Nb₃Sn multicell cavity coating system at Jefferson Lab

Superconducting radio frequency niobium cavities are the building blocks of modern accelerators for scientific applications. Lower surface resistance, higher fields, and high operating temperatures advance the reach of the future accelerators for scientific discovery as well as potentially enabling cost-effective industrial solutions. We describe the design and performance of an Nb₃Sn coating system that converts the inner surface of niobium cavities to an Nb₃Sn film. The niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl₂ vapor during the heat cycle, which results in about 2 μm Nb₃Sn film on the niobium surface. Film composition and structure as well as radio frequency properties with 1-cell R&D cavities and 5-cell practical accelerator cavities are presented.