Effect of mild baking on superconducting niobium cavities investigated by sequential nanoremoval

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Abstract

The near-surface nanostructure of niobium determines the performance of superconducting microwave cavities. Subtle variations in surface nanostructure lead to yet unexplained phenomena such as the dependence of the quality factor of these resonating structures on the magnitude of rf fields---an effect known as the ``$Q$ slopes''. Understanding and controlling the $Q$ slopes is of great practical importance for particle accelerators. Here we investigate the mild baking effect---$120\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ vacuum baking for 48 hours---which strongly affects the $Q$ slopes. We used a hydrofluoric acid rinse alternating with oxidation in water as a tool for stepwise material removal of about $2\text{ }\text{ }\mathrm{nanometers}/\mathrm{step}$ from the surface of superconducting niobium cavities. Applying removal cycles on mild baked cavities and measuring the quality factor dependence on the rf fields after one or several such cycles allowed us to explore the distribution of lossy layers within the first several tens of nanometers from the surface. We found that a single HF rinse results in the increase of the cavity quality factor. The low field $Q$ slope was shown to be mostly controlled by the material structure within the first six nanometers from the surface. The medium field $Q$ slope evolution was fitted using linear ($\ensuremath{\propto}B$ peak surface magnetic field) and quadratic ($\ensuremath{\propto}{B}^{2}$) terms in the surface resistance and it was found that best fits do not require the quadratic term. We found that about 10 nanometers of material removal are required to bring back the high field $Q$ slope and about 20--50 nanometers to restore the onset field to the prebaking value.

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