Study On The Welding Technologies And The Mechanical Properties Of Stainless Steel Vessel Of Superconducting Radio-frequency Cavities

The rf superconducting cavity has the advantages of low loss,high acceleration gradient and high beam stability,which are widely used in the world's cutting-edge particle accelerators,and become the core component to guarantee the quality of particle accelerators.In general,rf superconducting cavities need to be immersed in liquid helium containers(also known as helium vessels)to ensure their operational stability.Therefore,the study about preparation processes,physical properties,vacuum performance,remnant magnetic field properties is critical for the development of new helium vessels.The complex welding processes,material shaping difficulties and high preparation cost faced by the traditional titanium helium vessels had seriously restricted the development of noval superconducting cavity and thus affect the progress of the entire particle accelerator community.This Ph.D.dissertation puts forward the idea of adopting stainless steel material which features low price,relatively simple welding requirements and easy molding process as an alternative material for titanium.The stainless steel helium vessels need to be jointed with metal niobium to ensure the stability of the superconducting cavity structure and the leak-tight of the whole cavity,and that the welding site(also known as the welding joint)is subjected to the thermal stress and tuning load generated by multiple hot and cold cycles during the operation of the superconducting cavity.In addition,the stainless steel material may experience phase change during the welding and later assembly process,along with the generation of remnant magnetic field,resulting in an increase in the rf loss of superconducting cavity.Therefore,the study about the properties of stainless steel to niobium welding joints,including material properties,welding processes of samples or prototypes,mechanical,vacuum and remnant magnetic field properties has become a key scientific problem in the application of stainless steel helium vessels.This Ph.D.dissertation focuses on these key issues and obtains the following research results:First of all,the preparation of stainless steel to niobium joints through explosive welding technology was studied.The welding processes,mechanical properties,and vacuum properties of the joints were also analyzed qualitatively or semi-quantitatively.The forming mechanism of the interface is qualitatively explained by Bahrani mechanism and specific strength.The author firstly proposed that the specific strength can predict the interface morphology.The hardness,shear and tensile strengths of the joints prepared by this technology were studied,and the change rule of mechanical properties of the joint surface during the welding process was qualitatively explained from macroscopic mechanics and microscopic structures.Secondly,the technical processes and mechanical properties of vacuum brazing technology for welding joint were studied,and the influence of solder on the mechanical properties of welded joints was summarized.There exists intermetallic brittle compound on the bonding surface of the welded joint prepared with oxygen-free copper solder.The existence of such compound degrades the mechanical properties of the welded joint in liquid nitrogen environment.It is pointed out that the generation of intermetallic brittle compound can not be avoided by shortening the holding time.The utilization of Pd Ag Cu solder with different Pd concentration is an effective way to get rid of such compound.The mechanical properties of the joints made from this way are strengthened under liquid nitrogen environment.Therefore,it was confirmed at the same time that the existence of intermetallic brittle compounds is the main cause for the mechanical properties degradation of Nb and stainless steel welding joints.The welding problems caused by thermal mismatch were analyzed theoretically and the solution is put forward.A vacuum performance evaluation system was established for the welded joints prepared by two welding methods,and its reliability is studied.In addition,the residual magnetization produced in the process of stainless steel manufacturing was studied,and the influence of different treatment processes on the residual magnetization of stainless steel were quantitative studied,which provided significant guidance for the subsequent research of stainless steel helium vessels.The best processing technology of stainless steel helium tank could be determined by measuring the residual magnetization of joints processed under different annealing temperature,different types of raw materials,different heat treatment profiles,different welding processes,and joint welding structure.Finally,the structure and mechanical properties of stainless steel helium vessels and welding parts were studied by numerical simulation.The structure design and mechanical calculation of stainless steel helium tank were carried out,and the vacuum and mechanical properties of welded joint for helium tank were characterized,which provides theoretical and experimental basis for the actual manufacturing design of stainless steel helium tank prototype.A structure of stainless steel helium tank prototype for a 6-cell SRF cavity was designed and the mechanically simulated under different working condition,and the stress distribution was also studied.