A Superconducting Nb3Sn Coated Multicell Accelerating Cavity

We report on the first results, obtained with a five cell Nb
₃
Sn coated accelerating structure of 3 GHz. A uniform layer of Nb3Sn was formed by processing a niobium structure in a tin atmosphere at 1170°C. At 4.2 K a resonator Q of 7·10
⁹
was measured. This corresponds to a residual surface resistance of 27 nΩ which is the lowest ever achieved with a Nb
₃
Sn cavity in the GHz range. The rf losses of this resonator are by a factor of 55 lower than those of an equivalent niobium cavity at 4.2 K. The maximum accelerating field at this temperature was 4.0 MV/m. It is similar to the quench field of the niobium structure before coating. The spatial distribution of the rf losses was measured at different field levels using the temperature mapping technique. We describe the characteristic features of the Nb
₃
Sn layer, we report on residual losses specific to Nb
₃
Sn coated niobium structures and we discuss the observed field limitations.