Based on the data from beam position monitors (BPMs) using a high-speed oscilloscope, the transverse position and arrival time, equivalent to the longitudinal phase, of beam bunches, including the injected bunch have been measured and analyzed on a bunch-by-bunch basis. We present the results of applying this method to study the injection process at the High Energy Photon Source (HEPS). The beam transfer and injection in different areas, e.g., from the linear accelerator (LINAC) to the booster, from the booster to the storage ring (SR), and from the storage ring back to the booster—were analyzed using the oscilloscope. Valuable information, such as amplitude and frequency of the oscillation of the injected bunch, as well as the damping time are presented. This method exhibits the high sensitivity required for the detection of the longitudinal phase, allowing unambiguous identification of the longitudinal tune even after damping and decoherence. The longitudinal tune was measured at different energy points throughout the booster's energy ramping process, showing excellent agreement with theoretical values with a relative error of less than 5%. Furthermore, we used this method to analyze the energy and phase stability of the LINAC. The initial phase of the first turn demonstrated that the timing jitter of the LINAC beam is below 1.6 ps, and the oscillation amplitude confirmed that the energy stability of the LINAC beam is better than 0.02%. The methodology based on the oscilloscope has been successfully implemented at HEPS, highlighting its broad applicability across different types of electron accelerators, with a broad analysis potential.

Polarized lepton beams can substantially extend the physics capability of the Circular Electron Positron Collider (CEPC). Transversely polarized pilot bunches enable resonant-depolarization beam-energy calibration at the Z and W energies, while longitudinally polarized colliding beams would provide an additional spin observable for precision electroweak measurements and searches for physics beyond the Standard Model. This paper reviews the polarization requirements for CEPC and presents an updated source-to-collider strategy for generating, preserving, manipulating and measuring beam polarization. The proposed scheme combines a polarized electron source, a dual-purpose positron damping and polarizing ring, spin-preserving transport through the injector chain, booster-lattice optimization to avoid strong spin resonances, solenoid-based spin rotators in the collider ring and Compton polarimetry. Recent studies indicate that polarization transmission above 70% through the injector chain is achievable for applications at the Z and W energies, whereas polarization at the Higgs and t (t) over bar energies remains challenging because of stronger spin-resonance effects. The associated R&D program is summarized, and the main open issues for the CEPC Engineering Design Phase are identified.

The High Energy Photon Source (HEPS) is designed to be one of the world’s brightest synchrotron light sources and to provide ultra-stable beam quality. During the initial commissioning phase of the HEPS storage ring, significant orbit fluctuations induced by large-scale astronomical and geophysical phenomena were observed. From the perspective of beam dynamics, this study quantifies the impacts of the four dominant partial tides of the solid earth tides on the energy and horizontal orbit of the HEPS. Furthermore, this work also yields key spatiotemporal parameters of seismic events detected by the HEPS. This study bridges celestial motion, terrestrial internal dynamics, and the particle orbit behavior in accelerators used for probing the microcosm, thereby revealing the intrinsic homology of dynamic principles that govern physical processes across macroscopic and microscopic scales.

Fourth-generation synchrotron light sources employ ultralow-emittance storage rings with stringent injection requirements. On-axis swap-out injection alleviates the dependence on storage-ring dynamic aperture, but high-charge operation requires an efficient injector architecture capable of producing high-charge replacement bunches. This paper presents the accelerator physics design and performance analysis of a booster-based beam-recycling swap-out injection scheme implemented at the High Energy Photon Source (HEPS). In this approach, the full-energy booster serves as both an injector and a high-energy accumulator. An extracted storage-ring bunch is returned to the booster, merged with a low-charge bunch previously injected from the linac and accelerated to full energy. Following high-energy damping, the merged bunch is reinjected into the original storage-ring bucket. The scheme avoids the need for a dedicated accumulator ring while enabling high-charge bunch replacement. The recycling scheme was commissioned through staged machine studies. Full recycling-chain simulations, commissioning studies, and measured performance analysis are presented. The measured results characterize the recycling operation and quantify the transmission efficiency and performance limitations of the complete recycling loop. These results demonstrate the feasibility of the booster-based beam-recycling architecture and establish its operational basis for high-charge swap-out injection in future fourth-generation synchrotron light sources.

Orbit stability is a critical performance metric for modern synchrotron radiation facilities and colliders, necessitating effective orbit correction and slow orbit feedback systems. In recent years, innovative orbit feedback methods leveraging neural networks have been progressively implemented, demonstrating improved performance. BEPCII has also attempted to implement an orbit feedback system based on neural networks. Furthermore, we have noted that orbit stability is significantly affected by noises. To address the impact of noises, we introduced the Kalman filtering method into the orbit feedback system and combined it with the existing neural network framework, creating a dual neural network orbit feedback system. This innovative approach has led to further improvements in overall orbit stability.

The diffraction-limited storage rings (DLSRs) can provide an electron beam emittance below 100 pm center dot rad, approaching the hard X-rays diffraction limit with better transverse coherence and higher brightness compared to the current light sources, to the synchrotron light users. A round beam with equal horizontal and vertical emittances can effectively mitigate intra-beam scattering effects and increase the Touschek lifetime, however a horizontally elongated flat beam is preferable for some beamline applications with brighter photon beam. Depending on the specific requirement on the experiment, some users prefer high brightness with flat beam while others prefer high horizontal coherence with round beam. To address the limitation of being able to provide only one type of operation mode as either round beam or flat beam, this paper proposes two approaches for the simultaneous operation of both modes with several bunches in flat beam and the other in round beam, namely the flat beam generation in round-beam mode and the round beam generation in flat-beam mode. As an example to demonstrate its feasibility, the lattice design, dynamic aperture, and numerical simulations of these two approaches on the latest lattice of the High Energy Photon Source (HEPS) storage ring, are presented.

The High Energy Photon Source (HEPS) is designed to be one of the world’s brightest synchrotron light sources. In this paper, we provide an overview of the initial commissioning process of the HEPS storage ring, from the start of commissioning to the achievement of the first beam storage. We introduce the possible challenges that may arise during this critical phase of commissioning and detail the efforts made to achieve this important milestone in the HEPS storage ring commissioning. The commissioning process began with transportation through the high-energy transfer line BR (Booster-to-Ring), followed by beam injection and the first-turn transportation. This was followed by iterative trajectory correction and optimization, primarily utilizing a self-developed trajectory correction program based on the independently developed high-level application framework, Pyapas. Through iterative manual adjustment of the variable parameters, we successfully advanced the beam further. Subsequently, the RF cavities and sextupole magnets were gradually powered on. Through multi-turn trajectory correction and parameters optimization in a larger variable space, beam storage in the HEPS storage ring was achieved. On July 23, the first beam injection and the first-turn beam transportation were achieved within a few hours of starting the storage ring commissioning. By July 29, the beam circulation exceeded 10 turns, and by August 4, the beam circulated in the storage ring more than 1000 turns. On August 6, the first beam storage in the HEPS storage ring was achieved. Furthermore, the storage ring beam current reached approximately 60 microamperes on the same day. The successful beam storage is a significant milestone in the HEPS storage ring commissioning and a solid step toward the completion of the HEPS construction. It is hoped that the process to reach this achievement, as presented in this paper, will provide a useful reference for the commissioning of similar facilities both domestically and internationally.

For double-ring colliders, it is a vital task to make the two beams meeting each other properly at the interaction point. BEPCII is a double-ring collider that operates in the decay mode, as the beam currents decrease over time, the beam orbits need to be continuously adjusted to maintain the optimum collision conditions. Originally, this task was carried out manually, with operators adjusting three offset control knobs (x, y, y') according to the luminosity. There is a crucial necessity to implement advanced automated control approaches. However, the feedback methods are ineffective due to the constraints imposed by machine characteristics. Nevertheless, the optimization methods are also limited by the slow response speed of BEPCII and have intrinsic limitations. Reinforcement learning (RL), which learns from past experiences, provides a new approach for handling such problems. In this paper, we implemented two automated control methods: the dither method and the deep Q-network (DQN) RL method. The dither method is a numerical optimization method used to provide more online data for DQN training. With the help of the historical data from the dither method, we successfully trained a DQN agent to control the offset knobs to optimize the luminosity. The DQN method exhibited significantly better performance than the dither method, achieving faster optimization speeds and yielding higher integral luminosity. Furthermore, the DQN method has been integrated into the daily operations of BEPCII, achieving long-term stable deployment and effectively substituting manual labor.

The Goubau line method has been recently proposed and used for the longitudinal beam coupling impedance measurement of vacuum components. To extend the application of this method, the measurement of transverse impedance using the Goubau line method is proposed in this work. The feasibility of measuring the transverse impedance using the Goubau line method has been tested using a pillbox cavity. Additionally, the frequency shift induced by the dielectric-coated wire in the Goubau line measurements has been investigated analytically. The contribution of the dielectric coating to the frequency shift has been studied. Finally, the Goubau line setup has been optimized and used to evaluate the longitudinal and transverse beam coupling impedance key vacuum components in the High Energy Photon Source.

Several analytical formulas have been developed to calculate the Touschek lifetime of beam in electron storage rings under different assumptions. Among them, Piwinski’s formula is the most general and precise for unpolarized beams, widely applied in the mainstream accelerator simulation software. Lee’s formula is conventionally adopted to evaluate the influence of beam polarization on the Touschek lifetime, but it is less general than Piwinski’s formula when applied to unpolarized beams. In this paper, a new formula for the Touschek lifetime of ultrarelativistic polarized beams is derived in the theoretical framework of Piwinski. Its applications to several lattices of electron storage rings reveal that the influence of beam polarization on the Touschek lifetime is weaker than that estimated with Lee’s formula especially when the vertical emittance is large.

Purpose The physics design of the High Energy Photon Source (HEPS) was finished after many times of iteration. Hereby, the typical equilibrium electron beam parameters corresponding to the proposed two baseline operation modes in the baseline design of HEPS are presented. Methods To compute the equilibrium parameters of the electron beam, the lattice parameters, RF parameters, and the parameters of the insertion devices (IDs) were determined first. Furthermore, it is more precise to use the full-current electron beam parameters in the estimations of the performance of the synchrotron light. Therefore, not only the single-particle dynamics but also the current-dependent collective effects need to be considered in the computations of the full-current, equilibrium parameters of the electron beam. Both analytic computations and multi-particle tracking simulations were carried out. Results The full-current, equilibrium parameters of the electron beams in the HEPS storage ring are presented in this paper. Moreover, the main beam parameters in the injector (the booster and the LINAC), corresponding to the two baseline operation modes of the storage ring, are also presented. Conclusion The typical electron beam parameters corresponding to the two baseline operation modes are given in detail in this paper.

PurposeTo reduce the beam coupling impedance of the vacuum chamber made of poorly conducting material, a layer of high-conductivity metal, such as copper, is often coated on its inner surface. As the natural bunch length of modern accelerators is about several millimeters, its beam spectrum can reach tens of GHz. In this case, the skin depth of copper is of the same order of magnitude as its surface roughness, and its electrical properties can be different from that in DC, which will influence the beam coupling impedance. Therefore, the electrical property of copper coating at high frequency needs to be investigated.MethodsIn this paper, the method of resonant cavity is adopted to measure the coating conductivity, which is based on the relation between the quality factor of the cavity and material conductivity.ResultsThree different resonant modes are tested in the measurement, among which the H011 mode shows the best performance. The results also indicate that surface roughness of copper can have an influence on its effective conductivity at high frequency.ConclusionThe H011 mode is suitable for measuring high-conductivity materials. When the skin depth of copper is comparable to or larger than its surface roughness, its effective conductivity will be significantly reduced.

Purpose Round beam, i.e., with equal horizontal and vertical emittance, is preferable than a horizontally flat one for some beamline applications in Diffraction-limited storage rings (DLSRs), for the purposes of reducing the number of photons getting discarded and better phase space match between photon and electron beam. Conventional methods of obtaining round beam inescapably results in a reduction of dynamic aperture (DA). In order to recover the DA as much as possible for improving the injection efficiency, the DA optimization by using Non-dominated sorting genetic algorithm-II (NSGA-II) to generate round beam, particularly to one of the designed lattice of the High Energy Photon Source (HEPS) storage ring, are presented. Method According to the general unconstrained model of NSGA-II, we modified the standard model by using parallel computing to optimize round beam lattices with errors, especially for a strong coupling, such as solenoid scheme. Results and conclusion The results of numerical tracking verify the correction of the theory framework of solenoids with fringe fields and demonstrates the feasibility on the HEPS storage ring with errors to operate in round beam mode after optimizing DA.

Background:Long-term orbit stability is a key performance indicator in synchrotron radiation facilities and colliders nowa-days,in which the orbit correction and corresponding slow orbit feedback system are indispensable.Conventional method of orbit correction uses response matrix based on SVD algorithm,which becomes less effective after a long operation due to the fact that response matrix measurements cannot be taken during normal operation.Purpose:The purpose of this paper is to integrate machine learning model into the slow orbit feedback process and to automatically update the model online to better correct the orbit shifts.Methods:In this paper,we propose a method for slow orbit feedback of storage ring based on machine learning.Training the neural networks by using online data sets,which can establish the mapping relation between BPMs and correctors,and being updated automatically,without using extra time to remeasure the response matrix.Results:The experiments in this paper are all conducted and verified in the upgrading project of Beijing Electron-Positron Collider.By the way of learning automatically,the updated neutral network is closer to the real machine model,and the orbit after correction shows a smaller fluctuation relative to the golden orbit.Conclusion:Using the online data sets which reflect the response of orbit to correctors in real time to update the neural network can increase the orbit stability.

Masks are commonly used in light sources to protect sensitive elements from synchrotron radiations. In the ultra-low emittance rings, small aperture vacuum chambers are adopted in order to reach the very high gradient in the quadrupoles, while many masks are required due to the high radiation power density. Therefore, the impedance of the masks becomes one of the dominant contributors to the impedance budget. In this paper, the impedance is evaluated among different mask designs. Meanwhile, the impedance cross talk between adjacent masks is discussed.

Diffraction-limited storage rings (DLSRs), with sub-100pm⋅rad electron emittances approaching the diffraction limit for hard X-rays, would provide unprecedented transverse coherence and much higher brightness than currently operational light sources to the synchrotron radiation scientific community. Instead of typical flat beam, some individual synchrotron light users prefer round beam, a beam with equivalent emittances between the horizontal and vertical planes, for the reasons of fully transverse coherent X-ray radiation and better photon–electron phase space match, and so on. To meet the requirements of different experimenters, obtaining a locally-round beam will be a significant subject in DLSR studies. We investigate the approaches of realizing locally-round beam in a storage ring, by means of a local emittance equalization of the transverse planes. In this paper, a novel method is proposed to achieve locally-round beam, performed with a combination of skew quadrupoles in insertion section of the storage ring. Theoretical analysis and application of this method to achieve a locally-round beam at DLSRs, particularly to High Energy Photon Source (HEPS) storage ring, are presented.

X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. To further improve the spectrum brightness, a self-seeding FEL scheme has been developed and demonstrated experimentally. As the next step, new-generation FELs with high repetition rates are being designed, built and commissioned around the world. A high repetition rate would significantly speed up the scientific research; however, alongside this improvement comes new challenges surrounding thermal management of the self-seeding monochromator. In this paper, a new configuration for self-seeding FELs is proposed, operated under a high repetition rate which can strongly suppress the thermal effects on the monochromator and provides a narrow-bandwidth FEL pulse. Three-dimension time-dependent simulations have been performed to demonstrate this idea. With this proposed configuration, high-repetition-rate XFEL facilities are able to generate narrow-bandwidth X-ray pulses without obvious thermal concern on the monochromators.

One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

Strong intra beam scattering (IBS) effect and short Touschek lifetime become common issues for ultra-low-emittance storage ring (USR). One efficient approach to reduce these effects is operating such a machine under round-beam mode, a beam with equal horizontal and vertical emittance. Operating on the coupling resonance to obtain round beam will be most likely the method of choice for many of the future diffraction limited storage ring light sources. In this paper, we carried out a systematic study on round beam generation by driving linear difference coupling resonance using skew quadrupoles with same phase difference between horizontal and vertical planes. Theoretical analysis and application of this method at High Energy Photon Source (HEPS) storage ring are presented. Based on the simulation results of HEPS, we find that round beam can effectively mitigate the horizontal emittance growth due to IBS and increase the Touschek lifetime. Furthermore, the dynamic aperture is still be accommodated using on-axis injection when introducing reasonably skew quadrupole fields along the ring and closing to the coupling resonance to achieve round beam.