Study On Nb₃Sn Coating Technology For Copper-based Superconducting RF Cavity

RF accelerating cavities are widely used in various high-current accelerators for their high quality-factor Q0,high acceleration gradient,large beam aperture,and continuous wave mode usable.At present,the RF accelerating cavities mainly employ bulk Nb.Due to low heat conduction,the wall thickness of the Nb cavity is generally only 3-4 mm,and this thin-walled structure will produce thermal instability,which is easy to induce the microphonic effect and Lorentz force detuning.In addition,with the continuous optimization of Nb smelting technology,cavity processing,and post-treatment technology,the performance of bulk Nb cavities has approached its theoretical limit,and the space for further development is limited.One of the practical solutions to the above problems is to coat a Nb3Sn thin film on a copper substrate.On the one hand,the copper cavity has good thermal conductivity,easy processing,low cost,and the wall thickness can reach 10 mm,which can effectively improve the stability of the cavity.On the other hand,Nb3Sn material has a higher superconducting transition temperature,superheated magnetic field and lower surface resistance compared with Nb.Moreover,a higher superheated magnetic field means higher acceleration gradient;for the 1.3 GHz ellipsoidal superconducting cavity,the theoretical upper limit of acceleration gradient and Q0 value of Nb3Sn thin-film cavity are twice and ten times that of bulk Nb cavity,respectively.The bronze method is an essential process in the preparation of Nb3Sn wire.The principle of preparing Nb3Sn by the bronze method is:to reduce the heat treatment temperature by copper catalysis in Cu-Nb-Sn ternary reaction,and grow Nb3Sn at around 700℃.Therefore,in this thesis,the Nb3Sn films were prepared by the bronze method.Firstly,we deposited Nb thin films onto the oxygen-free copper surface,then Cu-Sn precursors were electrodeposited on the Nb layer,and then reheated to form Nb3Sn thin films.Experiments showed that the bronze method is feasible for preparing the Nb3Sn thin-film cavity.Thus,the best type of precursor suitable for Nb3Sn thin-film cavity is bronze film precursor.Firstly,we characterized the Cu-Sn precursor prepared by the electrochemical method.The three-dimensional nucleation model and electrochemical kinetic parameters of the copper pyrophosphate system,tin methane sulfonic acid system,and citric acid bronze system were analyzed.The results showed that a diffusion step controls the above three kinds of coating systems,and the nucleation mode is three-dimensional instantaneous nucleation.In addition,the effects of ion concentration,current density,temperature and stirring rate on the morphology of the films were investigated,and the best parameters of electrochemical coating in different systems were obtained.Accordingly,Cu/Sn/Cu,Cu/Sn and bronze precursors with complete films,good adhesion and controllable thickness were successfully prepared.Secondly,we introduced the Cu-Sn interdiffusion model and Nb3Sn film growth model.The phase transformation processes of Cu/Sn/Cu,Cu/Sn and bronze precursors at 210℃,400℃ and 700℃ are studied.The SEM surface morphology,XRD phase,M-T,and R-T superconductivity of Nb3Sn thin films were analyzed.We found that tin content is the crucial factor affecting the superconducting properties of Nb3Sn.Through the above experiments,we successfully prepared copper based Nb3Sn samples with superconducting transition temperature above 17.0 K.Consequently,we summarized the phase formation law of Nb3Sn in the bronze method under different precursors,and obtained the optimum precursor type for Nb3Sn thin-film cavity as a bronze precursor.Subsequently,we tried Nb3Sn coating on φ110 mm large samples,and prepared high quality copper-based Nb3Sn films on φ110 mm discs.Then,we simulated the electrochemical coating process of different electrode structures,improved the Nb cavity’s activation formula and the opposite electrode’s processing test,and optimized the critical problems such as the adhesion and spatial uniformity of cavity coating.We found that the surface cleanliness of Nb cavity directly affected the quality of Nb3Sn film.In addition,the electrochemical coating patch experiment of bulk Nb cavity was carried out.By testing the thickness and superconductivity of the patch samples,the spatial distribution characteristics of the coating quality of the cavities were obtained,and the correctness of the electrochemical simulation results was verified.Finally,we finished the electrochemical coating experiment of a 1.3 GHz cavity.The specific method is as follows:the bronze precursor was prepared on the 1.3 GHz bulk Nb superconducting cavity by electrochemical process,and then the Nb3Sn film was synthesized by no more than 700℃ heat treatment,and finally the bronze impurity layer on the surface can be removed.The low temperature vertical measurement results show that the Q0 of the film cavity is 6 E+8 at 4.2K.By analyzing the QLT,Q0-Epk,magnetic field data,temperature data and surface morphology results,we found that the limiting factor of thin-film cavity performance is the excessive residual resistance caused by cavity coating defects and pollution.Hence,we got the following optimization scheme of cavity coating.This dissertation proves that the electrochemical bronze method is feasible for superconducting RF cavity,and has prominent potential to improve its performance further.Through the technical verification of Nb cavity electrochemical bronze Nb3Sn coating,subsequent research will carry out the preparation and engineering application of a more practical 1.3 GHz copper-based Nb3Sn superconducting thin film cavity.