Superconducting radiofrequency (SRF) cavities are essential for high-energy particle accelerators rendering ultralow power dissipation and a high acceleration gradient. We demonstrate a superior performance in niobium-based 1.3 GHz cavities via medium-temperature (Mid-T) baking with remarkable values up to 5.5 × 1010 for the quality factor (Q0) at 16 and 32.7 MV/m for the maximum acceleration gradient (Eacc). Through correlative in situ spectroscopy, mass spectrometry, and electron and tunneling microscopy, we establish that nanoscale spatial distributions of impurities (O, C, H) directly evidenced at the metal-oxide interface govern performance enhancement. In niobium, annealing at 300 °C drives uniform oxygen doping (10-100 nm depth) via diffusion from the native oxide, while an optimized Mid-T protocol in delivering high Eacc values suppresses interfacial NbO segregation through competitive C-O interactions. For the optimized protocol, post-treatment characterization reveals up to an 8.8% increased superconducting gap and 29.7% reduced quasiparticle broadening, corroborating strain-mediated defect-impurity interactions at proximity layers. These results provide a Mid-T baking recipe to simultaneously enhance Q0 and maximum Eacc in SRF cavities via competitive impurity interactions at the metal-oxide interface.

Recently, 1.3 GHz 3-cell superconducting cavities were proposed for the injector of the high-brightness free electron laser based on the energy recovery linac scheme, which holds remarkable potential for next-generation high-power light sources in Shanghai. In the injector section, three cavities are required to accelerate a 10 mA electron beam to 10 MeV. In this paper, the geometric design, fabrication, surface processing, and the vertical test results of the cavities are presented. The first bare 3-cell cavity is tested at 2 K and reaches an intrinsic quality factor of 2.0 x 1010 at 12 MV/m and a maximum accelerating gradient of 26 MV/m, which are well above its specifications. Frequency and external quality factor of higher-order modes are measured at room temperature and are consistent with the simulation results. Additionally, a simplified surface treatment eliminating final light buffered chemical polishing (BCP) after high-temperature furnace baking is demonstrated for the first time for BCP baseline cavity.

Superconducting radiofrequency (SRF) cavities are essential for high-energy particle accelerators rendering ultralow power dissipation and a high acceleration gradient. We demonstrate a superior performance in niobium-based 1.3 GHz cavities via medium-temperature (Mid-T) baking with remarkable values up to 5.5 x 1010 for the quality factor (Q 0) at 16 and 32.7 MV/m for the maximum acceleration gradient (E acc). Through correlative in situ spectroscopy, mass spectrometry, and electron and tunneling microscopy, we establish that nanoscale spatial distributions of impurities (O, C, H) directly evidenced at the metal-oxide interface govern performance enhancement. In niobium, annealing at 300 degrees C drives uniform oxygen doping (10-100 nm depth) via diffusion from the native oxide, while an optimized Mid-T protocol in delivering high E acc values suppresses interfacial NbO segregation through competitive C-O interactions. For the optimized protocol, post-treatment characterization reveals up to an 8.8% increased superconducting gap and 29.7% reduced quasiparticle broadening, corroborating strain-mediated defect-impurity interactions at proximity layers. These results provide a Mid-T baking recipe to simultaneously enhance Q 0 and maximum E acc in SRF cavities via competitive impurity interactions at the metal-oxide interface.

The Shanghai high repetition rate XFEL and extreme light facility linear accelerator employs two 3.9 GHz third harmonic cryomodules. The 3.9 GHz fundamental power coupler is one of the key components of the cryomodule and is designed to handle 2 kW of continuous-wave rf power, meeting operational requirements with a beam current of 0.3 mA and cavity gradient of 13.1 MV/m. A key innovation of the 3.9 GHz coupler is the incorporation of an adjustable antenna insertion depth, which enables online external quality factor (Q_{ext}) tuning and eliminates the need for a three-stub tuner, reducing system complexity and cost. Two 3.9 GHz coupler prototypes were fabricated and underwent comprehensive rf testing. The couplers achieved excellent performance with an insertion loss of −0.16 dB and return loss of −43.22 dB at 3.9 GHz after structural optimization. High-power rf conditioning demonstrated stable operation at 2 kW in traveling wave mode and 1 kW in standing wave mode. Sixteen production 3.9 GHz couplers were manufactured and integrated into two third harmonic cryomodules, which achieved maximum total cavity voltages of 63.6 and 59.8 MV, respectively, with average usable accelerating gradients of 23.0 and 21.6 MV/m. The cryomodules have been installed in the tunnel and initial beam commissioning has been completed successfully.

Superconducting radio-frequency niobium cavities processed using buffered chemical polishing (BCP) sometimes show typical W-shaped pits on their surface, which may greatly limit their performance. However, the causes of such pits and effective solutions are not fully understood. In this study, we reproduced the formation of W-shaped pits on the cavity surface through niobium sample BCP experiments, directly observed the sample surface’s evolution during the polishing process and the polished surface’s morphology, and analyzed the cause of W-shaped pits in detail: the formation and attachment of bubbles on the niobium surface during the BCP process. Then, we systematically investigated the effects of different process parameters on the bubbles and pits, including the acid ratio, temperature, and flow rate. We also investigated how the formation of bubbles and pits was affected by the Nb facing orientation and grain size. This study provides insights into the mechanisms by which bubbles and W-shaped pits are formed on niobium surfaces, and highlights possible directions for reducing pit defects in Nb cavities processed using BCP treatment.

Buffered chemical polishing (BCP) is an important and widely used polishing technique for superconducting radio-frequency (SRF) cavities made of niobium. A common problem encountered during BCP is the formation of bubbles and W-shaped pits on the cavity surface, which may severely limit the RF performance. We report a method to address the problem of W-shaped pits through optimizing the BCP acid ratio. We systematically investigate the effect of the BCP acid ratio through sample and cavity BCP experiments and determine an optimal ratio for the three acids. The new BCP recipe with the optimal acid ratio is verified through the development of niobium cavities with several different shapes, which are shown to be free of pits and demonstrate excellent RF performance; notably, several 3.9 GHz nine-cell cavities present unprecedented accelerating gradients. Furthermore, the findings suggest a simple pit-free BCP recipe that does not require H3PO4, using only HF and HNO3. The method proposed in this study is also appropriate for suppressing pit formation with other acid mixtures or when polishing other metal objects.

Superconducting radio frequency (SRF) cavities constitute the cornerstone of high-efficiency particle accelerators. While traditional bulk niobium cavities have dominated the field, copper substrates with niobium films deposited inside the cavity represent a transformative approach for cost reduction and thermal management. However, achieving conformal superconducting films on complex cavity geometries remains a fundamental challenge, especially on the adhesive behavior of the film. Here, we present a breakthrough high-power impulse magnetron re-sputtering/sputtering (HiPIMRS) system engineered for uniform Nb film depositions on 1.3 GHz copper cavity interiors. Through a re-sputtering process on the copper substrates prior to deposition, we achieve atomic-scale interfacial integrity and eliminate interfacial oxides or degradation. Energy-dispersive x-ray spectroscopy confirms an oxide-free Nb/Cu interface, and atomic force microscopy reveals ultra-smooth surfaces (Ra < 20 nm for 3 μm films). Crucially, electrical transport measurements show that the niobium film has a critical temperature of 8.5 K throughout the cavity interior. XRD demonstrates a (110)-oriented crystalline structure. This work establishes HiPIMRS as a viable pathway for next-generation SRF cavity production, with interfacial engineering protocols offering generational advancements in film conformity and superconducting performance.

We report the world-leading performance of a 1.3 GHz cryomodule equipped with eight 9-cell superconducting radio-frequency cavities that underwent a medium-temperature furnace baking process. During continuous wave horizontal testing, these cavities achieved unprecedented average intrinsic quality factors of 4.0 × 1010 at 20 MV/m and 3.2 × 1010 at 29 MV/m, with no instances of field emission. The cryomodule demonstrates near-complete preservation of ultra-high-quality factors and ultra-high accelerating gradients from vertical to horizontal testing, marking a significant milestone in continuous-wave superconducting radio-frequency accelerator technology. This letter presents the cryomodule development experience, including cavity preparation, cryomodule assembly, degaussing, fast cooldown, and performance testing.

Superconducting radio-frequency (SRF) cavities are widely used in particle accelerator facilities. The SRF-based energy recovery linac (ERL) equipped with special SRF cavities offers feasibility for application in high-power free electron lasers (FELs). This study aims to design and optimize a 3-cell cavity that serves as the accelerating structure for a 10 mA class injector of high-brightness ERL-FEL. We apply the middle-cell shape of the mature TESLA cavity, and the end-groups are optimized for high-current beam operation. The cavity is designed with two fundamental power coupler (FPC) ports, which enable high-power input by two FPCs. In addition, the beam pipe is enlarged to damp potentially strong higher-order modes (HOMs) induced by the high current beams. A multi-objective genetic algorithm is utilized to optimize the cavity geometry. The damping of HOMs, multipacting, and mechanical issues are also investigated to verify the rationality of the cavity design.

Shanghai HIgh repetitioN rate XFEL and Extreme light facility (SHINE) is a 3 km long advanced X-ray source facility. The main superconducting Linear Accelerator (Linac) of SHINE can increase the electron beam energy up to 8 GeV under superconducting (SC) continuous wave (CW) mode. SHINE Linac is mainly based on the seventy-five 1.3 GHz-cavity cryomodules which are connected in series in 1.4 km and operated at superfluid helium temperature of 2 K. Each cryomodule mainly consists of 8 Superconducting Radio Frequency (SRF) cavities, 8 high power couplers, 8 tuners, one cold Beam Position Monitor (BPM) and one superconducting quadrupole (SCQ) magnet. In order to ensure the cryomodules stable operation at 2 K in the Linac tunnel, all the key elements are required to be tested at cryogenic temperatures before their assembly into the cryomodule. A Multifunction Test Facility (MTF) is designed and fabricated, by referring to the 3-cryogenic circuit design for the cryomodule (2 K, 5 K and 45 K) as well as the compatible consideration for all other cryomodules key elements cryogenic tests. SCQ magnet, as designed to work at liquid helium temperature by conduction-cooling, is thus necessary to be carried out on the performance tests in the MTF. This paper will give a detailed description of the MTF, as well as the cryogenic commissioning and test results for the facility itself. The experimental issues related to cryogenic tests of the SCQ magnets, such as the conduction-cooling performance and the cryogenic stability for current tests, are also discussed.

Plasma cleaning is extensively employed in superconducting radio-frequency (SRF) accelerators to mitigate field emissions induced by hydrocarbon contaminants. This study explores the plasma physical characteristics and modulation techniques in a commonly utilized 1.3 GHz 9-cell tesla-shaped SRF cavity through the integration of optical emission spectroscopy and a plasma fluid model. Investigations of argon and neon plasma demonstrate that argon generates a higher electron density due to its lower ionization energy, while neon plasma exhibits higher electron temperature and potential that promotes the decomposition of oxygen and the rate of surface chemical reactions. Furthermore, the research reveals that the electron density, temperature, and electric potential of the neon plasma within the cavity exhibit a centrally symmetric distribution, with maximum values of 1 x 1015 m-3, 0.53 eV, and 24.5 V, respectively. Subsequently, the effects of power, gas pressure, and frequency on the physical properties of the plasma are systematically analyzed. Among these, the 640 nm spectral line was selected as a qualitative indicator of glow intensity and plasma state changes within the cavity due to its strongest peak intensity. Findings indicate that increasing power and gas pressure enhance electron density and 640 nm spectral line intensity while reducing electron temperature. Additionally, comparative analysis of plasma properties in cells 1-9 reveals that most cells exhibit higher electron temperature and electron density at frequency II. (Frequency I serves as the transfer mode for plasma transfer, while frequency II acts as the stable mode for plasma stabilization.) Consequently, plasma cleaning of the 1.3 GHz 9-cell cavity using neon gas at this frequency is anticipated to yield superior cleaning outcomes. These findings enhance our understanding and control of plasma behavior within the SRF cavity and provide valuable insights for optimizing plasma cleaning processes.

Electropolishing (EP) has become a standard procedure for treating the inner surfaces of superconducting radio-frequency (SRF) cavities composed of pure niobium. In this study, a new EP facility was employed for the surface treatment of both 1.3 GHz and 3.9 GHz single-cell cavities at the Wuxi Platform. The stable “cold EP” mode was successfully implemented on this newly designed EP facility. By integrating the cold EP process with a two-step baking approach, a maximum accelerating gradient exceeding 40 MV/m was achieved in 1.3 GHz single-cell cavities. Additionally, an update to this EP facility involved the design of a special cathode system for small-aperture structures, facilitating the cold EP process for 3.9 GHz single-cell cavities. Ultimately, a maximum accelerating gradient exceeding 25 MV/m was attained in the 3.9 GHz single-cell cavities after undergoing the cold EP treatment. The design and commissioning of the EP device, as well as the electropolishing and vertical test results of the single-cell cavities, will be detailed herein. These methods and experiences are also transferable to multi-cell cavities and elliptical cavities of other frequencies.

The nitrogen doping (N-doping) treatment for niobium superconducting radio-frequency (SRF) cavities is one of the key enabling technologies that support the development of more efficient future large accelerators. However, the N-doping results have diverged due to a complex chemical profile under the nitrogen-doped surface. Particularly, under industrial-scale production conditions, it is difficult to understand the underlying mechanism thus hindering performance improvement. Herein, a combination of spatially resolved and surface-sensitive approaches is employed to establish the detailed near-surface phase composition of thermally processed niobium. The results show that intermediate phase segregations, particularly the nanometric carbon-rich phase, can impede the nitridation process and limit the interactions between nitrogen and the niobium sub-surface. In comparison, the removal of the carbon-rich layer at the Nb surface leads to enhanced nitrogen binding at the Nb surface. Combining the RF test results, it is shown that the complex uniformity and grain boundary penetrations of impurity elements have a direct correlation with the mid-field quench behavior in the N-doped Nb cavities. Therefore, proper control of the nanometric intermediate phase formation in discrete thermal steps is critical in improving the ultimate performance and production yield of the Nb cavities. The origin of the omnipresent mid-field quench issue of nitrogen-doped Nb cavities for modern electron accelerators is traced down to the nanometric phase compositions at the niobium surface by unraveling the key elemental interplay and functioning mechanisms of carbon, nitrogen, and oxygen. Guided by the findings, N-doped 1.3 GHz single-cell Nb cavity with top-tier performance is successfully manufactured.image

Nb3Sn is emerging as one of the focal points in superconducting radio frequency (SRF) research, owing to its excellent superconducting properties. These properties hold significant possibilities for cost reduction and the miniaturization of accelerators. In this paper, we report the recent efforts of the Shanghai Advanced Research Institute (SARI) in fabricating high-performance Nb3Sn superconducting cavities using the vapor diffusion method. This includes the construction of a Nb3Sn coating system with dual evaporators and the test results of 1.3 GHz single-cell coated cavities. The coated samples were characterized, and the growth state of the Nb3Sn films was analyzed. The first coated superconducting cavity was tested at both 4.4 K and 2 K, with different cooldown rates passing through the Nb3Sn critical temperatures. The causes of Sn droplet spot defect formation on the surface of the first cavity were analyzed, and such defects were eliminated in the coating of the second cavity by controlling the evaporation rate. This study provides a reference for the preparation of high-performance Nb3Sn-coated cavities using the vapor diffusion method, including the setup of the coating system, the comprehension of the vapor diffusion process, and the test conditions.

Several state-of-the-art recipes have been studied to pursue high quality factor (Q) and higher accelerating gradient (Eacc) on superconducting radio-frequency (SRF) cavities for the Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) project. Based on the newly constructed SHINE facilities for cavity surface treatment, a higher gradient has been achieved while maintaining high Q on single-cell and 9-cell cavities through cold electropolishing (EP) and an optimized nitrogen doping (N-doping) recipe. During the N-doping process, the furnace was first heated to 800 ℃ and maintained for 30 min to stabilize the temperature. Then, nitrogen was injected into the furnace. On single-cell cavities with Q0>4.0×1010 at a medium gradient, an average maximum accelerating gradient higher than 35 MV/m is achieved; among them, one cavity reaches its maximum Eacc=38.3 MV/m. On 9-cell cavities with Q0>2.8×1010 at a medium field, an average maximum accelerating gradient higher than 26 MV/m is achieved, which meets the SHINE requirements. The vertical test results of the nitrogen-doped cavities and the experience to improve the accelerating gradient will be presented in this paper.

经过对兰州市安宁污水处理厂冬季曝气池的活性污泥筛选、驯化分离获得反硝化聚磷菌;对其进行生理生化鉴定及核糖体的RNA的亚基(16S rDNA)基因序列测序,鉴定为荧光假单胞菌属Pseudomonas sp.以模拟废水的出水总氮(TN)、总磷(TP)、pH等为指标,考察反应温度、原水pH、摇床转速、不同碳氮比(C/N)等单因素对反硝化除磷菌脱氮除磷性能的影响.使用软件design-expert8.0.6.1将数据进行分析,获得二次响应面回归模型,并探究pH、温度、转速与TN去除率、TP去除率的交互作用.在最佳反应条件,即反应温度为16℃,反应时间为10 h,原水pH为7,摇床转速为140 r·min-1,C/N质量浓度比为12时,TN与TP的去除率分别为82.7%与88.1%,为今后城市废水处理提供支持.

The third harmonic superconducting cryomodule is being designed for the Shanghai High repetition rate XFEL and Extreme light facility(SHINE)project,which is under construction.In contrast to the European X-ray Free Electron Laser(E-XFEL)project,the 3.9 GHz cryomod-ules in the SHINE project will operate in the continuous wave regime with higher radio frequency average power for both cavities and couplers.We propose a 3.9 GHz fundamental power coupler with an adjustable antenna length,for satisfying the SHINE project requirements.Here,we describe the 3.9 GHz fundamental power cou-pler's design considerations and power requirements for various operating modes of the SHINE Linac.We also present the results of the radio frequency simulation and optimization,including the studies on multipacting and thermal analysis of the proposed 3.9 GHz coupler.

采用内转录间隔区(internal transcription spacer,ITS)序列分析和形态学鉴定相结合的方法,从腐烂的植物根中分离得到霉菌,利用真菌通用引物扩增菌株rDNA ITS区序列并对其进行鉴定.将序列提交至GenBank中以确定物种分类信息,进行物种同源序列比对分析,并通过Figtree构建物种进化系统发育树.ITS分子测序以及形态学鉴定结果表明,菌株LJM-001隶属于木霉菌属(Trichoderma).同时,利用该菌株作为指示微生物,模拟重金属和抗生素复合污染的生存环境,并以霉菌的干重、还原型谷胱甘肽含量以及蛋白质含量为表征,分析复合污染下微生物的生理生化变化,并对其进行相关性分析.实验结果表明,抗生素胁迫下的重金属抗性变化主要为协同杀菌和协同抗性,重金属离子浓度和种类均对菌株LJM-001耐抗生素性能有显著影响.

The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created. The first operation of the European X-ray free-electron laser facility accelerator based on superconducting technology is reported. The maximum electron energy is 17.5 GeV. A laser average power of 6 W is achieved at a photon energy of 9.3 keV.