In order to increase the operating power of beam, China Spallation Neutron Source phase II (CSNS-II) will use two families of superconducting cavities (double-spoke cavity and elliptical cavity) to increase the energy of H− from 80 MeV in the phase I to 300 MeV. In January 2024, the project team successfully completed the prototype verification of the double-spoke superconducting cavity cryomodule. The maximum acceleration gradient of the double-spoke resonator (DSR) reached 15.2 MV/m, significantly surpassing the design target of 9 MV/m. The development of the elliptical cavity prototype began in March 2021. At present, the elliptical cavities have completed the prototype design, manufacturing, post-processing, and vertical testing, achieving a maximum gradient of 25.7 MV/m and a Q exceeding 1 × 1010, far surpassing the engineering requirement of 14 MV/m and the Q exceeds 5 × 109. This paper details the design, fabrication, post-processing, and vertical testing of β=0.62 elliptical cavities, covering challenges and solutions.

Dynamic Lorentz force detuning (LFD) in the 324 MHz superconducting double-spoke cavities for the China Spallation Neutron Source Phase-II (CSNS-II) linac is investigated through combined modeling, simulation, and cryomodule experiments. A modal ponderomotive coupling model is established and validated against multiphysics finite-element analysis and measurements up to 9 MV/m with 1.2 ms pulses at 50 Hz. Intra-pulse detuning reconstructed from low-level RF (LLRF) waveforms agrees with full-pulse detuning inferred from piezo-sensor signals, and FFT analysis identifies dominant mechanical modes consistent with simulated eigenfrequencies. Beyond single-cavity behavior, inter-cavity mechanical coupling through a shared cryomodule and support system is experimentally verified: RF excitation of one cavity induces measurable vibration and frequency perturbation in the adjacent cavity, confirmed independently by LLRF and piezo diagnostics. Active compensation is implemented with piezo tuners using a parameterized half-sine feedforward waveform. Parameters are first optimized by systematic scanning and then refined on-line using a Proximal Policy Optimization (PPO) agent interacting with the real cavity via a gymnasium/pyepics interface. After 104 training steps, the PPO-optimized waveform reduces the effective detuning metric by about 37% relative to the initial policy, matching or surpassing the best manually scanned setting while improving robustness to operating-condition variations.

With the energy upgrade of the China Spallation Neutron Source (CSNS-II) and the increasing demand for isotope production, the RF repetition frequency of the Drift Tube Linac (DTL) accelerator must be elevated from 25 Hz to 50 Hz. However, the frequency shifts induced by RF heating exceed the current tuning range. Traditional tuning methods cannot be applied during DTL operation, so online tuning technology is explored in this study. Multi-physics simulations were used to analyze the electromagnetic and temperature distribution, frequency shifts, and compensation strategies. It was found that adding tuning blocks on the side of the cavity is the optimal solution. Two types of tuning blocks were developed and installed during the summer maintenance, achieving a beam transmission rate of over 98%. Experimental results showed that the new tuning blocks effectively compensated for the frequency deviation without affecting the cavity's stability or field distribution. This solution improves the high-frequency performance of the DTL accelerator and meets the upgraded requirements. It is practical approach that offers valuable insights for the online tuning of DTL accelerators, as well as other high-frequency cavities.

The China Spallation Neutron Source (CSNS) is the fourth pulsed accelerator-driven neutron source in the world, and it achieved its design target of 100 kW in 2020. The planned China Spallation Neutron Source Phase II (CSNS-II) commenced in 2024. The CSNS-II linac design primarily involves the addition of a radio-frequency ion source and a section of a superconducting linear accelerator composed of two types of superconducting cavities, namely double-spoke and six-cell elliptical cavities, after the drift tube linac (DTL). The development of the double-spoke superconducting cavity began in early 2021, and by January 2023, the welding, post-processing, and vertical tests of two 324 MHz double-spoke cavity prototypes were completed, with vertical test gradients of 11.6 and 15 MV/m, and Q_0 ≥ 3× 10^10 @ E_acc≤10 MV/m . The R D of the cryomodule began in January 2022. In October 2023, the clean assembly of the double-spoke cavity string and cold mass installation of the cryomodule commenced, with the installation of the cryomodule and valve box completing in two months. In January 2024, a horizontal test of the cryomodule was completed, making it the first double-spoke cavity cryomodule in China. The test results showed that the maximum gradients of the two superconducting cavities at a pulse width of 4 ms and repetition frequency of 25 Hz were 12.8 and 15.2 MV/m, respectively. This article provides a detailed introduction to the double-spoke superconducting cavity, tuner, coupler, and cryomodule, elaborates on the clean assembly of the cavity string and cold mass installation of the cryomodule, and provides a detailed analysis of the horizontal test results.

This study introduces a novel model predictive control (MPC) method for the automatic cooling of a 1.3 GHz 9cell superconducting radio frequency (SRF) cavity cryomodule. The research aims to optimize the cooling process and system performance. It provides a detailed process flow for the automatic cooling, emphasizing critical steps and components. The study analyzes the cryomodule's structure to understand cooling requirements and thermal management. The study employs a comprehensive thermo-hydro-mechanical model, specifically a thermalhydro-mechanical model calculation, to analyze and optimize the thermal dynamics and energy transfer within the cryomodule system. This approach helps in identifying areas for improvement and determining optimal cooling parameters. Surrogate-based control models are utilized, integrating data-driven insights with predictive techniques for real-time monitoring, adaptive control, and predictive maintenance. Experimental tests confirm the MPC method's effectiveness in automatic cooling, highlighting its practical benefits for 1.3 GHz 9cell SRF cavity cryomodule systems, leading to enhanced cooling efficiency and overall system performance.

The full-energy linac injector project of the Southern advanced photon source (SAPS) will utilize C-band traveling wave accelerating structures to achieve a high acceleration gradient, enabling the generation of higher-energy beams. For compact C-band accelerating structures, microwave measurements must be highly accurate and efficient, with the results being one of the most important indicators for subsequent tuning. However, existing method has difficulty in maintaining the accuracy of measurements after welding, and there is an urgent need to find a reliable solution. A novel, cost-effective probe design was proposed to measure single-cavity frequencies in welded accelerating structures without pre-tuned couplers, which is featured a thin shielding structure to minimize signal interference. Simulations experiment validated the theoretical framework, while experimental validation utilized the S-band and C-band accelerating structures. Based on the experimental data, thoroughly tuning for the C-band structure was implemented to further prove the performance of the probes. Furthermore, frequency measurements cross-verified against bead-pull method results. Experimental results of the plunger-probe method validated high precision with theoretical data. After tuning, frequencies of C-band structure improved, with quality factors aligning to simulations. Field flatness and local reflection coefficients (optimized to − 0.03 0.03) also confirmed probe accuracy and tuning efficacy. The proposed probe design simplifies post-welding cavity tuning by eliminating coupler-matching dependencies, offering a practical solution for high-gradient accelerators measurement.

Elliptical superconducting cavities are widely used in particle accelerators because they can provide stronger acceleration fields than regular cavities. Pre-tuning is required to improve their performance, but the process is time-consuming. This research introduces an automated pre-tuning system for multi-cell elliptical superconducting cavities. This platform integrates three main components: real-time electric field measurements, data analysis based on tuning algorithms, and automated mechanical adjustment systems. During copper cavity tests, the system achieved an electric field flatness of 96.5%. Results show the system can accurately tune cavity frequencies while keeping electric fields uniform. This solves the problem of low efficiency in manual tuning. The new system improves cavity reliability and offers useful solutions for precise tuning needs. It also offers a foundational framework for analogous system development in future projects.

The CSNS-II(China Spallation Neutron Source Phase-II) LINAC(Linear Accelerator) adds 44 SRF(Superconducting RF) cavities, accelerating particle energy from 80 MeV to 300 MeV. In order to promote the research and development of the SRF LLRF(Low Level Radio Frequency) control system for CSNS-II, LLRF algorithm validation was conducted on C-ADS(China Accelerator-Driven Subcritical system) injector I in Beijing. This article provides an overview of the SRF accelerators at CSNS-II and C-ADS injector I, outlines the framework of the C-ADS injector I LLRF, and presents the validation work and results of the LLRF algorithm. The validation work includes the transition from continuous wave (CW) operation to pulse operation, as well as Lorentz force detuning compensation, beam compensation, beam synchronization phase measurement, mechanical oscillation measurement. Ultimately, stable operation was successfully achieved with an RF pulse width of 1.0 ms and a 25 Hz repetition rate, meeting the requirements of the CSNS-II project as the amplitude and phase indicators approximately reached +/- 0.3% and +/- 0.3 degrees, respectively. This verification work will be of great significance for the successful implementation of LLRF in the CSNS-II LINAC project.

The Drift Tube Linac (DTL) is the medium-energy linear accelerator for the Chinese Spallation Neutron Source (CSNS), responsible for accelerating the beam from 3 MeV (provided by the Radio Frequency Quadrupole, RFQ) to 80 MeV, which is the required energy for ring injection. As a crucial component of CSNS [1], the DTL ensures the high-quality delivery of the beam to the downstream ring. A key subsystem of the DTL [2] is the movable tuner, which maintains cavity frequency stability against perturbations such as beam loading effects or thermal drifts. The current DTL uses a piston tuner with high-frequency springs to establish electrical contact between the copper and the cavity wall. However, this design poses a risk of sparking during operation. To support the isotope production upgrade project, the DTL's repetition frequency will be increased from 25 Hz to 50 Hz. To mitigate the sparking risk, a choke-type movable tuner [3, p. 59], has been developed, utilizing a non-contact lambda/2 short-circuited transmission line [3, p. 59]. Furthermore, the cavity now incorporates an auto-tuning system [4, 5], reducing the closed-loop power ramp-up time from 10 minutes to just 3 minutes. Once closed-loop stabilization [6] is achieved, beam extraction can proceed.

There are five sets of pulse type 324 MHz triode klystron power sources in the China Spallation Neutron Source (CSNS) Linac Radio Frequency (RF) system that provide high frequency power for the RFQ accelerator and four Drift Tube Linac (DTL) accelerators, respectively. They have a high frequency pulse width of 650 μs and a pulse repetition frequency of 25 Hz. Each klystron power source consists of a DC high voltage power supply, a crowbar, a solid-state modulator, and a 324 MHz klystron. At present, these five klystron power sources operate stably, which ensures the long-term stable operation of CSNS Linac. The development of each component of the klystron power source system is introduced in detail in this paper, and the specific problems and solutions are elaborated during the commissioning and operation of 324 MHz klystron power sources.

The China Spallation Neutron Source (CSNS), located in Dongguan city, Guangdong Province, is the country’s first and only spallation neutron source. It is also the fourth spallation neutron source in the world. Meanwhile, it is one of the core large-scale scientific facilities of the Guangdong-Hong Kong-Macao Greater Bay Area Comprehensive National Science Center. The China Spallation Neutron Source phase II (CSNS-II) upgrade design will increase the total beam power from 100 kW to 500 kW and boost the beam energy from 80 MeV to 300 MeV in the linac by adding a superconducting linear accelerator to the existing accelerator complex. The double spoke resonator is used in the energy range of 80 MeV to 165 MeV. At present, the double spoke resonator (DSR) prototypes have been designed, fabricated and tested. The two prototypes underwent vertical testing five times, resulting in a maximum accelerating gradient of 15 MV/m. Additionally,the value of Q0 exceeded 4× 1010 at an Eacc of 7.3 MV/m.

High quality factor 1.3 GHz 9-cell superconducting radio frequency (SRF) cavity cryomodule is the core technology of the international advanced accelerator. China's first high-quality factor 1.3 GHz 9-cell SRF cavity cryomodule was developed, assembled and tested at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences for the Dalian Advanced Light Source (DALS) and Circular Electron Positron Collider (CEPC) R&D. The 9-cell cavities in the cryomodule achieved a high average intrinsic quality factor (Q0) of 3.8 x 1010 at 16 MV/m and 3.6 x 1010 at 21 MV/m in the horizontal test. With such high specifications, the SRF cavity system has corresponding requirements for the cryogenic system. Key criteria for stable and reliable operation include a dependable cryogenic refrigeration system and process design, good thermal insulation, pressure control, temperature uniformity, and quench protection, etc. IHEP has successfully completed cryogenic testing development of the first 1.3 GHz 9-cell SRF prototype cryomodule. The whole cooldown period was 74 h, and the automatic cooldown was completed. The successful development and implementation of the 1.3 GHz superconducting cryomodule cryogenic system will provide a stable and reliable test environment for advanced superconducting cavity cryogenic testing, which will be critical in promoting the further development of the Free Electron Laser (FEL) and CEPC projects.

World's first 1.3 GHz cryomodule containing eight 9-cell superconducting radio-frequency (RF) cavities treated by medium-temperature furnace baking (mid-T bake) was developed, assembled and tested at IHEP for the Dalian Advanced Light Source (DALS) and CEPC R&D. The 9-cell cavities in the cryomodule achieved an unprecedented highest average Q0 of 3.8E10 at 16 MV/m and 3.6E10 at 21 MV/m in the horizontal test. The cryomodule can operate stably up to a total CW RF voltage greater than 191 MV, with an average cavity CW accelerating gradient of more than 23 MV/m. The results significantly exceed the specifications of CEPC, DALS and the other high repetition rate free electron laser facilities (LCLS-II, LCLS-II-HE, SHINE, S3FEL). There is evidence that the mid-T bake cavity may not require fast cool-down or long processing time in the cryomodule. This paper reviews the cryomodule performance and discusses some important issues in cryomodule assembly and testing.

According to the upgrade plan of CSNS-II (China Spallation Neutron Source Phase II), the beam power on target of CSNS needs to be increased to 400 kW in the future. In order to achieve this goal, a superconducting acceleration section will be added behind the LINAC (Linear Accelerator) section of CSNS-I. The superconducting acceleration section includes two superconducting cavity types operating at 324 MHz and 648 MHz. At present, the 324 MHz superconducting cavity couplers have been produced. In order to perform the RF conditioning and test the power transmission capability of the coupler, we designed and built a corresponding automatic RF conditioning control platform in the gallery of CSNS LINAC. The control system hardware is based on Micro TCA (Micro Telecommunications Computing Architecture, MTCA) LLRF (low-level radio frequency) control system, and the software development is based on EPICS (Experimental Physics and Industrial Control System). PLC (Programmable Logic Controller) based data acquisition and monitoring slow protection system, in-house developed interlocking fast protection system and some general equipment constitute the main hardware facilities, while EPICS IOC (input and output controller) and CSS (Control System Studio) OPI (Operator Interface) constitute a main pillar of control software. The control logic of the RF conditioning program mainly uses the vacuum pressure readings collected from the coupler as the judgment standard, and controls the power autonomously according to the real-time vacuum data, which can largely avoid human misoperation and greatly reduce the operator’s workload. At present, the system has started to run, and the actual work results show that its hardware and software performance are excellent, which ensures the safe and smooth progress of the RF condition process. In this article, we will first introduce the hardware architecture of the entire system, secondly introduce the software architecture and control logic of RF conditioning, and finally give the actual working results of the control system.

中国散裂中子源二期升级采用超导腔技术方案,其中在 165～300 MeV能量段采用 648 MHz 6-cell超导腔模组,每个模组中集成 3只 6-cell超导腔.超导腔工作在脉冲模式,为了保证超导腔 2K下的频率满足运行要求,每只超导腔需要一套低温调谐器对其频率进行精确调节控制.针对 648 MHz 6-cell超导腔的结构和运行特点进行了低温调谐器的设计,采用快慢组合机构补偿超导腔的频率偏移,对调谐器的基本性能和超导腔脉冲模式运行下的动态洛伦兹失谐进行了分析.

为了保证324 MHz超导腔高功率输入耦合器测试平台的安全可靠运行,基于PLC和FPGA控制器设计了联锁保护系统.整机系统主要包括速调管功率源部分和耦合器部分,联锁保护系统全面监测所有设备的工作状态,同时执行正常的时序逻辑和异常情况下的联锁保护逻辑及故障定位,确保整机系统运行的安全性.文章主要介绍联锁保护系统软硬件设计及运行情况,其长期运行测试结果表明:联锁保护系统保障了耦合器测试台系统的正常运行,满足设备状态监测显示、联锁保护及故障锁存等功能要求.

低电平射频控制系统主要用于对加速腔的高频场和谐振频率的控制,保证加速器的稳定运行并输出高品质的束流.低电平控制系统软件提供可视化操作界面,实现数据显示和存储、自动化算法等功能,提高了低电平系统的可操作性,减少了人工作业量和故障率.软件开发具有灵活性强、开发周期短的特点,适合控制速度要求相对不高而逻辑较为复杂的功能开发.本文介绍中国散裂中子源直线加速器射频低电平控制系统软件的开发设计,重点介绍前馈自动计算、加速腔的频率自动控制、频率失谐算法、一键升功率、界面优化等相关内容.最终低电平控制系统幅度相位稳定度指标优于要求的设计指标,且达到长时间稳定运行的要求.

The China Spallation Neutron Source drift tube linac (DTL) is a high current intensity linear accelerator operated in pulsed mode. Beam loss is the most important cause of DTL performance degradation. Therefore, control of beam loss is very crucial to the linac's commission and routine operation. The beam loss in beam pulse width (∼several hundreds of microseconds) can be regarded as an instantaneous process, which may cause the local temperature of the drift tubes (DTs) to exceed the melting point or the thermal stress to exceed the material yield strength, resulting in permanent damage to the DTs. RF heating and beam loss are two major heat sources of the DTs during operation. First, temperature rise due to RF power dissipation is evaluated in this paper. Then, the thermomechanical response due to beam loss at different energies is investigated. It is found that the 3-MeV beam loss has the greatest stopping power and causes the largest temperature rise to the DTs. The maximum beam current loss for different working conditions is analyzed by both analytical method and ANSYS simulations, and the details will also be discussed in this paper.

The design study of a 324 MHz double-spoke cavity is presented in this article. The geometric parameters are optimized to get reasonable RF properties (e.g., low E p / E acc , low B p / E acc , and high R / Q ). Then, the end cap and the spoke base of the cavity are deliberately optimized to minimize multipacting for the design gradient. Furthermore, the mechanical design is optimized to reduce the Lorentz coefficient and the frequency sensitivity of helium pressure fluctuation. In view of the pulsed operation characteristics of the cavity, the RF pulse spectrum and the corresponding amplitude of each spectrum are evaluated. Finally, the modal analysis is performed by the ANSYS software, and the first six modes are classified.