Superconducting RF (SRF) cavities made of Nb are an enabling device for future linear accelerators. Recently it has been demonstrated that hot spots in SRF cavities, which diminish performance, are... Show moreSuperconducting RF (SRF) cavities made of Nb are an enabling device for future linear accelerators. Recently it has been demonstrated that hot spots in SRF cavities, which diminish performance, are correlated with a high density of defects (etch pits) especially near grain boundaries. For a pit to cause local heating, it is likely that near-surface impurities, e.g. hydrides or oxides are leading to suppressed superconductivity. New probes are needed to measure such complexes. Here we present Raman spectroscopy. Raman is a fast, nonperturbative method that can measure the vibrational modes of Nb-O and Nb-H complexes by inelastic light scattering. These can then be compared to molecular dynamics simulations to identify oxide and hydride phases. The probing depth of Raman is estimated from the skin depth of the 785 nm laser in the bulk Nb ~ 10-20 nm. This is a reasonable fraction of the superconducting penetration depth ~ 45 nm. Simulating manufacturing processes of SRF cavities may shed light on the origins and composition of hot spots, and their relationship with defects in the material. Defects such as pits, whose origins are yet unknown, are found in the hot spots of completed cavities. Raman spectroscopy is used here to identify changes in the surface chemistry after manipulations such as creating artificial pits, exposing the material to chemical etching, or cold-working the material. BCP exposure and cold-working are common to the SRF manufacturing process. Show less
