Abstract
We propose a model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium thin film superconducting rf cavities. The physical mechanism that we explore is due to the constriction of surface electrical current flow at grain interface boundaries. This constriction creates an additional electrical contact resistance which induces localized punctual heat dissipation. The temperature at these spots is derived; and the electrical contact resistance is shown to depend on the magnetic field, on the grain contact size over which dissipation occurs, and on other key parameters, including the effective London penetration depth and the frequency. The surface resistance and the quality factors are determined using our model and are shown to be in excellent agreement with experimental data.
Highlights
Thin film superconductivity is an important pathway to improve the performance of superconducting rf (SRF) accelerators [1,2]
The performance of a SRF cavity is characterized by the quality factor which can be defined as Q0 1⁄4 G=Rs, where G is a constant that depends on the geometrical shape of the cavity and Rs is the total surface resistance
We have investigated heat dissipation in niobium thin film SRF cavities to explain the Q slope behavior with the accelerating fields
Summary
Thin film superconductivity is an important pathway to improve the performance of superconducting rf (SRF) accelerators [1,2]. The thermal and electrical transport in granular media are limited by the grain boundaries when the mean free paths, l of electrons and phonons become comparable to or greater than the size of grains. Porosity and film densification are an issue, and the model is worth considering In compliance with these ideas, we explore a new approach to explain the thermal behavior of Nb thin film SRF cavities for accelerators. We suppose that the grains are coupled together, and they form “tight links” since we consider the case where the grain boundary is superconducting This model does not require any hypothesis on the shape of the cavity, but it is sensitive to the frequency of the rf field. The quality factors are calculated using our model and compared to experimental data available for Nb thin films
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