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Title Degradation of Superconducting RF Cavity Performance by Extrinsic Properties
Type Poster SRF basics
Abstract 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, rf residual losses Rres increasing with rf electric field Ep or rf magnetic field Bp or heating Delta_T of the cavity surface relative to the He-bath. With the now available material qualities and surface preparations gross local defects became rare and global degradations show up. Aside of the rf residual losses Rres related to the Nb-Nb2O5 interface, extrinsic field dependencies are encountered reaching form Q-slope, i.e. delta_R prop. R_BCS(T,f)*(1 + gamma*(Brf/Bc)**2 + eta*(Brf/Bc)**4 + ...) with gamma > eta as the begin of a Taylor series and Bc(0)=0.2 T, from Q-drop, i.e. delta_R prop. exp(-c/Ep) without field emission, and for Nb films or heavily corroded Nb, to Rhys prop. omega*Brf/jcJ (T) with jcJ the critical current density of weak links. For carefully prepared and oxidized Nb Rres(T,f) prop. f**2/jcJ**3/2(T) > 2*(f/GHz)**2 nano-Ohm have been attained. With rf field in bulk Nb cavities immersed in He a Q-slope with gamma(T<2.17K = T_lambda) < 1 - 3 has been reached turning above T_lambda to gamma > 10 - 20 confirming Delta_T heating as main cause. By interface resistances, e.g., in sputtered cavities, gamma(T) = 20 - 50 have been found simulating an exponential R(B)-increase via the exponential R_BCS(T+Delta_T,f,B) increase. The Q-drop by interfacial tunnel exchange with Nb2O5 at the outside surface grows with defects and nanoroughnesses in 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 at rather low fields. Because of jcJ(T < Tc/2)= const, Rres(T<=Tc/2) and Rhys(T <= Tc/2) are T-independent for Nb, like the interface losses delta_R prop. exp(-c/Ep), whereas the Q-slope replicates via gamma(T) the BCS surface resistance in its T-dependence. All observations, up to date, can be explained in this frame work, pointing to the key interface material properties for further improvements of peak fields and for the reduction of rf losses.

List of authors...

Principle author first.

Last (Family) Name First Name (Initials only) Affiliation or Organization (abbreviations if possible)
Halbritter J. D. FZK, IMF 1

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