Towards one machine, two purposes: using a common SC linac for XFEL and ERL simultaneously

The linac based XFEL and ERL are advanced （or, say, 4th generation） light sources, with different electron beam parameters and different advantages. However, the linac used for XFEL and ERL should provide very advanced beams with high energy, high peak and/or average current, very low emittance and low energy spread, thus making the linac very complicated and expensive. To share the XFEL and ERL advantages and save the construction-operation budget, a proposal of using a common superconducting electron linac for hard X-ray XFEL and ERL is described in this paper. The interactions between the XFEL and ERL beams via the accelerating structure are studied and the result is positive.     

Constructions and Preliminary HV Conditioning of a Photocathode Direct-Current Electron Gun at IHEP

As one of the most important key technologies for future advanced light source based on the energy recovery linac, a photocathode dc electron gun is supported by Institute of High Energy Physics(IHEP) to address the technical challenges of producing very low emittance beams at high average current. Construction of the dc gun is completed and a preliminary high voltage conditioning is carried out up to 440 kV. The design,construction and preliminary HV conditioning results for the dc gun are described.     

光阴极注入器型能量回收射频加速器

 光阴极注入器型能量回收射频加速器（PERL）是新一代加速器，在高平均功率自由电子激光和下一代高亮度光源等研究中有很好的应用前景。分析了PERL的强流与高平均功率特性，对注入器输出束流品质的要求及光阴极注入器、超导加速腔等关键技术进行了研究，设计分析了一种特殊结构的高压DC Gun光阴极注入器，能有效地提高DC加速腔中的加速场强，当高压为1MV和加速场达到10MV/m时，产生的电子束流能够基本满足PERL应用要求。同一超导加速段中的束流加速和能量回收的数值模拟计算结果表明，能获得高效率电子束流能量回收效果。     

基于能量回收技术的光源——ERL光源

能量回收技术将使用后的电子束进行能量回收,用于加速后续束团,可大大减少加速器消耗的射频功率和有害辐射。基于能量回收技术的光源除节能环保外,还具有束团短、发射度低的特点,可有效提高光源的峰值亮度和相干性,是一种很有潜力的未来先进光源。介绍能量回收直线加速器技术的基本原理、相关关键物理问题和技术以及能量回收直线加速器发展现状,最后简要介绍几个国际上提出的典型能量回收直线加速器光源方案。     

An accelerator scenario for a hard X-ray free electron laser combined with high energy electron radiography

In order to study the dynamic response of the material and the physical mechanism of fluid dynamics,an accelerator scenario which can be applied to both hard X-ray free electron laser and high energy electron radiography is proposed.This accelerator is mainly composed of a 12 GeV linac,an undulator branch and an eRad beamline.In order to characterize a sample's dynamic behavior in situ and real-time with XFEL and eRad simultaneously,the linac should be capable of accelerating the two kinds of beam within the same operation mode.Combining in-vacuum and tapering techniques,the undulator branch can produce more than 1011 photons per pulse in 0.1%bandwidth at42 keV.Finally,an eRad amplifying beamline with 1:10 ratio is proposed as an important complementary tool for the wider view field and density identification ability.     

A compact warm ring for industrial applications

The ultrafast, high brightness X-ray free electron laser (XFEL) sources of the future have the potential to revolutionize the study of time-dependent phenomena in the natural sciences. These linear accelerator (linac) sources will generate femtosecond (fs) X-ray pulses with peak flux comparable to conventional lasers, and far exceeding all other X-ray sources. The Stanford Linear Accelerator Center (SLAC) has pioneered the development of linac science and technology for decades, and since 2000 SLAC and the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the development of linac based ultrafast electron and X-ray sources.     

Design study on the merger for BXERL-FEL

In North China, there is a preliminary proposal for ERL-FEL light source (BXERL-FEL) with its aim at "one machine, two purposes" (the XFEL and ERL work simultaneously). One of the key technologies is the merger section. In this paper, we give the physical design of the merger section for BXERL-FEL which merges three kinds of electron beam.     

New linear accelerator （Linac） design based on C-band accelerating structures for SXFEL facility

A C-band accelerator structure is one promising technique for a compact XFEL facility. It is also attractive in beam dynamics in maintaining a high quality electron beam, which is an important factor in the performance of a free electron laser. In this paper, a comparison between traditional S-band and C-band accelerating structures is made based on the linac configuration of a Shanghai Soft X-ray Free Electron Laser (SXFEL) facility. Throughout the comprehensive simulation, we conclude that the C-band structure is much more competitive.     

Recent Advancements of Undulator Technologies

Insertion devices—or more specifically, undulators—play a crucial role in actually producing intense light from a high-energy electron beam in synchrotron radiation (SR) and X-ray free electron laser (XFEL) facilities. Their function is quite simple; i.e., to let the electrons move periodically. Even so, there are many different concepts of undulators, and thus, technologies to realize these concepts are quite diverse. There are a variety of requirements dictated by the experimental users of SR and XFEL, mainly in terms of the polarization and wavelength of radiation. In addition, the specifications of undulators should meet these requirements under the given boundary conditions, such as the minimum operational gap, electron energy, and available area for installation. However, necessity is the mother of invention; undulator technologies and concepts have continuously advanced year by year to meet users' demands.     

Physical design study of the CEPC booster

A physical design study of the Circular Electron-Positron Collider(CEPC) booster is reported. The booster provides 120 GeV electron and positron beams for the CEPC collider with top-up injection. The booster is mounted above the collider in the same tunnel. To save cost, the energy of the linac injector for the booster is chosen as 6 GeV, corresponding to a magnetic field of 30.7 Gs. In this paper, the booster lattice is described and optimization of the cell length is discussed. A novel scheme of bypass near the detector of the collider is designed.The extremely low magnetic field caused by low injection energy is studied, and a new ideal of wiggling bands is proposed to mitigate the low-field problem. Beam transfer and injection from the linac to the booster are considered.     

Design study on the merger for BXERL-FEL

In North China, there is a preliminary proposal for ERL-FEL light source （BXERL-FEL） with its aim at ＂one machine, two purposes＂ （the XFEL and ERL work simultaneously）. One of the key technologies is the merger section. In this paper, we give the physical design of the merger section for BXERL-FEL which merges three kinds of electron beam.     

CRRFA-30L波段射频加速器

 主要介绍CRRFA-30L波段射频加速器结构和性能，论述了热阴极射频腔注入器、束流加速系统、微波功率源和控制等结构中主要技术及其研究进展，给出了加速器输出束流参数测量与测量结果分析，达到设计应用要求。     

Great progress in developing 500 MHz single cell superconducting cavity in China

Superconducting cavities have been adopted in many kinds of accelerator facilities such as synchrotron radiation light source, hard X-ray free electron laser linac, colliders and energy recovery linacs (ERL). The 500 MHz superconducting cavities will be a candidate to be installed in the high current accelerators and high current ERLs for their large beam aperture, low higher order modes impedance and high current threshold value. This paper presents great progress in the whole sequence of developing 500 MHz superconducting cavity in China. It describes the first in-house successful development of 500 MHz single cell superconducting cavity including the deep-drawing of niobium half cells, electron beam wielding of cavity, surface preparations and vertical testing. The highest accelerating gradient of the fabricated cavity #SCD-02 higher than 10 MV/m was obtained while the quality factor was better than 4×10⁸ at 4.2 K, which has reached the world level of the same kind of cavities.     

TRIUMF ARIEL 电子直线加速器设计

 TRIUMF 的三期升级工程(ARIEL) 计划建造一个 50 MeV 平均流强为10 mA的电子直线加速器作为注入器,通过光裂反应生成放射性核素。电子直线加速器包括两个主要部分：注入器和后加速器,注入器完成电子能量从100 keV到10 MeV的转换,随后的后加速器将电子能量从10 MeV加速到50 MeV。电子源拟采用重复频率为650 MHz的热电子枪提供初始能量为100 keV,束长为 171 ps的电子束。束流动力学模拟了几种不同的设计方案以获得最优化的设计,模拟显示通过对腔体以及聚焦元件的仔细设计以及电子枪出射电子的参数选择, 电子束能量在达到50 MeV时束长可以被聚焦到 11.75 ps (对应于1.3 GHz 频率下5.5°) ,并且可以使电子束在超导低温柜中的尺寸保持在1.26 cm以下。     

Grazing incidence off-plane lamellar grating as a beam splitter for a 1-Å free electron laser

A beam splitter based on the diffraction grating working in the grazing incidence conical diffraction is proposed for x-ray free-electron laser (XFEL) experiments with coherent beams in plasma and atomic physics. Such a beam splitter can provide undistorted wavefronts, high-power radiation scattering, and split beams with equal intensities, which propagate with delay along different paths to the target chamber of the XFEL end station. Using the PCGrate software based on rigorous electromagnetic theory and developed for the short-wavelength range, it is shown that the plane grating with lamellar groove profile of a certain depth, operating in grazing conical incidence mount (grooves are parallel to the incident beam), separates three beams in the ?1, 0, and +1 orders with close diffraction efficiency. Numerical simulation predicts 23–27% absolute efficiency for 0.1-nm incident radiation in each separated order of a bulk or multilayer grating, taking into account the atomic level roughnesses and interdiffusion. When using a multilayer coating based on Ru/C or Ru/B₄C pairs, the optimum grazing angle providing approximately equal efficiencies is ～1.038° which is four times higher than for the Pt-coated grating. Such optimization of radiation geometry, groove profile shape, and multilayer coating parameters can be performed for various XFEL wavelengths. The proposed grating, in addition to diffraction, technological, and design advantages over the beam splitter based on a set of perfect crystals, can be fabricated and tested using currently available methods.     

Expected thermal deformation and wavefront preservation of a cryogenic Si monochromator for Cornell ERL beamlines

Cornell energy‐recovery linac (ERL) beamlines will have higher power density and higher fractional coherence than those available at third‐generation sources; therefore the capability of a monochromator for ERL beamlines has to be studied. A cryogenic Si monochromator is considered in this paper because the perfect atomic structure of Si crystal is needed to deliver highly coherent radiation. Since neither the total heat load nor the power density alone can determine the severity of crystal deformation, a metric called modified linear power density is used to gauge the thermal deformation. For all ERL undulator beamlines, crystal thermal deformation profiles are simulated using the finite‐element analysis tool ANSYS, and wavefront propagations are simulated using Synchrotron Radiation Workshop. It is concluded that cryogenic Si monochromators will be suitable for ERL beamlines in general.     

Beam stability investigation for a free electron lithographic laser based on an energy-recovery linac

According to leading producers of microelectronic devices, lithography based on free electron lasers (FEL) could become the main technology for the mass production of elements with a scale up to 5 nm in the near future. One of the main hindrances in this path is the absence of working FEL with the required parameters. A feasibility study devoted to the production of such an FEL based on a superconducting energyrecovery linac (ERL) has been carried out at the Budker Institute of Nuclear Physics (BINP). The ERL average current is limited by longitudinal and transverse instabilities, caused by the interaction of an electron beam with the fields induced by it in the superconducting cavities. The estimations of the threshold currents and parameters of the ERL required for the operation of FEL are obtained.     

Monitoring the energy variation of an electron linac using a Cerenkov detector

A new method to monitor the energy variation of a multi-energy electron linac by combining a Cerenkov detector and a CsI(Tl) detector is reported. The signals in the Cerenkov detector show an appreciable but different dependence on the energy of the electron linac from the traditional CsI(T1) detector due to the particular response of the former to charged electrons with high velocity above threshold. The method is more convenient than the HVL (half-value layer) method which is commonly employed to calibrate the energy of an electron linac for real time monitoring. The preliminary validity of the method is verified in a dual-energy electron linac with 6 MeV and 3 MeV gears. Moreover, the method combining the Cerenkov detector and the CsI(Tl) detector is applicable to probe the X-ray spectrum hardened by the inspected material and may serve as a novel tool for material discrimination with effective atomic number in radiation imaging.     

Femtosecond X-ray generation through relativistic electron beam–laser interaction

Methods for generating ultra-short X-rays using the interaction of intense laser pulses with relativistic electron beams, and their application to measuring ultra-fast phenomena in solid state materials, are reviewed. Two different methods that use a long electron bunch and short laser pulse are discussed: Thomson scattering and optical slicing which have been implemented on linac and storage ring beams, respectively. The possibility of generating ultrashort electrons bunches from laser-plasma injectors is discussed.