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

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 fielddependent 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 carbonrich defects is associated with strong local field-dependent surface resistance. This paper summarizes RF and thermometry results correlated with material science findings.

Superfluid flow in disordered superconductors with Dynes pair-breaking scattering: depairing current, kinetic inductance, and superheating field

“…Strong local-heating (and resultant quenches) of SRF cavities are often attributed to geometrical defects on the surface [78][79][80][81][82][83][84][85][86]. Our theory suggest there can be another source of local heating.…”

Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators

“…Another benefit of the Cu outer layer is the increase in the mechanical stability of the cavity, making it less sensitive to microphonics and less susceptive to cracking of the Nb 3 Sn film. The occurrence of cracks in the film because of high stresses due to, for example, differential pressure, transportation or handling, would severely degrade R s [70,71].…”

Superheating fields of semi-infinite superconductors and layered superconductors in the diffusive limit: structural optimization based on the microscopic theory

“…These results can be tested using various techniques, e.g., high power rf pulse [58], rf characterization of samples [30,[59][60][61], third-harmonic voltage [25-27, 34, 62], magnetization measurements for ellipsoid samples [23], muon-spin-rotation technique [24,63], etc. Note that the rf heating of the cavity wall due to quasiparticles [38,44,64,65], vortices [66][67][68][69][70][71][72][73][74], topographic defects at the surface [75][76][77][78][79][80][81][82][83], and grain boundaries [84] can limit the achievable field.…”