We investigate “hot” regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-μSR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth ${\lambda }_{\mathrm{L}}=23\pm 2 \text{nm}$ . In contrast, a cutout sample from the 120 ∘C baked cavity shows a much larger $\lambda >100\hspace{0.17em}\text{nm}$ and a depth dependent mean free path, likely due to gradient in vacancy concentration. We suggest that these vacancies can efficiently trap hydrogen and hence prevent the formation of hydrides responsible for rf losses in hot regions.

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—$120°\mathrm{C}$ 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 $2\mathrm{nanometers}/\mathrm{step}$ 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 ($\propto B$ peak surface magnetic field) and quadratic ($\propto B^2$) 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.

Precipitation of lossy non-superconducting niobium hydrides represents a known problem for high purity niobium in superconducting applications. Using cryogenic optical and laser confocal scanning microscopy, we have directly observed surface precipitation and evolution of niobium hydrides in samples after different treatments used for superconducting RF cavities for particle acceleration. Precipitation is shown to occur throughout the sample volume, and the growth of hydrides is well described by the fast diffusion-controlled process in which almost all hydrogen is precipitated at T = 140 K within ∼30 min. 120 °C baking and mechanical deformation are found to affect hydride precipitation through their influence on the number of nucleation and trapping centers.

Niobium is an important technological superconductor used to make radio frequency cavities for particle accelerators. Using laser confocal microscopy we have directly investigated hydride precipitates formation in cavity-grade niobium at 77 and 140 K. We have found that large hydrides were usually formed after chemical or mechanical treatments, which are known to lead to a strong degradation of the quality factor known as $Q$ disease. From our experiments we can conclude that hydrides causing $Q$ disease are islands with a characteristic thickness of $\gtrsim 100\mathrm{nm}$ and in-plane dimensions $1–10\mu \mathrm{m}$. Our results show that mechanical polishing uploads a lot of hydrogen into bulk niobium while electropolishing leads to a mild contamination. Vacuum treatments at $600–800°\mathrm{C}$ are demonstrated to preclude large hydride formation in line with the absence of $Q$ disease in similarly treated cavities.