Residual surface losses in thin film coated RF superconducting cavities

Residual losses are known to be one of the main factors limiting the performance of RF superconducting cavities for particle accelerators. In particular, in thin-film sputter-coated cavities, residual terms that are strongly dependent on the RF field amplitude have been observed in the surface resistance. It is shown that RF losses at the grain boundaries can be one of the main sources of the residual surface resistivity in Nb and Nb-alloy thin-film-coated RF cavities. The effect of the film thickness on the surface impedance is also discussed.