Investigation on the fabrication of the 3rd harmonic superconducting cavity for the SSRF storage ring

A third harmonic superconducting niobium cavity has been proposed for installation in the Shanghai Synchrotron Radiation Facility （SSRF） storage ring to improve the Touschek lifetime. In order to investigate the feasibility of the superconducting cavity fabrication indigenously and the possibility to master the fabrica tion techniques, cavities were fabricated from copper and niobium sheets by deep drawing and electron-beam welding, and a series of measurements, such as resonant frequency, shape dimensions and wall thickness, were carried out during this process. After analysis of various problems existing in the fabrication process, technique improvements were proposed, and finally the precise shape as designed and resonant frequency within 1.2 MHz were achieved for the new completed cavities. In addition, full annealing was finally proved to be a good cure for niobium sheets＇ tearing up during deep drawing. By fabricating niobium cavities successfully, some problems to the next step were cleared. This paper introduces the process of cavity fabrication and its technique improvements towards forming, and the initial vertical test result of niobium cavity is also presented.     

Study on the 1.3 GHz low loss shape superconducting cavities at IHEP

As part of the international research program on the superconducting cavity for the International Linear Collider (ILC) R&;D on the 1.3 GHz low loss superconducting cavities has been carried out at the Institute of High Energy Physics (IHEP) since 2005. A design of 1.3 GHz low loss cavity shape was proposed and six single-cell cavities of different niobium material were successfully fabricated with standard technology. In this study our priority was on large grain (LG) cavities. The two LG cavities were treated with complete procedures of surface treatments based on chemical polishing (CP) without electro polishing (EP) at IHEP. The two LG cavities and a fine grain cavity were sent to KEK for vertical testing. All the three cavities reached accelerating gradients higher than 35 MV/m and the maximum gradient of 40.27 MV/m was achieved in the LG cavity. This paper presents the process of the vertical RF tests and the comparison of the LG and fine grain cavities's performance.     

Study on the 1.3 GHz low loss shape superconducting cavities at IHEP

As part of the international research program on the superconducting cavity for the International Linear Collider (ILC) R&D on the 1.3 GHz low loss superconducting cavities has been carried out at the Institute of High Energy Physics (IHEP) since 2005. A design of 1.3 GHz low loss cavity shape was proposed and six single-cell cavities of different niobium material were successfully fabricated with standard technology. In this study our priority was on large grain (LG) cavities. The two LG cavities were treated with complete procedures of surface treatments based on chemical polishing (CP) without electro polishing (EP) at IHEP. The two LG cavities and a fine grain cavity were sent to KEK for vertical testing. All the three cavities reached accelerating gradients higher than 35 MV/m and the maximum gradient of 40.27 MV/m was achieved in the LG cavity. This paper presents the process of the vertical RF tests and the comparison of the LG and fine grain cavities's performance.     

射频超导谐振腔中的电磁场研究

由高纯铌制成的射频超导谐振腔在高场时会出现Q值下降现象，这是高场下超导腔表面局部磁场增强所致. 为研究高场下超导腔表面的电磁场特性，用单cell超导腔设计了腔表面峰值磁场的研究实验. 通过对单cell超导腔表面峰值磁场的测量，可以确定超导腔焊缝及iris附近表面是否存在缺陷，以便进一步进行腔的处理来改进腔的性能.     

Manufacture of superconducting niobium cavity parts by hydropercussion punching

A complete cycle of manufacturing all parts of superconducting niobium cavities which comprise two half-cells, two drift tubes, and two flanges each has been developed. The cavity half-cells are manufactured by hydropercussion punching, which has a number of advantages over the instrumental stamping technique. For the first time, the diagram of formability of ultrapure niobium has been experimentally obtained for hydropercussion punching and the key parameters of the process have been determined that ensure complete deep-drawing of workpieces, viz., the value of the limit drawing ratio of ultrapure niobium that is 1.92 at a specific impact energy of 0.42 MJ/m². The deviations of the half-cell dimensions from the rated values do not exceed 0.1 mm. Production tools required for all manufacturing steps have been created. The parameters of machining of the niobium cavity parts prior to welding have been experimentally established.     

Computation of single-cell superconducting niobium cavity for accelerator of electrons and positrons

Computations of the accelerator section of the International Linear Collider (ILC), which consists of superconducting niobium cavities, are performed for conditions of the maximum energy transfer to electrons that travel along the cavity axis. A mathematical model and software packages are created for the computation of the electric characteristics and profile of a single-cell cavity. A computer-based synthesis of the cavity shape that yields the required electric characteristics is performed. The promising design variants of a single-cell cavity, with which a quality factor of 10¹⁰ is provided at a working frequency of 1.3 GHz, are found to optimize the construction and manufacture of a single-cell cavity. The electric characteristics of a chain of single-cell cavities are computed.     

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency (RF) cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode (HOM) couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF (TESLA Test Facility) HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project (PEFP) in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.     

Experimental study on SC RF cavities by using China large grain niobium for ILC

Large grain niobium has the potential of simplifying the production sequence and consequently reducing the cost of the superconducting RF cavities for ILC. To investigate the feasibility of fabrication and the possibility to achieve high gradient by large grain cavities, two 1.3GHz cavities were made of China large grain niobium and a series of vertical tests were carried out following several different surfaces treatment procedures. Two cavities have both reached the high gradient of more than 43MV/m repeatedly and the maximum accelerating field of 47.9MV/m has been achieved by China large grain niobium. This paper introduces the features of the fabrication and surface treatments on the large grain cavities and presents the preliminary results of the research.     

Experimental study on SC RF cavities by using China large grain niobium for ILC

Large grain niobium has the potential of simplifying the production sequence and consequently reducing the cost of the superconducting RF cavities for ILC.To investigate the feasibility of fabrication and the possibility to achieve high gradient by large grain cavities,two 1.3 GHz cavities were made of China large grain niobium and a series of vertical tests were carried out following several different surfaces treatment procedures.Two cavities have both reached the high gradient of more than 43 MV/m repeatedly and the maximum accelerating field of 47.9 MV/m has been achieved by China large grain niobium.This paper introduces the features of the fabrication and surface treatments on the large grain cavities and presents the preliminary results of the research.     

超导在加速器中的应用概况

文中简要回顾了世界上已建造的大型超导加速器,重点介绍未来的超导加速器方案,对超导新材料在加速器中的应用研究以及我国超导在加速器中的应用研究情况作了简要介绍.     

上海光源500MHz超导腔水平测试

上海光源是能量为3.5 GeV的第三代先进中能同步辐射光源,其储存环上安装了三台超导高频腔补偿电子因同步辐射等原因丢失的能量。为保障上海光源的长期稳定高效运行,中国科学院上海应用物理研究所和上海市低温超导高频腔技术重点实验室共同研制了具备低高次模损失参数和可承受更高入射功率的新型500 MHz超导腔,作为上海光源在线运行超导高频腔的备用腔。超导铌腔经低温垂直测试达到所需加速性能后,需要与高功率输入耦合器、高次模吸收器、低温恒温器等集成并完成水平测试,获得超导腔模组的加速性能、低温性能和真空性能。介绍了超导腔备用腔的研制、集成和测试过程,采用文丘里(Venturi)校准法获得模组的静态功耗反应模组的低温性能,并通过高功率测试获得了超导腔备用腔模组的加速性能。测试结果表明:自主研制的500 MHz超导腔备用腔满足上海光源的工作需求,在超导腔的加速腔压为2.0 MV时,无载品质因数为1.2×109 @4.2 K,且低温模组的静态热损耗为36.1 W。     

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency(RF)cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode(HOM)couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF (TESLA Test Facility)HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project(PEFP)in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.     

Great progress in developing 500 MHz single cell superconducting cavity in China

Superconducting cavities have been adopted in many kinds of accelerator facilities such as synchrotron radiation light source, hard X-ray free electron laser linac, colliders and energy recovery linacs (ERL). The 500 MHz superconducting cavities will be a candidate to be installed in the high current accelerators and high current ERLs for their large beam aperture, low higher order modes impedance and high current threshold value. This paper presents great progress in the whole sequence of developing 500 MHz superconducting cavity in China. It describes the first in-house successful development of 500 MHz single cell superconducting cavity including the deep-drawing of niobium half cells, electron beam wielding of cavity, surface preparations and vertical testing. The highest accelerating gradient of the fabricated cavity #SCD-02 higher than 10 MV/m was obtained while the quality factor was better than 4×10⁸ at 4.2 K, which has reached the world level of the same kind of cavities.     

Shape determination for deformed electromagnetic cavities

The measured physical parameters of a superconducting cavity differ from those of the designed ideal cavity. This is due to shape deviations caused by both loose machine tolerances during fabrication and by the tuning process for the accelerating mode. We present a shape determination algorithm to solve for the unknown deviations from the ideal cavity using experimentally measured cavity data. The objective is to match the results of the deformed cavity model to experimental data through least-squares minimization. The inversion variables are unknown shape deformation parameters that describe perturbations of the ideal cavity. The constraint is the Maxwell eigenvalue problem. We solve the nonlinear optimization problem using a line-search based reduced space Gauss–Newton method where we compute shape sensitivities with a discrete adjoint approach. We present two shape determination examples, one from synthetic and the other from experimental data. The results demonstrate that the proposed algorithm is very effective in determining the deformed cavity shape.     

北京大学国产大晶粒铌材超导腔的性能研究

大晶粒铌材是射频超导领域的研究热点之一. 采用大晶粒铌材的射频超导谐振腔, 其后处理工艺可以大大简化. 北京大学对此进行了深入细致的研究, 自主研制了采用国产大晶粒铌材的射频超导腔, 对这些超导腔进行了简单的表面处理, 包括标准的化学抛光(BCP)和120℃低温烘烤处理, 未进行非常复杂的电抛光处理, 低温性能测试结果表明其低温超导性能优越, 大晶粒1.3GHz超导腔的加速梯度达到了43.5MV/m, 为我国超导加速器的国产化打下了基础.     

铌基超导量子比特及辅助器件的制备

近年来超导量子计算的研究方兴未艾,随着谷歌宣布首次实现“量子优势”,这一领域的研究受到了人们进一步的广泛关注.超导量子比特是具有量子化能级、量子态叠加和量子态纠缠等典型量子特性的宏观器件,通过电磁脉冲信号控制磁通量、电荷或具有非线性电感和无能量耗散的约瑟夫森结上的位相差,可对量子态进行精确调控,从而实现量子计算和量子信息处理.超导量子比特有着诸多方面的优势,很有希望成为普适量子计算的核心组成部分.以铌或其他硬金属(如钽等)为首层大面积材料制备的超导量子比特及辅助器件(简称铌基器件)拥有其独特的优点以及进一步发展的空间,目前已引起越来越多的兴趣.本文将介绍常见的多种超导量子比特的基本构成和工作原理,进而按照器件加工的一般顺序,从基片选择和预处理、薄膜生长、图形转移、刻蚀和约瑟夫森结的制备等方面详细介绍铌基超导量子比特及其辅助器件的多种制备工艺,为超导量子比特的制备提供一个可借鉴的清晰的工艺过程.最后,介绍若干制备铌基超导量子比特与辅助器件的具体例子,并对器件制备的工艺与方法的优化做展望.     

CiADS超导直线加速器超导腔失效的分段补偿与冗余设计

加速器驱动次临界装置（ADS）对加速器运行稳定性和失束指标提出了前所未有的要求。对于超导直线加速器的研究发现,超导腔失效是失束的一个较大的来源,故针对超导腔的失效情况,本文提出分段补偿新方法,以提高高功率超导直线加速器的运行稳定性。提出的分段补偿方法与已有的全局补偿和局部补偿的方法相比,在保证加速器出口束流品质与无束损束流传输的同时,分段补偿束流能量,优化了参与能量补偿的超导腔数量,降低超导腔的备份功率源需求。论文最后针对CiADS的超导直线加速器的物理设计,做了分段补偿的多粒子模拟。结果表明,采用了分段补偿方法实现超导腔失效补偿的前提下,参与补偿过程中改变腔压的超导腔数量比例为48%,对功率源总的功率备份冗余需求小于20%。The accelerator driven subcritical system (ADS) has put forward unprecedented demands on the stability and beam trip of the accelerator operation. Depending on analysis, failure of the superconducting cavities is a major cause for beam trip of the superconducting cavity. Therefore, a new method of piecewise compensation is proposed to improve the stability of high power superconductivity linac. The piecewise compensation scheme proposed in this paper is compared with the existing global compensation and local compensation technology. While guaranteeing the beam quality of the accelerator and without beam loss transmission, the piecewise compensation method can optimize the number of superconducting cavities involved in energy compensation and reduce the demand for the backup redundancy of power sources of the superconducting cavities. At the end of the paper, the multi-particle simulation of piecewise compensation aims at the physical design of CiADS superconducting linac. The result shows that 48% of superconducting cavities modify the cavity's Epeak during the compensation process and the demanded redundancy of total power sources is less than 20% under the premise of successful compensation for the failure of superconducting cavities through the piecewise compensation method.     

射频超导腔加速性能的改进

 为提高超导加速腔的加速梯度和Q值，改进了薄膜型超导腔的加速性能。研究证明，对于铜铌溅射腔，在无氧铜衬底和铌膜之间加入NbN 层可以提高铌膜的超导转变温度，改善晶格结构；对纯铌超导腔提出了改进方法，在传统的纯铌超导腔表面制备多层的超导-绝缘-超导复合膜可以屏蔽Nb腔表面的界面场，提高超导腔的临界磁场，从而提高了铌腔的加速梯度。     

Researches and Preliminary Experiments on Copper-Niobium Sputtered Low-β Superconducting Resonator

Superconducting resonator is the important component of a superconducting accelerator. The best choice of the acceleration cavity for heavy ion superconducting accelerator is quarter wave resonator (QWR). By sputtering a layer of niobium films of several microns on the OFHC copper substrate, good superconducting properties and acceleration properties of the cavity can be obtained. However, it is very difficult to get uniform niobium films by sputtering because the inner surface of the QWR is complicated. To deal with this problem, we developed a multiparameter adjusting method to control the sputtering process of different parts of the QWR. A layer of uniform niobium film with nice superconducting performance is obtained. The low temperature experiments with liquid helium show that the Cu Nb QWR has good superconducting characters. The Q value of the cavity is about 5×10⁸.     

超导腔数字自激垂直测试系统

基于数字自激算法设计并实现的超导腔垂直测试系统提高了超导腔的垂直测试效率,为先进光源技术研发与测试平台(PAPS)的超导腔批量化生产提供了重要保障;垂直测试系统的射频前端和时钟分配系统采用了二次上下变频方案,可以在一定范围内灵活设置测试系统自激环路的工作频率,增大了该测试系统的工作带宽。利用此系统完成了1.3 GHz 9-cell超导腔的通带频率测试,结果表明,该测试系统能有效避免不同模式之间的串扰,具备较强的频率分辨能力(<800 kHz),保证多单元超导腔性能验证的进行。