Superconducting cavity with high acceleration gradient and quality factor is the core component of superconducting accelerator. At present, the radio frequency performance of superconducting cavity is close to the limit of pure niobium material, and the doping treatment of niobium material is expected to break through this limit. Based on the previous research of Peking University, this project improves the acceleration gradient and quality factor of superconducting cavity by adding a certain amount of specific elements into high purity niobium material. It includes: theoretically studying the effect of the elements in the modification of superconducting materials; Through experimental research to select the appropriate doping elements, optimize the doping ratio, reduce costs, improve quality; Research of modified niobium material and superconducting cavity surface doping process, selection of smelting method, ion implantation method, thin film thermal diffusion reaction and body heat diffusion doping, in order to obtain critical magnetic field under high and low surface resistance and mechanical properties, thermal conductivity, good niobium material, develop modified niobium superconducting cavity, the reprocessing study rf performance and low temperature test.
The theoretical and experimental research on the surface modification of superconducting cavity with dry high temperature gas at micro/nano scale was emphatic. The special niobium cavity doping high vacuum treatment device was established, which can conduct ion stripping, ion doping and reactive sputtering film formation on the surface of superconducting cavity. Through the research of the project, the doping modification mechanism of superconducting materials and superconducting cavity will be systematically understood, and a new method to improve the performance of superconducting cavity will be obtained. Modification of niobium to improve the performance of superconducting cavity.As the core component of the radio frequency superconducting accelerator which is widely used in large scientific equipment, the acceleration gradient and quality factor of the pure niobium superconducting cavity determine the engineering cost and operation cost to a great extent. In order to maintain the high acceleration gradient and greatly improve the quality factor of superconducting cavity, the nitrogen doped niobium material is the most promising project to be applied in engineering. Under the support of this project, Peking University has carried out theoretical and experimental studies on nitrogen doping of pure niobium superconducting cavity, which is the first in China to achieve the goal of maintaining a high acceleration gradient under various schemes while greatly improving the quality factor of superconducting cavity. The acceleration gradient at low temperature test can reach more than 20 mV /m, the quality factor at 2K temperature is more than 4×10^10, the quality factor at 1.8K temperature is more than 5×10^10, and has made the domestic leading and international advanced level of results for many times. At the same time, a set of repeatable and effective process flow is found out on the light-doping scheme of large crystal cavity, which provides a foundation for the mass production of pure domestic practical 9-cell nitrogen-doped superconducting cavity and the application in Shanghai hard X-ray free electron laser equipment and other large scientific projects.
