用于射频超导腔的Nb_(3)Sn镀膜技术进展

Progress on Nb_(3)Sn Coating Technique for Superconducting Radiofrequency Cavities

过去的五十多年,射频超导(SRF)技术取得了巨大的成功,以高纯度铌材(Nb)制作的射频超导腔已经大规模地应用到粒子加速器领域。然而进入21世纪以后,受材料本征性能的限制,Nb腔发展速度变缓慢,难以满足未来的大型超导加速器对其性能提出的更高要求。相比于Nb,铌三锡(Nb_(3)Sn)具有更高的超导转变温度、过热磁场和同温度下更低的表面电阻,因此,Nb_(3)Sn镀膜腔不仅拥有更高的性能,还可以把工作温度从2 K提升到4.2 K,降低系统的运行成本。在过去数十年中,发展出了多种制备Nb_(3)Sn薄膜的技术,如气相扩散、磁控溅射、青铜法和化学气相沉积。本文从制备过程、结果、影响性能的因素三方面对上述制备方法进行了总结并做了展望。目前,Nb_(3)Sn镀膜腔(1.3 GHz,椭球形单cell腔)的最高性能由Fermilab实验室用气相扩散法给出:4.4 K下,加速梯度达到~22.5 MV/m,品质因数达到~2×10^(10)@10 MV/m。其中,国内的Nb_(3)Sn镀膜腔最高性能为:4.2 K下,加速梯度达到~16.1 MV/m,品质因数达到~6×10^(9)@10 MV/m。理论上Nb_(3)Sn镀膜腔的加速梯度可达到~90 MV/m,因此Nb_(3)Sn镀膜腔的性能目前还存在很大的提升空间。

The past five decades have seen many successes in superconducting radio frequency(SRF)tecknique,and Nb SRF cavities have been widely used in frontier particle accelerators.However,in the twenty-first century,the development of niobium cavities has decelerated due to the limitation of intrinsic material properties,making it difficult to meet higher requirements for future accelerators.Nb_(3)Sn,with higher transition temperature,higher superheating field and lower surface resistance than Nb,is not only able to improve cavity performance,but also save operating costs by raising the operating temperature to 4.2 K.During the past few decades,several coating techniques of Nb_(3)Sn have been development to produce Nb_(3)Sn coated thin film superconducting radiofrequency cavities,including vapor diffusion,sputtering,bronze processing and chemical vapor deposition.Herein,this review article summarizes these coating techniques from the aspects of fabrication,results,properties and prospects.The Nb_(3)Sn cavity representing the-state-of-art performance was a single cell 1.3 GHz elliptical cavity fabricated with vapor diffusion technique on Nb substrate by FermiLab.It showed accelerating gradient E_(acc)reaches~22.5 MV/m at 4.4 K and a quality factor Q_(0)of~2×10^(10)at 10 MV/m and 4.4 K.By the way,the-state-of-art performace of Nb_(3)Sn cavity in China showed an E_(acc)of~16.1 MV/m at 4.2 K and a Q_(0)of~6×10^(9)at 10 MV/m and 4.2 K.The acceleration gradient of the Nb_(3)Sn cavities can theoretically reach 90 MV/m,thus there is still great improvement potential in terms of performance.