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 Beijing Electron–Positron Collider (BEPC) is a circular collider operating in the τ-charm energy region, which has been in operation since 1988 and underwent a major upgrade (BEPCII) that enhanced its beam energy to 1.89 GeV. To meet the further requirements of high-energy physics research in the τ-charm energy region, the linear injector is required to stably deliver an electron–positron beam with an energy range of 2.35 GeV–2.8 GeV. To address the beam energy requirement, two additional radio frequency (RF) power source systems developed based on the solid-state modulator scheme were integrated into the linear accelerator. For the purpose of meeting reliability specifications, the modulators based on the pulse forming network (PFN) were subjected to systematic upgrading and modification, which specifically involved enhancing repetition stability, conducting in-depth research on service life and reliability, and establishing and implementing fault prediction protocols. Following the system upgrade, the linear RF power source system has maintained long-term stable operation, which fully complies with the latest energy and stability requirements of the linear accelerator. Furthermore, the localized replacement of key core equipment and components has been successfully achieved, thereby providing crucial technical support for the enhancement of BEPCII’s collision energy and the advancement of subsequent physics research projects.

Low-density silica aerogel is an ideal medium for capturing cosmic dust in space, as the perforated tracks provide valuable insights into the characteristics of interplanetary particles. However, the mechanism underlying track formation remains unclear. This study examines the skirt-shaped or cone-in-cone cracks commonly observed in both ground experiments and returned aerogel samples. Similitude impact tests using various techniques are conducted to compare the cone cracks formed at different velocities. Results show that cone cracks typically form at lower impact velocities. Additionally, impact tests with hard spherical projectiles at speeds below 50 m s-1, conducted using an electromagnetic coilgun, produce multiple tracks with single cone cracks. The impact process is captured in real-time using a high-speed camera, and the formation mechanism is investigated. A theoretical model, based on contact mechanics and energy methods, is developed to quantify the relationship between cone crack morphology and projectile impact parameters. The model accuracy is validated through experimental results. This study reveals the formation of cone cracks in penetration tracks within silica aerogel. The proposed model identifies the energy absorption mechanism during single cone crack formation, potentially improving the understanding of key parameters (initial size, composition, velocity distribution, and astrophysical source) from the perforated tunnels in cosmic dust aerogel collectors.

In large-scale particle accelerator radio frequency (RF) power source systems, the power level can reach hundreds of megawatts with annual electricity consumption on the terawatt-hour (TWh) scale. The inherently low microwave conversion efficiency leads to significant energy losses. While improvements in klystron efficiency through novel materials, structural optimization, and advanced manufacturing techniques yield limited gains, the successful application of depressed collector technology in high-power klystrons could substantially enhance overall system efficiency and reduce energy waste. To investigate the feasibility of applying depressed collector technology to high-efficiency continuous-wave (CW) klystrons, this study developed a 30-kV low-power prototype system based on the CW klystron designed for the circular electron-positron collider (CEPC). The prototype, which excludes RF output and directly connects the electron gun to a single-stage depressed collector, was integrated with a high-voltage power supply system and tested on a high-power experimental platform. Key technical challenges were addressed, including thermal management under high-voltage operation, electrode insulation, coordinated control between cathode and collector power supplies, and floating potential sampling. Furthermore, the structural design of the depressed collector was optimized to achieve a more compact configuration, facilitating its integration into high-power klystron systems. Experimental results showed excellent agreement with theoretical simulations, achieving an efficiency of 87.2% under rated operating conditions. These findings establish a solid foundation for the practical application of depressed collector technology in high-power klystrons.

A laser-microwave synchronization system using an all-fiber optical-microwave phase detector (AFOM-PD) has been developed and experimentally validated at the Institute of High Energy Physics (IHEP). A Yb-doped fiber amplifier (YDFA) was employed to suppress the relative intensity noise (RIN) of the reference laser. And the root-mean-square (RMS) integrated RIN was reduced from 7.78 × 10 -4 before suppression to 6 × 10 -5 after suppression in the interval of 1 Hz to 1 MHz. The performance of this system has been demonstrated using a mode-locked fiber laser with a repetition rate of 29.1 MHz, in conjunction with L-band and S-band microwave signals. Residual phase noise levels of -118 dBc/Hz and -116 dBc/Hz at the 1-Hz offset frequency have been achieved for the L-band and S-band microwave signals, respectively. The integrated RMS timing jitters measured within the frequency range of 1 Hz to 1 MHz during the same experiments are 18.6 fs for the L-band signals and 6.0 fs for the S-band signals. Tests conducted over a 6-hour period have shown that this system can achieve a long-term RMS timing drift of less than 16 fs. This system has potential applications in the plasma wakefield accelerator currently being developed at IHEP.

In Linacs, achieving higher accelerating gradients inevitably demands higher-power RF sources and RF loads. These loads are crucial for absorbing the residual RF power. While IHEP has developed a series of SiC dry loads covering several frequency bands, their peak power capacity is limited to only several tens of megawatts. Therefore, this paper presents the development and proposal of pure stainless steel loads that are capable of withstanding power levels in the hundreds of megawatts range. Simulations and optimizations of the pure stainless steel load were conducted in CST with the aim of attaining lower input reflection and a broader operating frequency band. Low-power RF tests and high-power conditioning were carried out to verify the design. The length of the pure stainless steel load was optimized, and it was ultimately reduced to less than 1 m. Throughout the processes of fabrication, welding and high-power conditioning, the load achieved a VSWR of less than 1.1 at the operating frequency and a bandwidth of over 100 MHz (with S11 < -20.0 dB). During the high-power conditioning stage, the load operated stably at a peak power of 177.8 MW. This paper comprehensively details the design, simulation, fabrication, cold test, and high-power conditioning of all-metal loads. The obtained results are satisfactory, demonstrating that the loads exhibit strong absorption and thermal stability during high-power conditioning.

The Circular Electron Positron Collider (CEPC) is a large scientific project initiated and hosted by China, fostered through extensive collaboration with international partners. The complex comprises four accelerators: a 30 GeV Linac, a 1.1 GeV Damping Ring, a Booster capable of achieving energies up to 180 GeV, and a Collider operating at varying energy modes (Z, W, H, and ttbar). The Linac and Damping Ring are situated on the surface, while the Booster and Collider are housed in a 100 km circumference underground tunnel, strategically accommodating future expansion with provisions for a Super Proton Proton Collider (SPPC). The CEPC primarily serves as a Higgs factory. In its baseline design with synchrotron radiation (SR) power of 30 MW per beam, it can achieve a luminosity of 5e34 /cm^2/s^1, resulting in an integrated luminosity of 13 /ab for two interaction points over a decade, producing 2.6 million Higgs bosons. Increasing the SR power to 50 MW per beam expands the CEPC's capability to generate 4.3 million Higgs bosons, facilitating precise measurements of Higgs coupling at sub-percent levels, exceeding the precision expected from the HL-LHC by an order of magnitude. This Technical Design Report (TDR) follows the Preliminary Conceptual Design Report (Pre-CDR, 2015) and the Conceptual Design Report (CDR, 2018), comprehensively detailing the machine's layout and performance, physical design and analysis, technical systems design, R&D and prototyping efforts, and associated civil engineering aspects. Additionally, it includes a cost estimate and a preliminary construction timeline, establishing a framework for forthcoming engineering design phase and site selection procedures. Construction is anticipated to begin around 2027-2028, pending government approval, with an estimated duration of 8 years. The commencement of experiments could potentially initiate in the mid-2030s.

Superconducting cavity is the key equipment of the superconducting accelerator, which provides higher acceleration voltage and higher frequency power per unit length, and saves equipment space. Superconducting cavities need to be gradually cooled from ambient temperature (300 K) to the superconducting temperature (4.2 K or below) during the test and operation. The temperature difference on the cavity must be strictly limited during the cooldown process to prevent excessive thermal stress on the surface of the superconducting cavity. Since this cooldown process for the superconducting cavity is a typical large hysteresis, non-linear process that is difficult to control automatically using decoupled proportion integral derivative (PID) methods directly, a less efficient manual control scheme is normally adopted. In this paper, 3D numerical simulation, 1D pipe and 0D tank model with artificial neural network (ANN) were combined to generate a two -layer surrogate model that can balance computational accuracy and speed, to improve the automation and cooling efficiency of the superconducting cavity cooldown process. In order to achieve automatic control of the cooling procedure for the superconducting cavity, a model predictive control (MPC) approach was also built on the basis of this two -layer surrogate model. According to the results of the experiment test, the improved method could realize a quick and smooth cooldown process of the superconducting cavity, during which the temperature difference on the cavity could satisfy the requirements. Additionally, the improved automatic cooldown method was more adaptable and saved 29 % more time than the original manual control method. The foundation for a more intelligent automated control of future large cryogenic systems or other system with the large hysteresis, non-linear properties, was laid.

For improving a beam quality of the linear accelerator and decreasing the influence of failure recovery on the accelerator available time, a high-voltage pulse modulator system is required to enhance the stability of high-voltage pulses and maintain a low failure rate as well as a low failure recovery time, in order to implement a high-performance and stable operation. This paper systematically analyzes the current status and operating conditions of the modulator equipment of BEPCII linear accelerator and simulates the typical parameters of the modulator considering the high-voltage DC charging power supply, DC charging cables and components and pulsed hydrogen thyratron. The technical route and retrofit scheme are proposed to decrease the failure rate and its recovery time. The stability of the high-voltage pulse modulator has increased by 1000 ppm, the failure rate has been decreased by two orders of magnitude, and the averaged recovery time from failures has been reduced by 94

The 650 MHz RF system with 240 cavities will be adopted for the collider ring of the Circular Electron-Positron Collider (CEPC). The prototype test cryomodule (TCM) has been designed, constructed and commissioned at IHEP since the beginning of 2017. The thermal performances of the cryomodule are analyzed based on the thermo-electrical analogy, and the experimental investigations of the cryomodule are performed in the superfluid helium cryogenic system, meanwhile, the theoretical calculations and experimental measurements are compared. The results are shown that the calculation of the gas-cooling power coupler are in good agreement with experimental measurements, which indicates that thermal calculation models are more accurate, moreover, the deviations of static heat load of the cryomodule at 2 K temperature level between the theoretical calculations and experimental measurements are analyzed in the paper.

In theory, a 180° hybrid bridge can be used as a variable power divider and combiner. This study focuses on applying the variable power divider and combiner to a high-vacuum and high-power environment, aiming at realizing future applications in particle accelerators. Beginning from theoretical derivation, this article reports the design of an $S$ -band variable waveguide power divider and combiner based on a Magic Tee. The simulation results are consistent with those predicted by theory. A prototype of the variable power divider was fabricated for the microwave cold test, and the measurement results verify the variable power division. The power division ratio curve is consistent with the theoretical and simulated curves. The variable power combiner was experimentally verified by combining the Magic Tee and coaxial components. The measurement results are consistent with those of numerical analysis. Simulations evaluating the high-power performance reveal that the matching probe of the Magic Tee and the phase shifting plate tip of the straight waveguide phase shifter could not satisfy the high-power requirement. Thus, the structure of the Magic Tee is improved in the proposed design, and a 3-dB bridge waveguide phase shifter was adopted. The simulation results show qualified performance for high-power applications, and other microwave performance characteristics also show great improvement. The study lays a good foundation for application in future high-vacuum and high-power environments.

The interleaved parallel technique has been widely investigated for its flexibility in extending the output power of DC-DC converters while effectively reducing the input and output ripple. This paper focuses on the research of high-power interleaved three-level boost converter topology with low ripple and high efficiency. Firstly, the application of the overall system, control strategy, modulation method are introduced, and its effect on output voltage ripple is verified and compared by simulation. It is planned to build a 20kW prototype platform for subsequent experiments. The results will build a technical foundation and provide a reference for the application of the interleaved parallel three-level boost converter topology to subsequent research in related fields.

Purpose The High-Energy Photon Source (HEPS) is a 6 GeV storage ring-based light source under construction in Beijing, China. Its accelerator consists of a 6 GeV storage ring, a full-energy booster, an S-band normal-conducting 500 MeV linac, and three transport lines. As the beginning of HEPS, a stable linac is quite important, which needs a qualified waveguide distribution system to transmit microwave power from klystrons to accelerating structures. Methods Installation and high-power conditioning of the HEPS linac were performed from February to September 2022. The assembly and conditioning of the waveguide distribution system were completed in April and July, respectively. The design of the waveguide distribution system began in 2018 and was finalized at the end of 2020 after multiple iterations. Results Owing to careful design, fabrication, and assembly preparation, the assembly and conditioning of the system proceeded smoothly and considerable time was saved. Conclusion The entire process from the design to the ultimate realization of the system is introduced in detail in this paper. Satisfactory measurement results were obtained for some waveguide components.

High power radio frequency (RF) conditioning is a key step for the RF components to gain full performance. In the construction of high energy photon source (HEPS), upgrading Beijing Electron-Positron Collider (BEPCII), preresearch of circular electron positron collider (CEPC), and other projects at Institute of High Energy Physics (IHEP), there will be a lot of RF conditioning tasks within a very tight schedule. To improve the conditioning efficiency, an optimized automatic control has been developed. The automatic control system is developed based on the Experimental Physics and Industrial Control System (EPICS). The control hardware platform adopts a combination of the Micro Telecom Computing Architecture (MTCA) based low-level control and programmable logic controller (PLC) interlocking unit, which integrates a variety of measurements to ensure safe and reliable unattended operations during high-power (up to 160 MW) and high gradient (up to 33 MV/m) testing. The finite state transfer based on multiple event-triggered interrupt processes is used to improve conditioning efficiency. Taking the conditioning of the HEPS 3-m-long acceleration structure as an example, the breakdown rate (BDR) at high power is less than one out of 180 000 pulses. The time consumed for each accelerating structure is reduced by a factor of nearly 2/3. Experiments show that the system realizes the unattended operation of whole process, and improves conditioning efficiency.

The positron source is one of the most important parts of the circular electron–positron collider (CEPC) linear accelerator. The design goal is to obtain a 3 nC bunch charge positron beam from a 4 GeV primary electron beam with an intensity of 10 nC/bunch. In this study, an all-solid-state, high-current pulse power system was developed to drive a flux concentrator to capture positrons by generating a magnetic field with a peak value of 6 T. Based on the inductive load topology circuit, the optimized parameters were designed to generate a pulse current with a high peak value and a high rise rate pulse current. In addition, a non-reverse voltage was realized to protect the solid-state switch components. For the characteristics of the long-distance transmission between the pulse power supply and the load in the system, the influences of the cable distribution parameters on the pulse were analyzed, and the design of the damping circuit was proposed to suppress the high-frequency ripples of the peak pulse current. An optimized coaxial discharge structure was designed to reduce the stray parameters in the main circuit. The design was verified through simulations and experiments, and the results demonstrated that the developed prototype successfully generated an ideal half-sinusoidal pulse current with a peak value of 15 kA, and a front pulsewidth of $2.5 ~\mu \text{s}$ at a repetition rate of 50 Hz. The pulse repetition stability is better than 0.1%. The final output was consistent with the theoretical analysis. The system can be applied in the CEPC positron source, which also provides a reference for the subsequent development of a positron source system at research facilities.

Positron sources are one of the most important components of the injector of a circular electron positron collector (CEPC). The CEPC is designed as an e(+)e(-) collider for a Higgs factory. Its accelerator system is composed of 100-km-long storage rings and an injector. The design goal of the positron source is to obtain positron beams with a bunch charge of 3 nC. The flux concentrator (FC) is one of the cores of the positron source. This paper reports the design, development, and measurements of an FC prototype system. The prototype includes an FC and an all-solid-state high-current pulse modulator. Preliminary tests show that the peak current on the FC can reach 15.5 kA, and the peak magnetic field can reach 6.2 T. The test results are consistent with the theoretical simulation. The FC system fulfills the requirements of the CEPC positron source as well as provides a reference for the development of similar devices both domestically and abroad.

The CEPC includes a main ring and an injector. The injector consists of a booster and a linac. In order to meet the requirements of the booster, the baseline design of the linac is a 10 GeV electron and positron linac. Two alternative linac designs have also been introduced in this paper. For the linac baseline design, one-bunch-per-pulse is adopted. A 1.1 GeV damping ring is used to reduce the transverse emittance of positron beam. The main RF system of the linac adopts normal conducting S-band structure. Some key technologies of linac are development. The S-band structure and pulse compressor have been researched and studied. In the damping ring, two cavities used to provide 2 MV voltage. The preliminary cavity design has finished.

Circular Electron-Positron Collider (CEPC) is a 100 km ring e+ ecollider for a Higgs factory. The injector is composed of a Linac and a Booster. The baseline design of CEPC Linac is a normal conducting S-band linear accelerator with frequency in 2860 MHz, which can provide electron and positron beam at an energy up to 10 GeV and bunch charge up to 3 nC. To reduce the design difficulty of booster and booster magnet, an alternative design of the Linac with C-band accelerating structure at high energy part is proposed and the energy is up to 20 GeV. In this paper, the physics design of this scheme is presented. INTRODUCTION In September 2012, Chinese scientists proposed a Circular Electron Positron Collider (CEPC) in China at 240 GeV centre of mass for Higgs studies [1]. It could later be used to host a Super Proton Proton Collider (SppC) in the future as a machine for new physics and discovery. After that a great effort have been made in physics design [2]. The injector of CEPC is composed of a Linac and a full energy booster. The first part of the injector is a normal conducting S-band Linac with frequency in 2860 MHz and provide electron and positron beams at an energy up to 10 GeV [3, 4]. The main parameters of the CEPC linac are shown in Table 1 and layout is shown in Fig.1. The Linac is composed of electron source and bunching system (ESBS), the first accelerating section (FAS) where electron beam is accelerated to 4 GeV, positron source and pre-accelerating section (PSPAS) where positron beam is produced and accelerated to more than 200 MeV, the second accelerating section (SAS) where positron beam is accelerated to 4 GeV, the third accelerating section (TAS) where electron beam and positron beam are accelerated to 10 GeV, the electron bypass transport line (EBTL) where the electron beam is bypass the PSPAS and SAS section and one damping ring (DR) to reduce the emittance of positron beam. The electron Linac consists of ESBS, FAS, EBTL and TAS. The positron Linac consists of ESBS, FAS, PSPAS, SAS, DR and TAS. The horizontal distance between EBTL and SAS is 2.0 m. The Linac should be have potential to meet higher requirements and upgrade in the future, the designed bunch charge is larger than 3 nC both for electron and positron beam. Based on this consideration, the energy of electron beam for positron production is chosen as 4 GeV and the positron yield of positron source with some cut-off condition is 0.55 [5]. Table 1: Main Parameters of CEPC Linac Parameter Unit Value e-/e+ beam energy GeV 10 Repetition rate Hz 100 e-/e+ bunch population nC >1.5 Energy spread (e-/e+) <2×10-3 Emittance (e-/e+) nm <120 The CEPC booster [6] provides 120 GeV electron and positron beams to the CEPC collider and is in the same tunnel as the collider, which of circumference is 100 km. The electron beam is ramped from 10 GeV to 120 GeV in the booster. The magnetic field of dipole magnet is about 30 Gs at injection energy. It’s very challenging for magnet design and operation, on the other hand, the operating mode of magnet power supply is ramping and dynamic, so it’s a very critical issue. In order to reduce the design difficulty, increasing the energy of the Linac is a good way. Based on this idea, an alternative 20-GeV Linac scheme is proposed. High gradient accelerating structure is needed for high energy linac to reduce the length and cost. Considering the bunch charge is not very high and the emittance is small, the C-band accelerating structure is introduced to the alternative design. The C-band accelerating structure is used to replace S-band accelerating structure in the TAS and the energy of the Linac is increased to 20 GeV. The alternative design is presented and discussed in this paper. Figure 1: The layout of the CEPC Linac. ___________________________________________ *Work supported by National Natural Science Foundation of China (No. 11705214) and Youth Innovation Promotion Association CAS. † mengc@ihep.ac.cn 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPTS065 MC1: Circular and Linear Colliders A08 Linear Accelerators MOPTS065 1005 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I

The CEPC injector consists of linac and booster. To meet the requirement of the booster, the linac should provide 10 GeV electron and positron beam at a repetition frequency of 100 Hz. In this paper, the overall design of the linac has introduced. For the linac one-bunch-per-pulse is adopted and bunch charge should be larger than 3 nC in the design. A 1.1 GeV damping ring with 75.4 m circumference has adopted to reduce the transverse emittance of positron beam to suitably small value.