Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities

The 'Q-slope' problem has so far strongly limited the application of niobium thin film sputtered copper cavities in high field accelerators. In the present paper, based on experimental evidence, we consider the hypothesis that the Q-slope is related to enhanced thermal boundary resistance R _Nb/Cu at the Nb/Cu interface, due to poor thermal contact between film and substrate. We have developed a simple model that directly connects the Q versus E _acc curves to the distribution function f(R _Nb/Cu) of R _Nb/Cu values at the Nb/Cu interface over the cavity surface. Starting from different Q versus E _acc experimental curves from different sources, using typical 'inverse problem' methods, we deduce the corresponding distribution functions generating those curves. The results show, for all the examined cases, very similar functional dependences of f(R _Nb/Cu) and prove that, to describe the experimental Q versus E _acc curves, it is sufficient to assume that only a small fraction of the film over the cavity surface is in poor thermal contact with the substrate. The whole body of information and data reported seems to indicate that the main origin of the Q-slope in thin film cavities is related to bad adhesion at the Nb/Cu interface. Strategies to solve the Q-slope problem improving the film adhesion are finally delineated. (paper)