Free Electron Laser.The aim of the study is to achieve high-precision time measurement for minimizing bunch fluctuations. A readout electronics is developed to fulfill the requirements of the BAM system. The readout electronics is mainly composed of a signal conditioning circuit, field-programmable gate array(FPGA), mezzanine card(FMC150), and powerful FPGA carrier board. The signal conditioning circuit converts the laser pulses into electrical pulse signals using a photodiode. Thereafter, it performs splitting and low-noise amplification to achieve the best voltage sampling performance of the dual-channel analog-to-digital converter(ADC) in FMC150. The FMC150 ADC daughter card includes a 14-bit 250 Msps dual-channel high-speed ADC,a clock configuration, and a management module. The powerful FPGA carrier board is a commercial high-performance Xilinx Kintex-7 FPGA evaluation board. To achieve clock and data alignment for ADC data capture at a high sampling rate, we used ISERDES, IDELAY, and dedicated carry-in resources in the Kintex-7 FPGA. This paper presents a detailed development of the readout electronics in the BAM system and its performance.

ReLaX:the Helmholtz International Beamline for Extreme Fields high-intensity short-pulse laser driver for relativistic laser-matter interaction and strong-field science using the high energy density instrument at the European X-ray free electron laser fac

High-energy and high-intensity lasers are essential for pushing the boundaries of science.Their development has allowed leaps forward in basic research areas,including laser±plasma interaction,high-energy density science,metrology,biology and medical technology.The Helmholtz International Beamline for Extreme Fields user consortium contributes and operates two high-peak-power optical lasers using the high energy density instrument at the European X-ray free electron laser(EuXFEL)facility.These lasers will be used to generate transient extreme states of density and temperature to be probed by the X-ray beam.This paper introduces the ReLaX laser,a short-pulse high-intensity Ti:Sa laser system,and discusses its characteristics as available for user experiments.It will also present the first experimental commissioning results validating its successful integration into the EuXFEL infrastructure and viability as a relativisticintensity laser driver.

The MING proposal at SHINE:megahertz cavity enhanced X‑ray generation

The cavity-based X-ray free-electron laser(XFEL)has promise in producing fully coherent pulses with a bandwidth of a few meV and very stable intensity,whereas the currently existing self-amplified spontaneous emission(SASE)XFEL is capable of generating ultra-short pulses with chaotic spectra.In general,a cavity-based XFEL can provide a spectral brightness three orders of magnitude higher than that of the SASE mode,thereby opening a new door for cutting-edge scientific research.With the development of superconducting MHz repetition-rate XFEL facilities such as FLASH,European-XFEL,LCLS-II,and SHINE,practical cavity-based XFEL operations are becoming increasingly achievable.In this study,megahertz cavity enhanced X-ray generation(MING)is proposed based on China’s first hard XFEL facility-SHINE,which we refer to as MING@SHINE.

Compositional change in human enamel irradiated with MIR free electron laser

The purpose of this study was to investigate compositional changes in human enamel irradiated with the free electron laser (FEL). The exposure on dental enamel at the wavelength of 9.64 μm was observed with the Beijing free electron laser. The distribution of elements in the irradiated or non-irradiated enamel was measured by scanning electron microscope (SEM) with energy-dispersive spectroscopy and synchrotron radiation X-ray fluorescence (SRXRF) in Beijing Synchrotron Radiation Facility (BSRF). The results showed that the P/Ca ratio in the ablation region of enamel at the maximum wavelength of infrared absorption was obviously smaller than that at the non-maximum wavelength. In the ablation region the ratios of P/Ca and Ca/Sr were smaller than those in the non-ablation region. The distributions of P, Ca and Sr in the ablation region were heterogeneous due to the element change caused by FEL irradiation.

基于超导直线加速器的XFEL初步方案

基于超导直线加速器的X射线自由电子激光（XFEL）是自由电子激光领域未来发展的一个重要方向。介绍了国际上Euro—XFEL及LCLS—II等超导XFEL装置的设计建造现状。在此基础上提出基于一台7GeV超导直线加速器的超导XFEL初步方案，给出主要设计参数，并讨论实现100～1000GW量级峰值输出功率的技术路线，包括提高峰值流强、采用波荡器磁场渐变技术等。

曙光一号自由电子激光自发辐射放大的理论计算

曙光一号自由电子激光装置用自发辐射放大(ASE)机理,在毫米波波段测得了数百毫瓦的光信号输出。利用三维WAGFEL程序,结合曙光一号直线感应加速器型自由电子激光放大器参数,详细地分析了自发辐射放大的增益、输出功率随电子束的质量(包括束流能散度、流强、发射度等因素)的变化情况,给出了相应的定标关系;并且考察了摇摆场随机误差对自发辐射放大功率的影响。最后对自发辐射放大的实验结果进行了分析。

磁性材料在同步辐射及自由电子激光中的应用

同步辐射与自由电子激光都是相对论高能电子束产生的电磁辐射光,在国民经济、科学技术研究和国防军事等领域中有着广泛应用。各种永磁插入件是同步辐射光源及自由电子激光装置的关键设备之一,磁性材料特别是永磁体磁特性及质量对插入件的磁场品质、磁场峰值、磁场稳定性和运行方案等都有着重要影响。本文综述了同步辐射及自由电子激光特征,介绍了磁性材料在同步辐射及自由电子激光插入件中的应用情况。

DC-SC光阴极注入器的束载实验的调试进展

北京大学射频超导实验室设计了新型超导光电子枪———DC SC光阴极注入器,目标是为自由电子激光平台提供能量在2～3MeV,脉宽小于10ps,脉冲重复频率为81.25MHz,平均流强约为1mA的低发射度电子束。现在已经建成了DC SC光阴极注入器实验平台,包括激光驱动光阴极系统,Pierce直流高压加速结构,1.3GHz1+1/2纯铌超导腔,恒温器低温系统,4.5kW连续波微波系统,1/16分频与同步控制系统,束流诊断系统和能量分析系统等。并且完成了超导腔的静态实验,直流加速结构也经过了100μA低电流测试。实验结果符合设计要求,整体调试后即可以进行束载实验。

Fully coherent hard X-ray generation by two-stage phase-merging enhanced harmonic generation

Cascading stages of seeded free electron lasers（FELs） is a promising way to produce fully coherent X-ray radiation. We study a new approach to produce coherent hard X-rays by cascading the recently proposed phasemerging enhanced harmonic generation（PEHG） The scheme consists of one dogleg and two PEHG configurations,and may be one of the leading candidates for the extracted undulator branch in future X-ray FEL facilities. FEL physics studies show that such a scheme is feasible within the present technology and can provide high brightness Xray radiation pulses with narrow bandwidth and full coherence The radiated peak power at 1 ？A wavelength converted from an initial 200 nm seed laser is over 2 GW

Charge-state populations for the neon-XFEL system

The interaction between neon and x-ray free-electron lasers with different laser parameters is systematically studied by solving a set of coupled rate equations. As an example, the evolution of 1s^12 s^22 p^6 configuration is given under different incident photon numbers, pulse widths, and photon energies. We have also determined all of the charge-state populations as a function of three laser pulse parameters by averaging over time. The result shows that the variations of these charge-state populations demonstrate a pattern when the pulse width is shorter than 10 fs: some of the charge-states decrease rapidly,while the others rise but remain relatively constant for pulse width larger than 10 fs. The variation of the average charge with three parameters has also obtained. The average charge decreases for a pulse width shorter than 10 fs but remains basically unchanged for a pulse width longer than 10 fs.

基于束流环形高斯模型对上海硬X射线自由电子激光装置的束流刮束器进行活化分析

在建的上海硬X射线自由电子激光装置(Shanghai HIgh repetition rate XFEL aNd Extreme light facility,SHINE)的加速器隧道内,采用束流刮束器刮掉束晕外围张角比较大的电子,刮束器的束流损失率为0.1‰。本文采用蒙特卡洛程序FLUKA,用SOURCE程序实现电子束在3σbeam外的高斯分布,以尽量真实模拟电子在束流刮束器处的丢失情况,解决了低功率刮束情况下蒙特卡洛模拟无统计性结果的难题。利用束流环形高斯模型对刮束器的活化进行了分析,结果表明环形高斯模型可以更真实地再现电子在束流刮束器的丢失情况;5年运行后距刮束器30 cm处的剩余剂量率达到了数百μSv/h至数mSv/h,和国外类似装置计算结果相当;通过对放射性核素的种类分析,结果证明刮束器(钨)的放射性废物处置难度不大。

X-ray diffraction performance of thermally distorted crystals

The development of high-brightness X-ray free electron lasers(XFELs),such as hard X-ray self-seeding free electron lasers and XFEL oscillators(XFELOs),brings a severe challenge to the crystal monochromator due to a strong nonuniform thermal load.The distortion caused by spatial temperature gradients can severely affect the optical performance of crystals.Therefore,this paper presents a model to estimate the performance of non-uniform thermally distorted crystals.The model not only takes into account thermal strain,slope error and incident angle deviation,but also considers temperature-dependent factors such as the Debye-Waller factor and electric susceptibility.Our investigation indicates that the Debye-Waller factor reduces the height and bandwidth of rocking curves,and the impact of the electric susceptibility is tiny.The proposed model can describe the distortion of the reflectivity and transmissivity curves of nonuniform thermally loaded crystals and can be applied in the design of crystal monochromators,crystal splitters,crystal compressors and XFELOs.

低温环境对超导腔位移振动特性影响的实验研究

利用高精度磁电式速度传感器可以测量超导腔在低温环境下的振动。定量描述低温环境对超导腔位移振动特性的影响,是实现超导直线加速器亚微米级束流稳定性要求并抑制机械振动导致腔频偏移的基础,这对模组机械和低温系统设计具有重要意义。上海光源机械团队以目前承建的“硬X射线自由电子激光装置(Shanghai HIgh repetitioN rate XFEL and Extreme light facility,SHINE)”1.3 GHz超导加速模组为研究对象,在超导腔常温振动测试基础上,采用频谱分析方法研究了超导腔在300.0 K、125.0 K、2.0 K三种温度下的位移功率谱密度、位移均方根和频响函数。测试结果表明:在上海光源地面本底振动下,2.0 K低温环境引起的超导腔垂向振动影响为本底的9.4%,横向振动影响为本底的4.5%。低温环境下新增振源为冷质流动,流体工质的状态对超导腔垂向和横向振动具有不同的影响。研究成果可用于指导模组测试以及结构设计优化。

Maximizing magnetic field generation in high power laser-solid interactions

In order to understand the transport of fast electrons within solid density targets driven by an optical high power laser,we have numerically investigated the dynamics and structure of strong self-generated magnetic fields in such experiments.Here we present a systematic study of the bulk magnetic field generation due to the ponderomotive current,Weibel-like instability and resistivity gradient between two solid layers.Using particle-in-cell simulations,we observe the effect of varying the laser and target parameters,including laser intensity,focal size,incident angle,preplasma scale length,target thickness and material and experimental geometry.The simulation results suggest that the strongest magnetic field is generated with laser incident angles and preplasma scale lengths that maximize laser absorption efficiency.The recent commissioning of experimental platforms equipped with both optical high power laser and X-ray free electron laser(XFEL),such as European XFEL-HED,LCLS-MEC and SACLA beamlines,provides unprecedented opportunities to probe the self-generated bulk magnetic field by X-ray polarimetry via Faraday rotation with simultaneous high spatial and temporal resolution.We expect that this systematic numerical investigation will pave the way to design and optimize near future experimental setups to probe the magnetic fields in such experimental platforms.

一种新型硬X射线超高能量分辨弯晶光谱仪

基于目前世界上先进光源的硬X射线光谱仪的设计算法,采用圆柱面强弯曲布拉格晶体分光实现高能量分辨。提出了一种新型二次衍射分光光谱仪结构,可在3~25 keV光子能量范围内实现单发脉冲2×10^(5)的能量分辨率。运用理论分析和数值计算验证了光谱仪的能量分辨性能,并通过机械扫描得到带宽大于1%的重组光谱。该技术可以用于测量上海硬X射线自由电子激光装置(SHINE)硬X射线波段自放大自发辐射光谱中的细锐结构以及用户科学实验中的高分辨光谱的探测和应用,具有重要的研究价值和意义。

自由电子激光器腔长的调节装置

本文论述了自由电子激光器腔长调节的基本原理及方法。用微机控制的精密机械调整装置实观了精细调节;用微机控制的电致伸缩器件实现了超精细调节。精细调节的范围为±3cm,精度优于0.5/Am;超精细调节的范围为±5/4m,精度优于0.1/4m。