Measurements of superconducting Nb3Sn cavities in the GHz range

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Abstract
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 temperaturesT\min{c}\max{\ast} < 1K andT\min{c}\max{\ast} \simeq 2.5K. The normal conducting spotsT\min{c}\max{\ast} \lsim 1K cause the large rf residual lossesR'_{res} \propto f^{2}observed up to date. The spots withT\min_{c}\max_{\ast} \simeq 2.5K cause temperature dependences ofR'(T)between 2 and 6K, where RBCS(Nb3Sn) is still negligible. In line withR_{res} \propto f^{2}, the lowest rf lossesR_{res} < 2.10^{-9}\Omegaand the highest field strengthB_{crit} = 83 m^{T}(\wedgeE_{peak} = 29</texMV/m)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 withT\min{c}\max{\ast} < 2.5K. The origin and the chemistry of the weak spots withT\min{c}\max{\ast} \lsim 1K, which cause the largeR_{res} \propto f^{2}and the lowB_{crit} (T) \simeq const, are still not clear. They seem related to the Nb3Sn surface. The weak spots withT\min{c}\max{\ast} \simeq 2.5K 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.