Copper-based Nb3Sn film is a promising material for applications in the field of superconducting radio frequency (SRF). However, the interdiffusion of copper (Cu) from the substrates into the Nb3Sn films affects the superconductivity of the samples, and the preparation of Nb3 Sn–Nb–Cu composite films is a good solution. In this paper, the effects of the niobium (Nb) layer thickness on Cu interdiffusion and the critical temperature of Nb3Sn film were investigated. Nb3 Sn–Nb films were deposited on Cu substrates by magnetron sputtering. The obtained samples have been characterized via focused ion beam (FIB), time of flight secondary ion mass spectrometry (TOF-SIMS), and magnetic property measurement system (MPMS). It was found that the thicker the Nb isolation layer, the stronger the hindrance effect on the Cu interdiffusion and the higher the critical temperature. A suitable Nb isolation layer thickness was 2.6μm, which effectively hindered the Cu interdiffusion, corresponding to a critical temperature of 16.6 K for the samples.

As a promising material, in the field of superconducting radio frequency (SRF), Nb3Sn could be prepared via bronze route, which is fairly mature for preparing the superconducting cable. This paper studies how different annealing temperature and time affect the superconducting properties of Nb3Sn films made by bronze route. Niobium films with mu m thickness were deposited on bronze substrates by magnetron sputtering. The prepared samples were annealed at 700 C and 750 C for several hours to allow the diffusion of Sn from the bronze substrate to the niobium layer and the formation of the Nb3Sn phase. The obtained films have been characterized via X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy analysis (EDS), and magnetic property measurement system (MPMS). The peaks of the Nb3Sn phase were found by XRD and the EDS result showed the Sn% of the cross-section fluctuates around 23at.%. The average crystallite size is estimated to be about 82 nm. The highest critical temperature of Nb3Sn films prepared by this method is up to 17.6 K.

Nb3Sn is an alternative material for future superconducting accelerators. With its higher critical temperature and superheating field than that of niobium, its application in the field of SRF is a great advantage. In this paper, the magnetron sputtering method is applied to prepare the Nb3Sn thin film on the oxygen-free copper substrate by the stoichiometric target—the ratio of 3:1 of niobium to tin, so as to explore the effect of different annealing air pressures on the properties of the film in detail. The film crystal structure, morphology, composition, and superconducting critical temperature were characterized by XRD, SEM, EDS, and MPMS. After in-situ annealing, XRD and MPMS reveal the existence of Nb3Sn crystal. The compact film with good surface properties is free of cracks and tin islands. The superconducting critical temperature reaches 13.6 K. The results show that if annealing was conducted in Ar atmosphere at 1 Pa, the obtained film exhibits better performance with a sharp transition from the superconducting state to the normal state.

The Nb3Sn thin film cavity, having the potential to be operated at a higher temperature and higher gradient compared to the cavity made from bulk niobium, is one of the most promising key technologies for the next-generation radio-frequency superconducting accelerators. In our work, several 1.3 GHz single-cell TESLA-shaped Nb3Sn thin film cavities, coated by the vapor diffusion method, were tested at Peking University and Institute of Modern Physics, Chinese Academy of Sciences. It was observed that the performance of the Nb3Sn thin film cavities in the tests without the slow cooling down procedure and the effective magnetic field shielding was significantly improved by using a low temperature baking at 100 °C for 48 hours. Although the peak electric field of the cavity remained unchanged, the rapid drop of the unloaded Q value (Q 0) with the increasing accelerating field (Q-slope) was effectively eliminated, resulting in an improvement of the Q 0 in the intermediate field region by ∼ 8 times. Furthermore, under better test conditions with the shielded magnetic field less than 5 mG and the slow cooling down procedure in the temperature range of 25–15 K, the Q 0 was still improved by about 20%. Our study shows that the low temperature baking can be an effective supplement to the effective post-treatment for the Nb3Sn thin film cavity.

Niobium sputtered copper cavities were proposed as a kind of promising next generation superconducting cavities, but were still challenged by the Q-slope effect under high acceleration gradients. Current solutions focus on improving the film quality to make it more bulk-like, for which a higher substrate temperature is required. However, due to the limitation of the melting point of the copper substrate, both the deposition process and the post-annealing process cannot be performed at a high temperature. The laser annealing mentioned in this paper uses nanosecond pulsed laser as the heat source, and the local temperature field generated within the thickness scale of the niobium film can anneal the film without affecting copper substrate. Laser annealing system has been set up in Peking University, and experiments with niobium thin film on copper (Nb/Cu) samples have been carried out. Superconducting performance, surface topography and other properties of Nb/Cu samples before and after annealing are compared. Recrystallization of niobium films happened and various factors that may cause Q-slope have been suppressed according to the results. All these indicate the effectiveness of laser annealing and the possibility of being used in niobium sputtered copper cavities in the future.

Superconducting Nb3Sn thin films have been produced on copper substrate by dc magnetron co-sputtering technique successfully. XRD and magnetic moment measurement demonstrate the exist of Nb3Sn crystal. By changing the ratio between niobium target sputtering current to tin target, the tin concentration in different films is varied. However, the experiment results indicate critical temperature TC of different Nb3Sn thin film are nearly invariable while the composition of Nb-Sn of those are changed. Highest TC of those films annealing at 650°C are around 12K, while 750°C annealing increase it to 15K. The SEM and element analysis results manifest inhomogenous distribution of tin atoms, where some tin islands appear over the film. The exist of tin-rich areas like tin islands is responsible for such particular performance of Nb3Sn films.

With the development of radio frequency superconducting accelerator, research on thin film coated copper cavity becomes increasingly attentive. High Power Impulse Magnetron Sputtering (HiPIMS) is a kind of promising emerging PVD technique in comparision with dc magnetron sputtering already used on thin film coated copper cavities. In this paper, the physical properties of niobium films condensed by conventional dc magnetron sputtering (DCMS) and by high power impluse magnetron sputtering (HiPIMS) are evaluated and compared. The dc superconductivity of films with these two techniques is similar and all transition temperature is 9.5K. Further more, the microstructure of niobium films deposited by HiPIMS at 150 °C belong to zone T in contrast to that by DCMS in zone I. From AFM results, the surface roughness by HiPIMS and DCMS is 3.92 ± 0.14 nm and 6.67 ± 0.39 nm individually. Adhesion strength is obtained by scratch tests and the films critical load LC by HiPIMS is 5.53 ± 0.93 N and that by DCMS is 1.99 ± 0.13N. Films by HiPIMS have better structure-properties including morphology, surface roughness, and adhesion strength, which are beneficial to Q-slope mitigation in niobium film coated copper superconducting cavities.

The results from DC magnetization measurements and direct observation of hydrides precipitation of rectangular bulk nitrogen doping niobium (Nb) samples used for the fabrication of superconducting radio frequency (SRF) cavities were presented. The surface treatments of the Nb samples consisted of heavy electropolishing (EP), nitrogen doping and the subsequent EP with different amounts of material removal. The critical fields and the superconducting parameters were calculated from the magnetization data and their dependence on the subsequent EP material removal was presented. Unexpected pinning behavior of the N-doped Nb samples with subsequent EP material removal was observed and the Nb hydrides were suspected to be the possible contributor to the unexpected pinning effects of the N-doped Nb samples. Direct observation of Nb hydrides precipitation on both the N-doped and the un-doped Nb samples has been carried out using Scanning Electron Microscope (SEM) with a cold stand at 80 K, providing direct evidence of the N-H interaction that the amounts of Nb hydrides precipitation on the N-doped samples can be reduced to varying degrees with different amounts of material removal. Based on the study of magnetic properties and Nb hydrides precipitation observation, a possible physical image of N-H interaction affecting the magnetic properties of the N-doped Nb samples was proposed. The effects of subsequent EP material removal dependence of magnetic properties and Nb hydrides precipitation reveal the important role of N-H interaction in the understanding of the physical mechanism of nitrogen doping phenomenon.

磁控溅射镀膜电源是磁控溅射系统中的关键设备之一.根据铌靶和锡靶溅射处理装置的技术要求,研制了一套输出电压0～800 V可调、脉冲宽度5～200μs可调、频率0～60 Hz可调、在脉冲电流最大幅值约150 A的磁控溅射镀膜电源,分别给出了该电源在铌靶负载和锡靶负载下的实验结果.设计上采用高压短脉冲预电离一体化高功率双极性脉冲形成电路方法,解决了高功率磁控溅射在重复频率工作下有时不能成功溅射粒子、电离时刻不一致、溅射起弧打火靶面中毒、溅射效率低等问题,降低了磁控溅射装置内气体的工作气压,实现低气压溅射镀膜,提高了靶材的溅射效率,减小薄膜表面粗糙度.通过大量实验论证,该电源达到了理想的溅射效果,满足了指标要求.

China Academy of Engineering Physics terahertz free electron laser (CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which was jointly built by CAEP, Peking university and Tsinghua university. The stimulated saturation of the CTFEL was reached in August, 2017. CTFEL consists of a GaAs photocathode high-voltage DC gun, a superconducting RF linac, a planar undulator and a quasi-concentric optical resonator. The terahertz lasers frequency is continuously adjustable from 2 THz to 3 THz. The average power is about 20W and the micro-pulse power is more than 0:3MW.

China Academy of Engineering Physics tera-hertz free electron laser (CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which was jointly built by CAEP, Peking university and Tsinghua university. The stimulated saturation of the CTFEL was reached in August, 2017.

China Academy of Engineering Physics tera-hertz free electron laser (CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which was jointly built by CAEP, Peking university and Tsinghua university. This THz FEL facility consists of a GaAs photocathode high-voltage DC gun, a superconducting RF linac, a planar undulator and a quasi-concentric optical resonator. The terahertz lasers frequency is continuous adjustable from 2 THz to 3 THz. The macro-pulse average power is more than 10 W and the micro-pulse power is more than 0.5 MW.

Micro-pulse electron gun (MPG) is a novel electron source which can produce narrow-pulse, high-repetition rate electron current. Theoretical and experiment work have been done to study physical properties and steady operating conditions of MPG. Proof-of-principle work has been finished and the next work is to research the parameters of the MPG electron beam and master the MPG work property deeply. Thus, a high voltage accelerating platform which can supply 100 kV direct voltage was designed. Furthermore, electromagnetic and mechanism designs were operated to adapt the high voltage platform and measure beam parameters.

A novel S-Band Micro-Pulse electron Gun (MPG) which works at the left crossover point energy EcI on the Secondary Electron Yield (SEY) curve was proposed. The working principles of the MPG were presented. The maximum output beam current limited by space charge effects and beam loading effects was investigated by theoretical analysis. The result shows that the maximum beam current is decided by the parameters of the MPG such as resonant frequency, cavity length, shunt impedance and the secondary emission property of the cathode and grid, having nothing to do with the power flowed into the cavity. The low shunt impedance and the low slope of the SEY curve can help increasing the maximum beam current. According to the principles, a MPG with the frequency of 2.856 GHz has been designed and constructed. The steady working state was achieved by using oxygen free copper and molybdenum grid with different transmission coefficient. It was found a good agreement between the analysis and the experiments.

China Academy of Engineering Physics terahertz free electron laser (CAEP THz FEL,CTFEL) is the first THz FEL oscillator in China,which is jointly built by CAEP,Peking University and Tsinghua University.It is designed as a high-repetition-rate and high-duty-cycle linac-based FEL facility. This THz FEL mainly consists of a gallium arsenide (GaAs) photocathode high-voltage direct current (DC) gun,a superconducting radio frequency (RF) linac,a planar undulator,and a quasi-concentric optical resonator. The DC gun provides a high-brightness electron beam with the bunch charge of about 100 pC and the repetition rate of 54.167~MHz.The normalized emittance of the electron beam is less than 10m,and the energy spread is less than 0.75%.A 24-cell superconducting RF accelerator provides an effective field gradient of about 10 MV/m and energizes the electron beam to 6-8~MeV.The beam then goes through the undulator and generates the spontaneous radiation,which is reflected back and forth in the optical resonator and then stimulated by the electron beam. The first stimulated saturation of CTFEL in the macro-pulse mode was obtained in August,2017.In this paper,the THz spectrum is measured by a Fourier spectrometer (Bruker VERTEX 80 V).The macro-pulse energy is measured by an absolute energy meter from Thomas Keating Instruments.The longitudinal beam length is preliminarily calculated by the auto-correlation curve from the time-domain signal of the spectrometer.The macro-pulse duration is captured by a GeGa cryogenic detector from QMC Instrument.The measurement results indicate that the terahertz laser frequency is continuously adjustable from 2 THz to 3 THz.The macro-pulse average power is more than 10 W and the micro-pulse power is more than 0.3 MW.The single-pass gain is larger than 2.5%. This facility is now working in macro-pulse mode in the first step,also called step one.The minimum macro-pulse duration is about 50s and the maximum is about 2 ms.The macro-pulse repetition is 1 Hz or 5 Hz.The typical pulse duration and repetition rate are 1 ms and 1 Hz,respectively.In the middle of 2018,the duty cycle will upgrade to more than 10% as step two.And the continuous wave (CW) operation will be obtained in step three by the end of 2018.The spectrum adjustment range will also be expanded to cover from 1 THz to 4 THz by then. Some application experiments have been carried out on the platform of CTFEL.This facility will greatly promote the development of THz science and its applications in material science,chemistry science,biomedicine science and many other cutting-edge areas in general.

Nitrogen doping study on niobium (Nb) samples used for the fabrication of superconducting radio frequency (SRF) cavities was carried out. The samples' surface treatment was attempted to replicate that of the Nb SRF cavities, which includes heavy electropolishing (EP), nitrogen doping and the subsequent EP with different amounts of material removal. The surface chemical composition of Nb samples with different post treatments has been studied by XPS. The chemical composition of Nb, O, C and N was presented before and after Gas Cluster Ion Beam (GCIB) etching. No signals of poorly superconducting nitrides NbNx was found on the surface of any doped Nb sample with the 2/6 recipe before GCIB etching. However, in the depth range greater than 30 nm, the content of N element is below the XPS detection precision scope even for the Nb sample directly after nitrogen doping treatment with the 2/6 recipe.

Bipolar pulsed power supply is one of the key equipment in magnetron sputtering system.Based on the working principle and technical characteristics of magnetron sputtering,a bipolar pulsed power supply with output voltage of 0-800 V, pulse width of 20-200 μs,frequency of 0-60 Hz,maximum pulse current of 150 A was developed,and the experimental results for the power supply with the water resistance load and the plasma load were obtained.The charging to the energy storage capacitor in the bipolar pulsed power supply was controlled by the DSP control mode.Integrating FPGA,PLC and touch screen man-ma-chine exchange system,the bipolar pulses were got.A large number of experimental demonstration shows that the power supply solves the problems of plasma load discharge et al.,and the designed bipolar pulsed power supply achieves the desired sputtering effect and fulfills the requirements of the technical index.

China Academy of Engineering Physics tera-hertz free electron laser (CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which was jointly built by CAEP, Peking university and Tsinghua university. The stimulated saturation of the CTFEL was reached in August, 2017. This THz FEL facility consists of a GaAs photocathode high- voltage DC gun, a superconducting RF linac, a planar undulator and a quasi-concentric optical resonator. The terahertz laser's frequency is continuous adjustable from 2 THz to 3 THz. The average power is more than 10 W and the micro-pulse power is more than 0.5 MW.