Measurements of superconducting Nb3Sn cavities in the GHz range

Superconducting Nb3Sn Cavities have potential advantages over rf cavities with Nb surfaces To test possible applications and to improve the understanding of Nb3Sn coatings on Nb, rf cavities have been measured between 1.5 and 8K and between 0.1 and 7GHz. The temperature dependence of the surface resistance R(T) indicates weak superconducting spots with transition temperatures K and K. The normal conducting spots K cause the large rf residual losses observed up to date. The spots with K cause temperature dependences of between 2 and 6K, where RBCS(Nb3Sn) is still negligible. In line with , the lowest rf losses and the highest field strength have been observed at the lowest frequency 0.1GHz measured. Surface resistance and penetration depth measurements have shown that grain boundaries or hydrogen clusters do not cause the weak spots observed with K. The origin and the chemistry of the weak spots with K, which cause the large and the low , are still not clear. They seem related to the Nb3Sn surface. The weak spots with K consist most likely of Nb6Sn5, which in cooling below 950°C precipitates due to the excess Sn present in Nb3Sn coatings grown in Sn vapor.