Surface Superconductivity in Niobium for Superconducting RF Cavities

Abstract

A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for di erent frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above Bc2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range between Bc2 and Bcohc3, and (2) incoherent surface superconductivity with disconnected sc domains between Bcohc3 and Bc3. The coherent critical surface field separating the two phases is found to be Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.

15 pages, 21 figures, DESY-Report 2004-027

References in corpus (5)

• The Superconducting TESLA Cavities
• Achievement of 35 MV/m in the Superconducting Nine-Cell Cavities for TESLA
• Improved surface treatment of the superconducting TESLA cavities
• Superconductors in realistic geometries: Geometric edge barrier versus pinning
• Superconductivity Above H_{c2} as a Probe for Niobium RF-Cavity Surfaces

Cited by in corpus (25)

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• Electromagnetic properties of impure superconductors with pair-breaking processes
• Spatial decoherence near metallic surfaces
• Field of first flux entry and pinning strength of superconductors for RF application measured with muon spin rotation
• Flux pinning characteristics in cylindrical ingot niobium used in superconducting radio frequency cavity fabrication
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• Depth-resolved measurements of the Meissner screening profile in surface-treated Nb
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• In-situ Synchrotron X-Ray Photoelectron Spectroscopy Study of Medium-Temperature Baking of Niobium for SRF Application
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• Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted $\beta$-emitter ${}^8$Li
• Measurements of the first-flux-penetration field in surface-treated and coated Nb: Distinguishing between surface pinning and an interface energy barrier
• Enhancement of the critical current by surface irregularities in Fe-based superconductors
• Depth-resolved Characterization of Meissner Screening Breakdown in Surface Treated Niobium
• Study of impurity distribution in mechanically polished, chemically treated and high vacuum degassed pure Niobium samples using TOFSIMS technique