Abstract
Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.
Highlights
The performance of Nb as a material for superconducting radiofrequency (SRF) cavities is reaching fundamental limits in terms of surface resistance and peak surface magnetic field
The results listed in table 2 are in good agreement with data derived from low field surface impedance measurements [11]
In this study the maximum RF and DC fields of Nb3Sn prepared for SRF application have been measured using a quadrupole resonator and the muon spin rotation technique
Summary
The performance of Nb as a material for superconducting radiofrequency (SRF) cavities is reaching fundamental limits in terms of surface resistance and peak surface magnetic field. One potential alternative material is Nb3Sn, a Type-II superconductor with a relatively high Ginzburg–Landau parameter κ (»40 close to stoichiometry [1]) as opposed to about 1.4 for high purity Nb. Nb3Sn has a relatively high critical temperature of about 18 K, twice that of Nb which allows, for example, to operate RF cavities at 4.2 K with the same losses as for Nb at 2 K, if dominated by intrinsic losses from thermally activated quasiparticles, so called BCS losses. To obtain accelerating gradients above ≈10 MV m–1 for β = 1 elliptical cavities Nb3Sn has either to be operated in the mixed phase, with the penetrated vortices being pinned to avoid strong dissipation, or in a metastable vortex free state This state can potentially be sustained up to the superheating field Hsh which is about 440 mT for Nb3Sn, compared to about 240 mT for Nb. Superconducting material parameters at T = 0 K relevant to this work of Nb and Nb3Sn are presented in table 1. The values of κ and Hc1 are for clean Nb and Nb3Sn close to stoichiometry, while Hsh is computed
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