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

Nb₃Sn (T _C ≈ 18 K and H _Sh ≈ 400 mT) is a prospective material to replace Nb (T _C ≈ 9 K and H _sh ≈ 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 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. (paper)