Latest Progress in Nb-Coating Synthesized on Wall of Superconducting RF Cu-Cavity:A Review Study

The latest progress in deposition of Nb-coatings on superconducting RF copper-cavity wall was tentatively reviewed. The discussions with specific examples focused on:i) the advantages over bulk Nb-cavity, including high thermal/mechanical stability, insensitivity to DC magnetic field, separation of RF/thermal performance, optimization of BCS-resistance and low fabrication cost; ii) the existing problem of severe Q-slope, particularly in medium acceleration gradient(10~15 MV/m), became worse with an increase of frequency; iii) the advanced coating technologies, including the high power magnetron sputtering, cathode arc deposition, e-beam plasma evaporation deposition, laser pulse deposition, for reduction of defects, increase of Nb compactness, improvement of interfacial adhesion and for synthesis of multilayers and composite coatings; and iv) the basic studies, such as the influence of trapped O₂/H₂/N₂ on surface residual resistance and Q-slope formation mechanisms. The Nb-coated cavities have been successfully operating in LEP and LHC accelerators for many years.