The Technical Study Of Sputtering For Superconducting Radio Frequency Cavity On Copper Substrate

Superconducting radio frequency（SRF）cavities are widely used in state-of-the-art accelerators,due to their advantages of larger aperture,lower energy loss and higher gradient operation in CW mode.At present,SRF cavities are mostly made of high pure niobium.Limited by the low thermal conductivity of niobium,the thickness of bulk niobium cavity cannot exceed 3～4 mm to ensure that niobium on the inner surface of the cavity can maintain in superconducting state.Because of the poor mechanical properties of niobium,the thin wall thickness reduces the mechanical strength of bulk niobium cavity,leading to poor operation stability.Thin Film Superconducting RF（TFSRF）cavities with a thick copper substrate can benefit from the excellent thermal conductivity of copper,thus solving the problem of low stability,improving the cooling efficiency,and enhancing mechanical stability of the superconducting cavity without affecting the RF performance.Furthermore,the performance of bulk niobium cavity is close to the theoretical limit predicted by the material properties of the niobium.Researchers hope to find new RF superconducting materials to replace pure niobium in order to obtain higher acceleration gradient,higher operating temperature and lower operating cost.Nb₃Sn is considered as one of the most promising next generation RF superconductors due to its double superconducting transition temperature,1.75 times accelerating gradient potential and smaller RF loss compared with pure niobium.If a bulk niobium cavity is upgraded to a copper-based Nb₃Sn film cavity,it can not only solve the problem of low stability,but also improve the accelerating gradient of the film cavity,improve the operating temperature of the superconducting accelerator,and reduce the cost of the accelerator operation.Thus,this dissertation hopes to develop a new technique for fabricating copper-based Nb₃Sn thin-film-coated superconducting radio-frequency cavities.Firstly,the pre-treatment process of copper substrate cavity are studied.Both surface quality and film adhesion were improved.Secondly,in view of the problem of non-uniform spatial distribution of sedimentary niobium film in QWR cavity,a tri-electrode structure was proposed and applied.Other deposition parameters in the coating process were also studied,including:the effect of working gas pressure,the bias and the temperature on film quality during sputtering.The optimal sputtering parameters were determined through3 iterative experiments.The results showed that the obtained niobium film was dense and had low roughness,the superconducting transition temperature T_creached 9.3 K,and the critical magnetic field was more than 1000 Oe,which met the basic requirements from superconducting RF application.Thirdly,a real copper cavity was coated with the established equipment and technique,and a 325 MHz QWR Cu/Nb cavity was obtained.In order to study the performance of the Cu/Nb cavity,a vertical test was carried out at 4.2 K.Experimental results revealed that field leakage between the flange surface of the outer conductor and the tuning plate would seriously affected the performance of the superconducting cavity.In view of the vertical test,the improvement scheme of the outer conductor’s fresh surface coating and the extension of the length of the outer conductor was proposed and simulated,and the lessons were summarized for the preparation of the QWR copper substrate film cavity in the future.Besides,on the basis of sputtering deposited niobium technology,we successfully plated Nb₃Sn film on top of tin-rich bronze surface.The technology only requires low temperature annealing of niobium coated bronze surface to generate Nb₃Sn thin film with superconducting transition temperature above 16.5 K.Finally,to verify the feasibility of the online operation of the thin film cavity,a1.5 Me V proton irradiation experiment was conducted.The experimental results show that the superconducting transition temperature T_cof niobium thin films decreases with high dose of proton injection.Due to the lack of RF measurement on samples,the effect of proton injection on the RF performance of Nb thin films needs further experimental verification.In this paper,a QWR copper-based Nb thin-film-coated cavities was prepared by sputtering.Thus,the technique of sputtering Nb film on the inner surface of the copper cavity was obtained.Besides,it was proved that Nb₃Sn films were synthesized via the ex-situ annealing of Nb-coated bronze precursors.Therefore,this paper explores a new technique for fabricating copper-based Nb₃Sn thin-film-coated superconducting radio-frequency cavities.This technique will promote the application of copper-based Nb₃Sn thin-film-coated superconducting radio-frequency cavities in RF superconducting accelerators.