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.     

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.     

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.     

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.     

The First Nine-Cell TESLA Cavity Made in China

A totally home-made 9-cell TESLA type superconducting cavity is mane at Peking University. The cavity abrication is according to DESY specification. The cavity is made of high purity niobium from OTIC, Ningxia. The electron beam welding is carried out at Harbin Institute of Technology, Harbin. By the cooperation, the cavity is tested at Thomas Jefferson National Accelerator Facility, USA. The preliminary result shows the acceleration gradient Eacc is 23 MV/m without quench and has potential for improvement.     

射频超导腔的研究新进展

 射频超导谐振腔已经大规模地应用到粒子加速器领域，其优越之处在于它可以在CW模式或长宏脉冲模式下，提供高的加速梯度。射频超导已经成为自由电子激光和能量回收直线加速器的关键技术。经过30多年的研究发展，解决了超导腔的热崩溃、场致发射等诸多关键问题，目前加速梯度已经超过40 MV/m。高加速梯度的获得是射频超导领域的前沿热点，电抛光＋低温热处理技术使射频超导腔的加速梯度提高3～4 MV/m。最新发展起来的超导腔的干式处理可以改善超导腔的表面状况，提高超导腔的Q值，抑制次级电子发射效应，有可能成为提高超导腔性能的又一有效手段。     

用于射频超导腔制造的大晶粒铌材热导研究（英）

大晶粒铌材由于声子峰效应在2 K温度具有良好的热导,这有助于提高超导腔的热稳定性。对宁夏东方钽业股份有限公司（OTIC）生产的大晶粒铌材在不同热处理条件下的热导和晶格缺陷开展了研究,结果表明经过超过800 ℃的热处理,OTIC的大晶粒铌材能够恢复声子峰,这与之前DESY的测量结果不同。认为用其加工的超导腔有望具备较好热稳定性。     

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 Shang-hai 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, tech-nique 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.     

Cold RF test and associated mechanical features correlation of a TESLA-style 9-cell superconducting niobium cavity built in China

The RF performance of a 1.3 GHz 9-cell superconducting niobium cavity was evaluated at cryogenic temperatures following surface processing by using the standard ILC-style recipe. The cavity is a TESLA-style 9-cell superconducting niobium cavity, with complete end group components including a higher order mode coupler, built in China for practical applications. An accelerating gradient of 28.6 MV/m was achieved at an unloaded quality factor of 4×10⁹. The morphological property of mechanical features on the RF surface of this cavity was characterized through optical inspection. Correlation between the observed mechanical features and the RF performance of the cavity is attempted.     

Physical calculation for an X-band hybrid dielectric-iris-loaded accelerator

A small, high performance X-band hybrid dielectric-iris-loaded travelling-wave linac with the length of 1.47 m and the maximum accelerating gradient of 45 MV/m has been designed. The beam energy of 33 MeV, the energy spread of 0.5%, the beam emittance of about 5.7 πmm·mrad and the capture efficiency of 40% are reached by adjusting the sizes of the accelerating cavities and the phase velocity. The attenuation per unit length of structure, the shunt impedance Rs, the quality factor Q, the group velocity and the phase velocity are 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.     

Frequency and temperature dependence of electrical breakdown at 21, 30, and 39 GHz

A TeV-range e(+)e(-) linear collider has emerged as one of the most promising candidates to extend the high energy frontier of experimental elementary particle physics. A high accelerating gradient for such a collider is desirable to limit its overall length. Accelerating gradient is mainly limited by electrical breakdown, and it has been generally assumed that this limit increases with increasing frequency for normal-conducting accelerating structures. Since the choice of frequency has a profound influence on the design of a linear collider, the frequency dependence of breakdown has been measured using six exactly scaled single-cell cavities at 21, 30, and 39 GHz. The influence of temperature on breakdown behavior was also investigated. The maximum obtainable surface fields were found to be in the range of 300 to 400 MV/m for copper, with no significant dependence on either frequency or temperature.     

Status of the IHEP 1.3 GHz superconducting RF program

The 1.3 GHz superconducting radio-frequency (SRF) technology is one of the key technologies for the ILC and future XFEL and ERL projects in China. With the aim to develop 1.3 GHz SRF technology, IHEP has started a program to build an SRF Accelerating Unit. This unit contains a 9-cell 1.3 GHz superconducting cavity, a short cryomodule, a high power input coupler, a tuner and a low level RF system. This program also includes the SRF laboratory upgrade, which will permit the unit to be built and tested at IHEP. The unit will be used for the 1.3 GHz SRF system integration study, high power horizontal test and possible beam test in the future. In this paper, we report the recent R&D status of this program. The first large grain low-loss shape 9-cell superconducting RF cavity made by IHEP reached 20 MV/m in the first vertical test in July, 2010. The prototype tuner and low level RF (LLRF) system are under test. The high power input coupler and cryomodule are under fabrication. Several key SRF facilities for 9-cell cavity surface treatment and pre-tuning were successfully commissioned and are in operation.     

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.     

射频超导技术与ERL技术新进展

射频超导谐振腔可以工作在连续波或长宏脉冲模式. 射频超导技术已发展为加速各种带电粒子束的重要手段. 射频超导技术发展的前期受材料性能、腔的处理以及加工安装水平等的限制. 经过几十年的不断改进, 射频超导技术获得了重大突破. 射频超导腔应用到超导加速器上并成功运行, 积累了腔的质量控制工艺和工业化制备的大量经验. 近期国际上面对未来大科学装置项目, 在射频超导技术方面进行了大量的研发工作, 主要包括提高超导腔加速梯度的新腔型研究和采用新型材料(大晶粒铌材)超导腔的研究. 能量回收直线加速器(ERL)技术是近年来获得发展的重要加速器技术. ERL具有高效、节能、稳定性好、低辐射水平等优势, 被越来越多地应用到先进光源和自由电子激光装置中.     

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。     

Flattop operation of the ILC accelerating cryomodule

A 500 GeV center-of-mass International Linear Collider (ILC), currently under R&D development, is foreseen as the next-generation high-energy physics (HEP) instrument [1]. The achievement of a 31.5 MV/m average operational accelerating gradient in a single cryomodule is a proof of principle for the ILC project. However, individual cavity performance may have a large spread in operating gradients, up to 20% of the nominal value [2, 3]. In case of cavities performing below the average, the design parameters could be achieved by tweaking the RF distribution accordingly. We present a simple theoretical analysis of the ILC cryomodule operation with a gradient spread. The difference in the gradients breaks the synchronism of a transient processes in each cavity and causes nonuniform acceleration along the bunch train. A proper solution was found to maintain flattop operation of the accelerating module. Finally, we perform numerical efficiency estimations for the proposed RF distribution scheme based on real data of the gradient spread of actual cavities. The text was submitted by the authors in English.     

用于自由电子激光的S波段J型波导馈电加速管

随着波导式耦合行波加速管设计梯度的日益提高,为了防止加速管输入耦合器电场的横向动量在束流通过加速器耦合器时引起束流品质的下降,侧壁开有两个对称耦合孔的对称双馈圆柱腔获得了广泛的应用。研制的S波段J型波导馈电加速管即为双馈圆柱腔中的一种,研制样管在老练平台上老练时的最高加速梯度达到30 MV/m。然而因为四极场的存在,开有两个耦合孔的圆柱耦合腔内,仍然会引起轴向电场的幅度和相位在横向的梯度,从而使束流发射度变差。在理论上对J型波导馈电的跑道式耦合腔进行了研究,通过与圆柱腔进行比对模拟计算,证明跑道式耦合器可以很好地改善轴向电场在横向平面内非近轴区域沿圆周的场强一致性,从而减小四极场的影响。重要的是,J型波导馈电跑道式耦合腔的机械加工、测试比圆柱腔更加容易实现,是未来双馈加速器发展的一个理想方向。     

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.     

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.