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

A. Romanenko,F. Barkov,J. P. Ozelis,A. Grassellino

doi:10.1103/physrevstab.16.012001

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.

Highlights

The quality factor of superconducting niobium cavities exhibits a field dependence characterized by three distinct regions in the Q0ðBpeakÞ curve
While there are no established methods to control low and medium field Q slopes, it was discovered that the high field Q slope (HFQS) can be removed by a so-called mild baking—an in situ ultrahigh vacuum annealing of cavities at 90–145C for the duration of 12–48 hours [2]
We report rf measurements of the quality factor versus temperature after nanoremoval steps, from which we extract how superconducting parameters change as a function of depth, and Q0ðBpeakÞ curves, which provide information on the change in Q slopes

Summary

Introduction

The quality factor of superconducting niobium cavities exhibits a field dependence characterized by three distinct regions in the Q0ðBpeakÞ curve (see Ref. [1] for review). One of the major obstacles is the absence of a full nanoscale understanding of the material changes in the magnetic field penetration depth (& 100 nm) brought about by different treatments applied on cavities. Such treatments include electropolishing (EP), buffered chemical polishing (BCP), 120C baking, and 600–800C baking in vacuum furnaces. While there are no established methods to control low and medium field Q slopes, it was discovered that the high field Q slope (HFQS) can be removed by a so-called mild baking—an in situ ultrahigh vacuum annealing of cavities at 90–145C for the duration of 12–48 hours [2]

Objectives

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Discussion

Conclusion