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.C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.     

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.     

超导腔垂直测量系统中的磁屏蔽

简要介绍空间磁场对纯铌超导加速腔性能的影响,以及在超导腔垂直测量时对空间磁场进行有效屏蔽的方法.由于多数磁性材料对应力和温度变化非常敏感,而且国内缺乏在低温下相关磁屏蔽材料性能的数据,为此对8种国产铁磁和软磁材料在低温下的初始磁导率进行了测量,并给出了相应的测试结果.最后介绍了作者研制的1.3GHz超导腔垂直测量低温恒温器内置式磁屏蔽装置及其性能.     

1.3 GHz超导腔的电化学青铜法铌三锡镀膜技术

由于射频超导腔具有高品质因数Q₀,大束流孔径等诸多优势,已被加速器行业广泛应用。目前纯铌腔的性能已经接近理论极限,使用Nb₃Sn薄膜腔代替纯铌腔是突破这一限制的有效手段。铌三锡具有较高的超导转变温度和过热磁场,理论预期可以大幅度提高SRF腔体工作温度和加速梯度。目前,Nb₃Sn薄膜制备技术蓬勃发展,其中锡蒸汽扩散法已经比较成熟,已制备出初步满足工程需求的铌基铌三锡薄膜射频超导腔。但是由于反应温度在1100℃以上,锡蒸汽扩散法无法摆脱纯铌基底,因此不可避免地在机械稳定性、导热性等方面有缺陷,难以满足未来高可靠性加速器的应用。青铜法广泛应用于铌三锡线缆的制备,热处理温度不高于700℃,具有制备铜基铌三锡镀膜腔的潜力。此外,电化学镀膜与其他方式相比,具有成本低、反应过程容易控制、常温常压等明显优势。本工作将上述两种工艺结合起来,研究了电化学方式在1.3 GHz铌基超导腔上镀青铜前驱体,之后热处理合成铌三锡薄膜腔。垂测结果表明,4.2 K下的薄膜腔本征Q₀为6×10⁸左右且仍具很大提升...     

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。     

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。     

BEPCⅡ超导腔静态热负荷和无载品质因数Q_0的测量

超导腔的静态热负荷和无载品质因数是表征超导腔低温恒温器以及超导铌腔性能好坏的最重要参数.BEPCⅡ超导腔采用的是液氨浸泡冷却方式,对两个超导腔在测试站分别进行了降温调试,在超导腔达到超导状态并稳定运行后,对其静态热损耗进行了测定.此外,超导腔Q_0的测量主要是采用热力学的方法测量其高频损耗然后经计算得出Q_0.介绍了BEPCⅡ超导腔静态热负荷和无载品质因数的测量原理及方法,并且给出了两个超导腔在不同高频加速电压下的测试结果.此测试结果已作为BEPCⅡ超导腔验收测试的重要依据.     

BEPCⅡ超导腔静态热负荷和无载品质因数Q0的测量

超导腔的静态热负荷和无载品质因数是表征超导腔低温恒温器以及超导铌腔性能好坏的最重要参数.BEPCⅡ超导腔采用的是液氦浸泡冷却方式,对两个超导腔在测试站分别进行了降温调试,在超导腔达到超导状态并稳定运行后,对其静态热损耗进行了测定.此外,超导腔Qo 的测量主要是采用热力学的方法测量其高频损耗然后经计算得出Qo.介绍了BEPCⅡ超导腔静态热负荷和无载品质因数的测量原理及方法,并且给出了两个超导腔在不同高频加速电压下的测试结果.此测试结果已作为BEPCⅡ超导腔验收测试的重要依据.     

BEPCⅡ超导腔静态热负荷和无载品质因数Q0的测量

超导腔的静态热负荷和无载品质因数是表征超导腔低温恒温器以及超导铌腔性能好坏的最重要参数. BEPCⅡ超导腔采用的是液氦浸泡冷却方式, 对两个超导腔在测试站分别进行了降温调试, 在超导腔达到超导状态并稳定运行后, 对其静态热损耗进行了测定. 此外, 超导腔Q₀的测量主要是采用热力学的方法测量其高频损耗然后经计算得出Q₀. 介绍了BEPCⅡ超导腔静态热负荷和无载品质因数的测量原理及方法, 并且给出了两个超导腔在不同高频加速电压下的测试结果. 此测试结果已作为BEPCⅡ超导腔验收测试的重要依据.     

DC-SC光阴极注入器的束载实验的调试进展

 北京大学射频超导实验室设计了新型超导光电子枪——DC-SC光阴极注入器，目标是为自由电子激光平台提供能量在2~3MeV，脉宽小于10ps，脉冲重复频率为81.25MHz，平均流强约为1mA的低发射度电子束。现在已经建成了DC-SC光阴极注入器实验平台，包括激光驱动光阴极系统，Pierce直流高压加速结构，1.3GHz 1＋1/2纯铌超导腔，恒温器低温系统，4.5kW连续波微波系统，1/16分频与同步控制系统，束流诊断系统和能量分析系统等。并且完成了超导腔的静态实验，直流加速结构也经过了100μA低电流测试。实验结果符合设计要求，整体调试后即可以进行束载实验。     

HOMs simulation and measurement results of IHEP02 cavity

In accelerator RF cavities, there exists not only the fundamental mode which is used to accelerate the beam, but also higher order modes(HOMs). The higher order modes excited by the beam can seriously affect beam quality, especially for the higher R/Q modes. 1.3 GHz low-loss 9-cell superconducting cavity as a candidate for ILC high gradient cavity, the properties of higher order mode has not been studied carefully. IHEP based on existing low loss cavity, designed and developed a large grain size 1.3 GHz low-loss 9-cell superconducting cavity(IHEP02cavity). The higher order mode coupler of IHEP02 used TESLA coupler's design. As a result of the limitation of the mechanical design, the distance between higher order mode coupler and end cell is larger than TESLA cavity.This paper reports on measured results of higher order modes in the IHEP02 1.3 GHz low-loss 9-cell superconducting cavity. Using different methods, Q e of the dangerous modes passbands have been obtained. The results are compared with TESLA cavity results. R/Q of the first three passbands have also been obtained by simulation and compared with the results of the TESLA cavity.     

Development of a China test cryomodule for ILC

Research and development of a 1.3 GHz 9-cell cavity test cryomodule were carried out by a collaboration group between IHEP (Institute of High Energy Physics) and TIPC (Technical Institute of Physics and Chemistry) in China. The cryomodule is a "test model" for the ILC cryomodule, and a key component of a superconducting accelerator test unit which will be built in the near future, also can be used as a horizontal test facility for 1.3~GHz 9-cell cavities. This paper presents the development status of the cryomodule, including structure design, cryogenic flow diagram, thermal and mechanical simulations, heat load estimation and etc.     

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。     

Development of a China test cryomodule for ILC

Research and development of a 1.3 GHz 9-cell cavity test cryomodule were carried out by a collaboration group between IHEP (Institute of High Energy Physics) and TIPC (Technical Institute of Physics and Chemistry) in China.The cryomodule is a "test model" for the ILC cryomodule,and a key component of a superconducting accelerator test unit which will be built in the near future,also can be used as a horizontal test facility for 1.3 GHz 9-cell cavities.This paper presents the development status of the cryomodule,including structure design,cryogenic flow diagram,thermal and mechanical simulations,heat load estimation and etc.     

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.     

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.     

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.     

Design studies on a 500 kV DC gun photo-injector for the BXERL test facility

BXERL is a proposal for a test facility (Beijing X-ray Energy Recovery Linac), which requires its injector to provide an electron beam of 5 MeV, 77 pC/ bunch at a repetition rate of 130 MHz (average current of 10 mA). In this paper, we present the design of the injector, which consists of a 500 kV photocathode DC gun equipped with a GaAs cathode preparation device, a 1.3 GHz normal conducting RF buncher, two solenoids, and one cryomodule containing two 1.3 GHz 2-cell superconducting RF cavities as the energy booster. The detailed beam dynamics show that the injector can generate electron bunches with a RMS normalized emittance of 1.49 πmm·mrad, a bunch length of 0.67 mm, a beam energy of 5 MeV and an energy spread of 0.72%.