Nb3Sn Multicell Cavity Coating System at JLAB

SRF 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 Nb$_{3}$Sn coating system that converts the inner surface of niobium cavities to Nb$_{3}$Sn film. Niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl$_{2}$ vapor during the heat cycle, which results in about 2 $\mu$m Nb$_{3}$Sn film on the niobium surface. Film composition and structure as well as RF properties with 1-cell R\&D cavities and 5-cell practical accelerator cavities are presented.