MEASUREMENTS OF SUPERCONDUCTING NB3SN CAVITIES IN THE GHZ RANGE

Superconducting NbgSn 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 T'OK and T2.5K* The normal conducting spoEs(VIK) ause the large rf residual losses RJ gf2 f 2 observed up to date. The spots with T4-5?5cause temperature dependences of R”^T) between 2 and 6K, where Rg^g(Nb3Sn) is still negligible. In line with R f2, the lowest rf losses R <;2 • 10 ^9?jeand the highest field strength Bre^t“83 tiT(”E^ea^-2 9MV/m) have been observed at tne 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 T 1 <; 2.5 K. The origin and the chemisery of the w6ak spots with T’GIK, which cause the large R af and theclow B. (T)-const, are sti lTenot clearr They crit seem related to the Nb3Sn surface I1 he weak Spots with T * 2 „ 5 K consist most likely of Nb 6Sn 5, which in cooling below 950°C precipitates due to the excess Sn present in Nb3Sn coatings grown in Sn vapor. © 1979 IEEE