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Thermal Boundary Resistance at the Nb-Cu interface as a possible limiting factor for sputtered thin film RF superconducting cavities

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30/7-018 - Kjell Johnsen Auditorium (CERN)

30/7-018 - Kjell Johnsen Auditorium

CERN

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Ruggero Vaglio
Description
The “Q-slope” problem has so far strongly limited the application of niobium thin film sputtered copper cavities in high field accelerators. Based on experimental evidences of film adhesion pronlems, in this talk we consider the hypothesis that the Q-slope is related to enhanced thermal boundary resistance RNb/Cu at the Nb/Cu interface, due to poor thermal contact between film and substrate. Based on extensive studies carried out in the seventies on the thermal contact between two solid surfaces, we will present a simple model that directly connects the Q vs Eacc curves to the distribution function f(RNb/Cu) of RNb/Cu values at the Nb/Cu interface over the cavity surface. Starting from different Q vs Eacc experimental curves from different sources, using typical “inverse problem” methods, we determine the corresponding distribution functions generating those curves. The results show in all examined cases a very similar functional dependence of f(RNb/Cu), resembling a log-normal distribution. We will show that, in order to describe the experimental Q vs Eacc 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.