SUPERCONCUCTING Nb RF CAVITY :

INTRINSIC VERSUS EXTRINSIC PROPERTIES

J. Halbritter

Forschungzentrum Karlsruhe, IHM

The intrinsic surface resistance of superconductors R_BCS(T,f,B) is degraded by extrinsic effects which actually limit applications, e.g., by rf breakdown or severe heat load. Examples for extrinsic effects are, NbOx(x<0.2)-cluster changing R_BCS(T,f,B), residual losses R_res, R(E,B) increasing with rf electric field E_p or with rf magnetic field B_p, or heating ∆T of the cavity surface relative to the He-bath. With the now available material qualities and surface preparation gross local defects are rare and global degradations show up. Aside of the rf residual losses R_res related to Nb-Nb₂O₅ interfaces at external surfaces or at weak links, extrinsic field dependencies are encountered reaching from Q-slope, i.e. dR µ R_BCS (T,f) (1 + g(B_rf/B_c)² + h (B_rf/B_c)⁴ +) with g > h as the begin of a Taylor series and B_c(0) ≃ 0.2 T, to Q-drop, i.e. dR^E µ exp(-c/E_p) without field emission, and to R_hys µ wB_rf/j_cJ with j_cJ the critical current density of weak links. For carefully prepared and oxidized Nb R_res(T,f)≳1(f/GHz)²nW have been attained. In bulk Nb cavities immersed in He a Q-slope with g (T < 2.17K=T_l) ≲ 0.2 – 3 has been reached turning above T_l to g ≳ 10 – 20 confirming ∆T heating by the Kapitza thermal boundary resistance as main cause. Thermal interface resistances in sputtered Nb cavities yield, e.g. g(T) ≃ 20 – 100, simulating an exponential R(B)-increase via the exponential R_BCS(T + ∆T,f,B) increase. The Q-drop is explained by interfacial tunnel exchange of Nb with Nb₂O₅. Internal surfaces, i.e., weak links, as they occur by oxidation along grain boundaries or at other defects, e.g., in cold worked Nb, yield hysteresis losses R_hys at rather low fields. Because of j_cJ(T ≲ T_c/2) ≃ const, R_res(T £ T_c/2) and R_hys(T £ T_c/2) are T-independent for Nb, like the interface losses d R µ exp (-c/E_p), whereas the Q-slope replicates via g(T)R_BCS(T) the BCS surface resistance in its T-dependence for R_BCS(T + ∆T) > R_res + R_hys + R^E_. All observations, up to date, can be explained in the above frame work, showing the importance of detailed analysis of R(T,f,B)-dependencies and pointing toward locally piled up rf losses transferred to interfaces and then to He being the key for further improvements of peak fields and for the reduction of rf losses.