  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.
