Measurements of superconducting Nb3Sn cavities in the GHz range

Superconducting Nb
3
Sn Cavities have potential advantages over rf cavities with Nb surfaces To test possible applications and to improve the understanding of Nb
3
Sn 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\min{c}\max{\ast} < 1
K and
T\min{c}\max{\ast} \simeq 2.5
K. The normal conducting spots
T\min{c}\max{\ast} \lsim 1
K cause the large rf residual losses
R'_{res} \propto f^{2}
observed up to date. The spots with
T\min_{c}\max_{\ast} \simeq 2.5
K cause temperature dependences of
R'(T)
between 2 and 6K, where R
BCS
(Nb
3
Sn) is still negligible. In line with
R_{res} \propto f^{2}
, the lowest rf losses
R_{res} < 2.10^{-9}\Omega
and the highest field strength
B_{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 with
T\min{c}\max{\ast} < 2.5
K. The origin and the chemistry of the weak spots with
T\min{c}\max{\ast} \lsim 1
K, which cause the large
R_{res} \propto f^{2}
and the low
B_{crit} (T) \simeq const
, are still not clear. They seem related to the Nb
3
Sn surface. The weak spots with
T\min{c}\max{\ast} \simeq 2.5
K consist most likely of Nb
6
Sn
5
, which in cooling below 950°C precipitates due to the excess Sn present in Nb
3
Sn coatings grown in Sn vapor.