Analysis of RF losses and material characterization of samples removed from a Nb3Sn-coated superconducting RF cavity

TL;DR

This study investigates RF losses in Nb3Sn-coated superconducting cavities, identifying defect-related loss regions through thermometry and material analysis, aiming to improve understanding of material properties and performance.

Contribution

It provides a detailed correlation between RF loss measurements, thermometry mapping, and material defects in Nb3Sn coatings on SRF cavities, highlighting defect impacts.

Findings

Loss regions linked to thin patchy and carbon-rich defects.

Field-dependent surface resistance observed at cryogenic temperatures.

Material analysis correlates defects with increased RF losses.

Abstract

Nb3Sn (Tc ~ 18 K and Hsh ~ 400 mT) is a prospective material to replace Nb (Tc ~ 9 K and Hsh ~ 200 mT) in SRF accelerator cavities for significant cost reduction and performance enhancement. Because of its material properties, Nb3Sn is best employed as a thin film (coating) inside an already built RF cavity structure. A particular test cavity noted as C3C4 was a 1.5 GHz single-cell Nb cavity, coated with Nb3Sn using Sn vapor diffusion process at Jefferson Lab. Cold measurements of the coated cavity indicated the superconducting transition temperature of about 18 K. Subsequent RF measurements indicated field-dependent surface resistance both at 4.3 K and 2.0 K. After initial cold measurements, the cavity RF loss distribution was studied with a thermometry mapping system. Loss regions were identified with thermometry and were cut out for material analysis. The presence of significantly thin patchy regions and other carbon-rich defects is associated with strong local field-dependent surface resistance. This paper summarizes RF and thermometry results correlated with material science findings.