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Raman Spectroscopy as a Probe of Surface Defects in Nb for SRF Cavities
Title
Raman Spectroscopy as a Probe of Surface Defects in Nb for SRF Cavities
Creator
Hommerding, Emily, Ford, Denise, Cao, Chaoyue, Bishnoi, Sandra, Zasadzinski, John
Date
2012, 2012
Description
Superconducting RF (SRF) cavities made of Nb are an enabling device for future linear accelerators. Recently it has been demonstrated that hot...
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.
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SURFACE CHARACTERIZATION OF NIOBIUM FOR SUPERCONDUCTING RF CAVITIES
Title
SURFACE CHARACTERIZATION OF NIOBIUM FOR SUPERCONDUCTING RF CAVITIES
Creator
Cao, Chaoyue
Date
2014, 2014-07
Description
Surface characterization techniques including point contact tunneling (PCT) spectroscopy and Raman spectroscopy have been employed to study...
Show moreSurface characterization techniques including point contact tunneling (PCT) spectroscopy and Raman spectroscopy have been employed to study the surface of niobium (Nb) superconducting radio frequency (SRF) cavities. PCT spectroscopy provides a direct means of measuring the surface superconductivity, which is closely correlated with the cavity’s performance characterized by the quality factor Q. Cavities with remarkably high Q show near ideal tunneling spectra with sharp coherent peaks and low zero bias conductance, consistent with the Bardeen-Cooper-Schrie↵er (BCS) density of states (DOS), and bulk gap parameter, " = 1.55 -1.6 meV. Cavities with Q-drop often exhibit strong non-uniform heating during RF operations, with high loss regions identified as hot spots. PCT spectra on hot spots reveal suppressed superconductivity, broadened DOS and Kondo tunneling, consistent with magnetic impurities on the surface. Raman spectra on hot spots indicate the presence of various impurities on the surface including amorphous carbon, C-H chain compounds and NbC, providing insights into the formation of hot spots. The origin of the impurities is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.
Ph.D. in Physics, July 2014
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