  Workbench-size particle accelerators, enabled by Nb3Sn-based superconducting
radio-frequency (SRF) cavities, hold the potential of driving scientific
discovery by offering a widely accessible and affordable source of high-energy
electrons and X-rays. Thin-film Nb3Sn RF superconductors with high quality
factors, high operation temperatures, and high-field potentials are critical
for these devices. However, surface roughness, non-stoichiometry, and
impurities in Nb3Sn deposited by conventional Sn-vapor diffusion prevent them
from reaching their theoretical capabilities. Here we demonstrate a seed-free
electrochemical synthesis that pushes the limit of chemical and physical
properties in Nb3Sn. Utilization of electrochemical Sn pre-deposits reduces the
roughness of converted Nb3Sn by five times compared to typical vapor-diffused
Nb3Sn. Quantitative mappings using chemical and atomic probes confirm improved
stoichiometry and minimized impurity concentrations in electrochemically
synthesized Nb3Sn. We have successfully applied this Nb3Sn to the large-scale
1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at
multiple operation temperatures, notably lower than vapor-diffused cavities.
Our smooth, homogeneous, high-purity Nb3Sn provides the route toward high
efficiency and high fields for SRF applications under helium-free cryogenic
operations.
