Elastic recoil detection studies of near-surface hydrogen in cavity-grade niobium

Journal of Applied Physics

Trenikhina

et al. 2013

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.

Section: Discussionsupporting

confidence: 72%

Precipitation of hydrides in high purity niobium after different treatments

Journal of Applied Physics

Trenikhina

et al. 2013

“…In particular, for cavity-grade niobium such hydrogen enrichment at the niobium/oxide interface was reported [4,19,20]. In most of the studies the near-surface hydrogen concentration was found to be only weakly affected by vacuum heat treatments at 600-1000 C typically applied on cavities to mitigate Q disease.…”

Section: Discussionmentioning

confidence: 99%

Direct observation of hydrides formation in cavity-grade niobium

Phys. Rev. ST Accel. Beams

Grassellino

2012

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 * 100 nm and in-plane dimensions 1-10 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 C are demonstrated to preclude large hydride formation in line with the absence of Q disease in similarly treated cavities.

“…[3]) may be due to the entering of hydrogen into niobium whenever the protective Nb 2 O 5 layer is not present. Some previous studies [10] addressed this issue but detailed understanding is not yet developed.…”

Section: Introductionmentioning

confidence: 99%

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

Grassellino

Phys. Rev. ST Accel. Beams

et al. 2013

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 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 nanometers=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 (/ B peak surface magnetic field) and quadratic (/ 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.