用铌材改性法提高超导腔性能的理论和实验研究

SRF cavity with high accelerating gradient and quality factor is the key component of SRF accelerator. The performance of cavity is close to the limit of pure niobium. The adoption of doped niobium is expected to exceed this limit. Superconducting cavities with doped niobium have become a hot topic. This project will improve the accelerating gradient and quality factor of superconducting cavities by doping some element into high pure niobium based on the previous studies at Peking University. We will study the effect of impurities in the superconducting material modification in theory; study the effect on the superconducting properties by preparing doped niobium samples so as to choose proper doping element, optimize the content and find out the doping method with low cost and high quality; study the doping technique of niobium or cavity with melting, ion implantation, film heat diffusion and gas heat diffusion methods to obtain doped niobium material, uniformity, high critical magnetic field and low surface resistance; fabricate doped cavities with doped niobium material, study the post-treatment method and carry out low temperature RF test; study the dry doping treatment method in the surface (micron and nanometer scale) of superconducting cavities with high temperature gases in both theory and experiments. High vacuum doping system for niobium cavity is constructed for this project. By the study of this project, deep understanding of mechanism of doping superconducting cavities and new method to improve the performance of niobium cavities will be obtained.

高加速梯度和品质因数超导腔是超导加速器的核心部件。目前超导腔的射频性能已经接近纯铌的材料极限，铌材掺杂改性处理有望突破这一限制。本项目基于北京大学的前期研究，通过在高纯铌材中掺入适量特定元素的方法提高超导腔的加速梯度和品质因数。包括：从理论上研究掺入元素在超导材料改性中的作用；通过实验研究挑选合适的掺杂元素、优化掺杂比例、降低成本、提高品质；研究改性铌材及超导腔表层的掺杂工艺，选取冶炼法、离子注入法、薄膜热扩散法和气体热扩散掺杂，以获得高的下临界磁场、低表面电阻且机械、热导性能良好的铌材，研制改性铌超导腔，进行后处理研究及低温射频性能测试；重点对超导腔表面微纳尺度干式高温气体掺入改性处理进行系统的理论及实验研究，建立了专用铌腔掺杂高真空处理装置，可以在线对腔体表面进行离子剥离、离子掺杂、反应溅射成膜等。通过项目研究，将对超导材料和超导腔掺杂改性机理有系统认识，获得提高超导腔性能的新方法。

纯铌超导腔作为在大科学装置上有广泛应用的射频超导加速器的核心部件，其加速梯度和品质因数在很大程度上决定了工程造价和运行成本。为保持高加速梯度的同时大幅提高超导腔的品质因数，对铌材进行掺氮处理是目前最有希望进入工程应用的方案。在本项目的支持下，北京大学对纯铌超导腔氮掺杂开展了理论和多种掺杂方案的实验研究，在国内率先实现了多种方案下都能保持高加速梯度的同时大幅提高超导腔品质因数的目标。在低温测试时的加速梯度可达20 MV/m以上，2K温度时中高场下的品质因数超过4×10^10，1.8K温度时的品质因数超过5×10^10，多次取得了国内领先及国际先进水平的成果。同时在大晶腔轻掺杂方案上摸索出了一套可重复的行之有效的工艺流程，为下一步纯国产实用型9-cell掺氮超导腔的大批量生产，以及在上海硬X射线自由电子激光装置等大科学工程上的应用提供了基础。