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 temperaturesTmin{c}max{} < 1K andTmin{c}max{} 2.5K. The normal conducting spotsTmin{c}max{} 1K cause the large rf residual lossesR'ᵣₑₛ ∝ f²observed up to date. The spots withTmincmax_{} 2.5K cause temperature dependences ofR'(T)between 2 and 6K, where RBCS(Nb3Sn) is still negligible. In line withRᵣₑₛ ∝ f², the lowest rf lossesRᵣₑₛ < 2.10⁻⁹ the highest field strengthBcrit = 83 mT(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 withTmin{c}max{} < 2.5K. The origin and the chemistry of the weak spots withTmin{c}max{} 1K, which cause the largeRᵣₑₛ ∝ f²and the lowBcrit (T) const, are still not clear. They seem related to the Nb3Sn surface. The weak spots withTmin{c}max{} 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.