The Beijing Electron Positron Collider II (BEPCII) has achieved a series of achievements in high-energy physics study. Along with the deepening of the research, more important physics is expected in higher-energy region (e.g., 2.35 GeV). As the upper limit of BEPCII design energy is 2.1 GeV, an urgent upgrade is required. To achieve a higher luminosity at higher energy, the number of RF cavities is increased from one to two in each ring, such that the delivered beam power could be doubled and the bunch length can be reduced. With a doubled beam current and reduced beta function at IP, a tripled luminosity at 2.35 GeV is expected after the upgrade. The magnets in RF region are replaced with new ones which have smaller sizes, and the RF region is rearranged to accommodate two RF cavities in each ring. The accelerator design code SAD is used to rematch the linear lattice and do the dynamic aperture (DA) tracking. The candidate positions of new skew quadrupoles (SKQs) are ranked by the coupling correction capability. The code Accelerator Physics Emulation Studio (APES) is used to remodel the lattice. The impedance is simulated by CST and beam instability is evaluated accordingly. The layout is modified and the linear lattice is rematched. The dynamic aperture tracking result show that the lattice could meet the injection and collision requirements of BEPCII upgrade with reasonable margin. The locations of the new SKQs have been strategically selected, and the new SKQs demonstrate a remarkable improvement in coupling correction. APES modeling indicates that the lattice requires further investigation. The impedance and the beam instability evaluated results show enough margin for high-current operation.

Precision beam polarization measurements based on Compton polarimeters are essential for the physics program of future high-energy colliders. In order to prepare for these and to extend the scope of physics measurements of the BESIII experiment at the BEPCII, a diagnostic of electron beam transverse polarization at BEPCII is of interest. The design and status report of the commissioning, until July 2025, of this device is reported in this paper. We report unambiguous observation of Compton interaction, discuss current limitations of the experimental setup and draw prospects for improvements and actual measurement of electron beam polarization in the near future.

The Beijing Electron Positron Collider (BEPC) upgrade scheme, BEPCII, has shown good performance in both synchrotron radiation and high-energy physics experiments since 2009. The BEPCII upgrade project was approved in July 2021, according to the new requirements for high-energy physics experiments. The luminosity optimization process of BEPCII from 2008 to 2024 and its upgrade project are described in detail. The peak luminosity during the data taking reached 1.1 × 1033 cm-2 s-1 at the beam energy of 1.884 GeV in January 2023. A total of 52.7 fb-1 of data were collected within the beam energy region of 0.920–2.472 GeV, and a series of achievements in the field of high-energy physics has been noted during the past 16 years. The upgrade scheme aims to increase the luminosity by a factor of three at the beam energy of 2.35 GeV and extend the maximum beam energy from 2.47 to 2.8 GeV. The installation of the upgrade project started on July 1, 2024, and the commissioning started on May 2, 2025.

The BEPCII upgrade project necessitates two new final focus superconducting magnets on both sides of interaction point to achieve a high luminosity at higher beam energy. Each superconducting magnet consists of a quadrupole magnet for final focusing the beam and an anti-solenoid for canceling the magnetic field of Detector solenoid. Compared with the BEPCII magnet, the magnetic field gradient of new superconducting quadrupole (SCQ) increases from 18.7 to 25 T/m. The field harmonics of SCQ are required to be less than 3 × 10–4. Use line current approximation and field simulation by OPERA, the magnetic design of serpentine coil of SCQ is determined, and the calculated field harmonics are less than 0.5 × 10–4. According to the theoretical trajectory of each turn conductor, the SCQ coil is wound layer by layer using direct winding technology. Room temperature magnetic field measurement and cryogenic vertical test at 4.2 K are used to check the field quality and validate the magnet fabrication process. Cryogenic horizontal test at 4.5 K shows that the magnetic field performance of SCQ and anti-solenoid of the two magnets meets the design requirements. The measured field harmonics of SCQ are less than 2 × 10–4 corresponding to the nominal field gradient 25 T/m. The measured magnetic field distribution of anti-solenoid is consistent with field simulation result. Two new final focus superconducting magnets have been successfully developed for BEPCII upgrade project. They were installed in upgraded BEPCII interaction region at the end of 2024, and have maintained stable operation under the Detector solenoid field without any quench since May 2025.

With the high current physical operations of the upgraded Beijing Electron-Positron Collider (BEPCII), thresholds on collision luminosity and beam current have been presented due to various factors such as collision background, noise, equipment stability under high power operation, and soon. One of the most serious influences on beam dynamics was beam instability which has been clearly exhibited. The comprehensive experimental investigation of beam instabilities in BEPCII is an indispensable part of beam physics research and can provide references for the upgrade project of BEPCII. Over the past two years, the experimental investigation of beam instabilities in BEPCII has been carried on with the single and multiple bunch filling in the storage ring. Various impedances, the sources of beam instabilities, including broadband and narrowband impedances in longitudinal and transverse planes are studied. The measured impedance values are compared with the initial design of BEPCII.

The CEPC booster has been designed to provide electron and positron beams at different energies for the collider. The latest booster design aligns with the TDR's higher luminosity objectives for four energy modes. The booster's optics have transitioned from FODO in the CDR to TME structure, resulting in a significant reduction in emittance to match the lower emittance of the collider in the TDR. Extensive efforts have been invested to address the challenge of error sensitivity for the booster, ensuring that the dynamic aperture with errors meets the requirements across all energy modes. Additionally, a combined magnets scheme (B + S) has been proposed to minimize the magnet construction costs and reduce the operation costs through lower power consumption. This paper discusses the design status of the CEPC booster in the TDR, encompassing parameters, optics, dynamic aperture, ramping scheme, and injection scheme.

CDEX-50 is a next-generation project of the China Dark Matter Experiment (CDEX) that aims to search for dark matter using a 50-kg germanium detector array. This paper comprises a thorough summary of the CDEX-50 experiment, including an investigation of potential background sources and the development of a background model. Based on the baseline model, the projected sensitivity of weakly interacting massive particle (WIMP) is also presented. The expected background level within the energy region of interest, set to 2--2.5 keVee, is $\sim$0.01 counts keVee$^{-1}$ kg$^{-1}$ day$^{-1}$. At 90\% confidence level, the expected sensitivity to spin-independent WIMP-nucleon couplings is estimated to reach a cross-section of 5.3 $\times$ 10$^{-45}$ cm$^{2}$ for a WIMP mass of 5 GeV/c$^{2}$ with an exposure objective of 150 kg$\cdot$year and an analysis threshold of 160 eVee. This science goal will correspond to the most sensitive results for WIMPs with a mass of 2.2--8 GeV/c$^{2}$.

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.

This paper describes the injection philosophy and the design of timing and filling scheme for the high luminosity Circular Electron and Positron Collider (CEPC) scheme under different energy modes. It is found that the RF frequency choice in Conceptual Design Report (CDR) cannot meet the injection requirements for the bunch number at the Z pole. A modified scheme was proposed to support the design luminosity, which basically meets our current design requirements and retains more flexibility for a future high-luminosity upgrade.

The Circular Electron Positron Collider (CEPC) is a proposed Higgs factory with center-of-mass energy of 240[Formula: see text]GeV to measure the properties of Higgs boson and test the standard model accurately. Synchrotron radiation (SR) generated from the final doublet (FD) magnets in the interaction region of CEPC double ring scheme is one of the typical issues. SR photons can contribute to the heat load of the beam pipe and cause photon background to the experiments. Furthermore, the radiation dose can damage detector components. In this paper, SR generated from FD magnets is analyzed when beam is with tails and offset. SR from the dipole leakage field of the FD superconducting magnets is also analyzed and the physics limit is given to protect the detector.

The Beijing Electron Positron Collider II (BEPCII) has achieved a series of achievements in high energy physics study. Along with the deepening of the research, more im-portant physics is expected in higher energy region (>2.1 GeV). As the upper limit of BEPCII design energy is 2.1 GeV, an urgent upgrade is required for BEPCII. To achieve a higher luminosity at higher energy, the number of RF cavities is expected to be doubled. In this paper, the lattice design for the upgrade of BEPCII is studied. The modification of the collider layout to accommodate more cavities is introduced. The linear lattice design and the Dynamic Aperture (DA) optimization results will also be shown. The dynamic aperture tracking result show that the lattice could meet the injection requirement of BEPCII beam with reasonable margin.

A quadratic increase of the synchrotron tune with beam current has been observed during the operation of the Beijing Electron Positron Collider Upgrade (BEPCII). This observation is unexpected considering the conventional theory of the impedance-driven collective effects that the tune shift varies linearly with the beam current. To explain this phenomenon, a simple physical model based on coupled bunch instability theory is developed. The coherent synchrotron frequency is solved directly with a clean form without taking the perturbative approach which is valid for ΔΩ<<ωs. With the model developed, the source of the quadratic increase of the synchrotron tune is found and identified as being due to the interaction with the fundamental mode of the RF cavities. On the other hand, our analysis also offers a new beam-based method to characterize the R/Q and the detuning rate of the fundamental mode of the RF cavity, which can be complementary to laboratory bench measurements.

The CEPC booster needs to provide electron and positron beams to the collider at different energy with required injection efficiency. At Higgs energy, only the on-axis injection from booster to collider can be fulfilled in CDR. With a consideration of keeping the off-axis injection scheme for safety and reliability, a new booster design based on TME lattice is considered to reduce the emittance by three times after CDR. The new booster design has reached an emittance of 1.3 nm at 120 GeV and the DA without errors is even better than CDR. The geometry of new booster is designed carefully in order to share the same tunnel with collider. The design status of CEPC new booster including parameters, optics, dynamic aperture and geometry is discussed in this paper.

With the discovery of the Higgs boson at around 125 GeV, a circular Higgs factory design with high luminosity ([Formula: see text]) is becoming more popular in the accelerator world. The CEPC project in China is one of them. Machine Detector Interface (MDI) is the key research area in electron–positron colliders, especially in CEPC, since the synchrotron radiation (SR) photons can contribute to the heat load of the beam pipe and radiation dose may damage the components. And the heat load can cause the temperature rise in some part, and if the temperature rise is too high, the beryllium pipe in the interaction region will melt and the superconducting magnet may quench. Thus, the heat load distribution from synchrotron radiation and beam loss in the interaction region are analyzed carefully and results are given in this paper.

With the discovery of the Higgs boson at around 125 GeV, a circular Higgs factory design with high luminosity [Formula: see text] is becoming more popular in the accelerator world. The Circular Electron and Positron Collider (CEPC) project in China is one of them. Machine Detector Interface (MDI) is the key research area in electron–positron colliders, especially in CEPC. Since the [Formula: see text] beams collide at the Interaction Point (IP) with a horizontal angle of 33 mrad, the horizontal trajectory will couple to the vertical. Due to the solenoid and anti-solenoid combined field strength quite high, the maximum could be up to 4.2 T, the transverse magnetic field component is also quite high. Thus synchrotron radiation (SR) from vertical trajectory in combined field should be taken into account. And also synchrotron radiation is an important influential factor in the collimator design of CEPC MDI. These two effects are analyzed in this paper.

For the past generation ${e}^{+}{e}^{\ensuremath{-}}$ storage ring colliders, we usually used natural bunch length or its impedance lengthened value in beam-beam simulations instead of considering the impedance directly. In the future colliders, such as FCC-ee and CEPC, the beam-beam interaction becomes essentially three dimensional. In order to increase the luminosity, the future accelerators will collide very intense beams of high energy with low emittances and small beta functions at the collision points exploiting the crab waist collision scheme with a large Piwinski angle. For these extreme parameters several new effects become important for the collider performance such as beamstrahlung, coherent X-Z instability, 3D flip-flop so that the longitudinal beam dynamics should be also treated in a self-consistent manner. In this paper we describe the numerical code for the self-consistent 3D beam-beam simulations including beamstrahlung and the longitudinal beam coupling impedance and study interplay of different effects arising in beam-beam collisions of the future colliders.

In the tracking of the future circular Higgs factories, significant reduction of dynamic aperture due to the synchrotron radiation has been observed. This paper will analyze the synchrotron radiation effects including the closed orbit and optics distortion, radiation damping and quantum excitation for the lattice of CEPC.

This paper reports some preliminary study into the implementation of longitudinally polarized e+/ecolliding beams in the Circular Electron Positron Collider, at a center of mass energy of 91 GeV as a Z factory and energies beyond.