Surface quality characterization of thin Nb films for superconducting radiofrequency cavities

Superconducting radiofrequency (SRF) cavities are vital components of particle accelerators nowadays. In order to minimise the energy dissipation, a perfect inner surface of the cavity, hindering the penetration of magnetic field, is required. In this work, we investigated ten planar samples differing in the surface quality of Nb film deposited on Cu substrate, and as a consequence exhibiting various levels of the first entry field, H (en), at which the magnetic field starts to enter the film. The observed surface defects are categorised as hills, pits and cracks. For a practical range of dimensions of these features, the factor beta, characterising the local magnetic field enhancement, was calculated by the numerical finite-element simulations. It is expected that the local field enhancement causes a premature penetration of the magnetic field, thus lowering H (en). Then, for each investigated sample, the range of beta values characterising defect type that cause the highest field enhancement, is identified and compared with the H (en) fields. We have found that the H (en) of the samples that contain multiple types of the surface features is indeed limited by those defects that cause the highest field enhancement. The H (en) vs beta dependence has shown a good match with linear fit for the set of investigated samples. Thus, the main result is that the local magnetic field enhancement, computed in a straightforward way for the most significant defects, is a strong indicator of the surface quality that is relevant for the superconducting film intended for SRF cavity application.

Automated summary

Superconducting radiofrequency (SRF) cavities are vital components of particle accelerators. This study investigates the surface quality of Nb film samples on Cu substrates, categorizing surface defects as hills, pits, and cracks. Through numerical finite-element simulations, the factor beta, which characterizes local magnetic field enhancement, is calculated. The study finds a linear relationship between the local magnetic field enhancement and the first entry field (H(en)), indicating that the magnetic field penetration is affected by the defects causing the highest field enhancement.