Synchrotron radiation from an electron storage ring holds many advantages such as stability, broad spectrum, multi-users supporting. However, it lacks coherence. When the coherent radiation is produced from a storage ring, not only the coherence of the light is enhanced, but also the flux, brightness and the energy resolution of the light can be remarkably improved. As the flux is further increased, the power of the radiation may reach the requirement of industry. This paper presents a review of various kinds of electron storage ring based coherent light sources and gives an outlook of its development.

Energy recovery linacs recapture the energy of the used electron beam into the electromagnetic field in RF cavities for subsequent acceleration, which can greatly reduce both the RF power supply required for high current acceleration and harmful radiation from the dumped beam. In addition to a high energy efficiency and reduced radiation from the dump, energy recovery linac light sources have the advantage of providing short-bunch, low-emittance electron beams for emitting high-brightness, highly coherent photons. These characteristics make them a very promising candidate for future advanced light source. This paper presents an introduction to energy recovery linacs, with the emphasis on basic principles and the most important physics and technical problems, as well as the activities in developing energy recovery linac facilities around the world. A brief introduction to some representative schemes of energy recovery linac light source is presented in the end.

A new drive laser system was developed to meet the requirement of DC-SRF-II photoinjector at Peking University. This modular drive laser system can operate from single pulse to 81.25 MHz CW mode, with green light output power 1.41 W @ 1 MHz. Longitudinal and transverse shaping were also implemented in the system, resulting in near flat-top 18 ps laser pulse with cut-Gaussian transverse profile. The stability measurement shows that the system output power stability is 1.8% RMS, and the pointing stability is less than 1 μrad.

The ultralow emittance storage ring light source based on multi-bend achromats is an important development direction of the new generation of synchrotron radiation light source. As the first fourth generation synchrotron radiation light source in China, High Energy Photon Source (HEPS) is under construction. The physics and engineering design has been finished for HEPS. The beam energy is 6 GeV, the beam current is 200 mA, and the horizontal natural emittance is less than 60 pm∙rad, promising radiation brightness of up to 1×1022phs·s-1·mm-2·mrad-2·(0.1%bw)-1 at typical hard X-ray regime. HEPS will be an important platform to support original and innovative research in the fields of basic science and engineering science. This paper will introduce the overall and physics design of the HEPS project.

The Hefei Advanced Light Facility (HALF) is a soft X-ray and VUV diffraction-limited storage ring light source, and the construction of HALF has just been approved by the Chinese government. The electron beam energy of the HALF storage ring is 2.2 GeV; the circumference is 480 m; the natural beam emittance is 86 pm·rad; and there are 20 long and 20 short straight sections in total. This paper will report the physics design progress of the HALF storage ring, including lattice design and optimization, simulation and calculation of beam injection and collective effects.

Over the past decade, the fourth-generation synchrotron light sources based on diffraction-limited storage rings (DLSRs) have been extensively designed and developed around the world. In China, two fourth-generation synchrotron light sources, the High Energy Photon Source and the Hefei Advanced Light Facility, are being or will be constructed. This paper will report the main issues and progresses in physics design and optimization of DLSRs, including lattice design and optimization, beam injection and collective effects, and will also introduce the current design and construction status of DLSR facilities around the world.

Hefei Advanced Light Facility (HALF) is aimed to be a world-class diffraction limited storage ring (DLSR) in the soft X-ray & VUV regime. The characteristics and law of beam emittance evolution due to intra-beam scattering (IBS) in HALF storage ring was studied based on equations of equilibrium emittance. The results show that, it is necessary to comprehensively optimize the key parameters to obtain excellent beam performance in physical design of middle to low-energy DLSR. Applying the key parameters optimization strategy and taking methods to release the emittance growth due to IBS into consideration, the new version of HALF lattice can meet the needs of soft X-ray diffraction limit.

Driven by the demand of science and technology, synchrotron radiation (SR) facilities continue to develop. At present, the development of SR has gone through three generations, and is in the vigorous development stage of the fourth generation (4th). SR based on diffraction-limited storage ring is one of the typical representative of the 4th synchrotron light sources. The mainstream of the 4th SR is to further reduce the electron beam emittance, so that the light source from the 4th SR exhibits excellent transversal coherence and is able to produce circular cross-section radiation. If the beam emittance drops to the optical diffraction limited “radiation wavelength/4π”, its brightness enhances to 2-3 orders of magnitude higher than that of the third generation SR light source. The qualitative leap in the performance of this SR light source will bring a substantive breakthrough to the SR-based experimental techniques, and bring new opportunities to the cutting-edge scientific and technological research, as well as the development of modern industry. Starting with the development trend of worldwide SR facilities, this review first introduces the characteristics and performance of low-energy diffraction limited storage ring (DLSR) light source, then introduces the progress of experimental techniques brought by DLSR, and illustrates the application of low-energy DLSR light source in material science, energy science, life science and environmental science, as well as its industrial opportunities. Finally, the technical breakthrough and potential application prospect of low energy DLSR light source are summarized and prospected.

The Hefei Advanced Light Facility (HALF) is a diffraction limited storage ring (DLSR).The extracted light of HALF has higher brightness resulting in higher heat load to the storage ring. The redundant synchrotron radiation is absorbed by the photon absorber located in the front-end to protect the ultrahigh vacuum system of DLSR. A special design of the photon absorber is required due to the compact physical design. Considering the toothed surface profile, cooling channel, and installation, we propose a photon absorber made of CuCrZr without additional positioning on the basis of the two-piece vertical absorber. The spot size and power of the radiation from the bending magnet with a bending angle of 2.74° are calculated. The thermal-mechanical simulations based on the finite element analysis method show acceptable results. The maximum thermal deformation, temperature, and stress are 0.05 mm, 80 ℃, and 20.8 MPa, respectively, indicating that the new absorber works in a safe range. The present study provides a critical theoretical basis for the design of the photon absorber in the front-end of HALF.

Flash X-ray radiography is a technique that uses short pulse X-rays generated by accelerators to perform fluoroscopic imaging of fast-moving dense objects. This paper summarizes the performance requirements for X-rays in flash X-ray radiography, then reviews the development of flash X-ray radiography and its accelerators, and finally analyzes the future research direction of accelerators for flash X-ray radiography.

Laser plasma accelerator can accelerate electron beams to GeV level within a few centimeters, which provides a driving source for the development of table-top free electron laser. However, due to the difficulties in laser plasma acceleration and the development of existing technology, the quality of electron beam is not high enough to realize free electron laser, especially in the aspects of stability, divergence and energy spread of electron beam, which present substantial obstacles to the development of compact free electron laser. In this paper, the latest research progress of free electron laser based on laser plasma accelerator is introduced, the main challenges in the experiment for high gain free electron laser are analyzed, then corresponding solutions and experimental progress are summarized, and the future development is prospected. A recent research shows that the high-quality electron beam produced by controlling and optimizing the injection and acceleration of laser plasma accelerator can achieve spontaneous emission amplification and produce high gain radiation in the exponential gain regime, which promotes the research for free electron laser based on laser plasma accelerator to a new stage.

Inverse Compton scattering X/gamma-ray source can produce quasi-monochromatic, continuously tunable, high brightness, small spot size, polarization precisely controllable, and ultrashort (ps or sub-ps) X-ray pulse in the energy regime ranging from tens keV to several MeV or even higher. Recently a 0.2-4.8 MeV quasi-monochromatic compact gamma-ray source with high peak spectral density based on the inverse Compton scattering has been proposed in the Department of Engineering Physics, Tsinghua University. This type of compact gamma-ray source will be used for advanced X/gamma-ray imaging application based on the nuclear resonance fluorescence. In this paper, we will present the optimization of the design.

Terahertz radiation has important prospects in fundamental science and industrial applications, but traditional electronic and optical methods can hardly generate high-power, narrow-band and continuously tunable coherent terahertz radiation at 1-10 THz. Accelerator-based terahertz sources using relativistic ultrashort electron beams or pre-modulated electron bunch trains have the possibility to generate tunable high spectral energy density narrow-band terahertz radiation in the above-mentioned range. This article reviews the recent theoretical and experimental progresses of the Tsinghua University Accelerator Laboratory in accelerator terahertz sources based on relativistic electron beams, as well as the terahertz radiation measurement, beam diagnosis and advanced acceleration technologies developed together with the accelerator terahertz sources.

The Chinese Academy of Engineering Physics Terahertz Free Electron Laser Facility (CAEP THz FEL, CTFEL) is the only high-average power free electron laser terahertz source based on superconducting accelerators in China, with the advantages of continuously adjustable frequency (0.1-4.2 THz), high peak power (＞0.5 MW), high average power (＞10 W), high repetition rate (54.17 MHz), short pulse (～ps), narrow bandwidth (～2%), full coherence and linear polarization. Since the first lasing in 2017, it has been running stably for more than four years, and many experimental studies have been carried out. To further meet the demands of users, an upgrade plan for an infrared terahertz free electron laser facility based on CTFEL is proposed, where the electron beam energy is increased to a maximum of 50 MeV, and the spectrum range is expanded to 0.1-125 THz. Meanwhile, two experimental stations for material spectroscopy and biomedicine will be built.

Superconducting accelerator, which uses SRF cavity working at cryogenic environment, can operate in macro-pulse or CW modes. Due to its large beam aperture, the interaction between beam and cavity can be reduced remarkably. After the development of more than a half century, SRF technology is quite advanced and has been applied in different kinds of light sources. In this paper, basic principle of SRF, fabrication of elliptical SRF cavity and structure of typical cryomodule of SRF accelerator are introduced.

The pre-tuning of frequency and field flatness is one of the most time-consuming post-processing procedures for 9-cell superconducting cavities, and will soon become the bottleneck of mass production of 9-cell cavities in domestic related major scientific projects. In this paper we firstly introduce two commonly used pre-tuning methods for 9-cell superconducting cavities, namely DESY method and Cornell method. Then we analyze and compare their calculation accuracy and error sources by modeling, and make a correction on the Cornell method’s tuning amount calculation. Verifing the pre-tuning of several cavities by the experimental research, we give a fast pre-tuning method in which DESY reconstruction algorithm is used for coarse-tuning as it has high precision and rapid tuning speed in low field flatness and Cornell perturbation algorithm is used for fine-tuning as it has high precision in high field flatness with faster measurement. Combining these two tuning algorithms, the pre-tuning is divided into two steps: coarse tuning and fine tuning, which can effectively improve the pre-tuning speed of the 9-cell superconducting cavity.

The photoinjectors are promising high quality electron sources for X-ray free electron laser (XFEL). The uniformity of the electron emission from the photocathode has big influence on the electron beam quality. Usually, the quantum efficiency (QE) mapping is used to evaluate the uniformity in experiments. The QE mapping imaging method used to measure the QE mapping has high resolution and it can be used in real-time measurement, so far it is only used in separated type photoinjectors. In our photoinjector, the electric field and magnetic field are overlapping in the cathode vicinity, according to theoretical analysis and simulations, we analyze the application of this method in overlapping field. Finally, we demonstrate that the imaging method is suitable for this overlapping field, and the results show that the QE mapping has higher resolution in the center of the photocathode than the outer part. We also analyze the imaging method used to measure the transverse momentum distribution, and one way is proposed to judge whether the rest magnetic field on the cathode is zero or not.

We present the design of a 100 W high repetition rate photocathode drive laser system for realizing high average current operation of the superconducting accelerator at Peking University. To achieve good beam quality and reliability, we choose photonic crystal fiber (PCF) as the gain medium of the main amplification unit. In addition, we address several key issues for the drive laser system, including the evaluation of the output power of each amplification unit, the design of pulse stretcher and compressor, the optimization of free space coupling setups for pump pulse and seed pulse, etc. We also combine a high-speed semi-conductor optical amplifier (SOA) optical switch with a low-speed acousto-optic modulator (AOM) to achieve the necessary diagnostic mode for the intense electron beam accelerator. This unique design is of importance for the photocathode drive laser with the repetition rate around or above 100 MHz.