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Title Superconductivity above the upper critical field as a probe for Niobium RF-cavity surfaces
Type Talk  
Abstract Since surface superconductivity (SSC) resides in the same sheath, where also microwave currents are flowing, SCC may be employed as an alternative probe to explore the effects of surface treatments, by which currently the performance of superconducting accelerator cavities were improved. We demonstrate this idea by investigating the linear complex conductivity between 10 Hz and 1 MHz and also the critical current density along the circumference of Nb-cylinders in magnetic fields beyond the upper critical field, applied parallel to the cylinder axes. The cylinders have been cut from Nb-sheets (RRR=300), being used for the cavity production for the TESLA project at DESY. They have been subjected to the same chemical (CP) and electrolytical (EP) surface polishing and subsequent low temperature baking (LTB) processes as the RF cavities.- Using the conventional definition of the surface critical field, i.e. the onset of an excess Ohmic conductivity, we find for all temperatures between 2 K and 7 K the ratio R between the surface critical field and the upper critical field to increase from 1.87 for CP to 2.07 by EP. After LTB R rises to even larger values, 2.13 and 2.51, respectively. The screening part of the conductivity reveals that long-range coherence of SSC appears at a field, which turns out to be about 25 percent lower than the surface critical field. There also a critical surface current starts to circulate around the cylinder. While EP enhances this current by a factor of eight, LTB has no effect.- We relate our results to two features of the surface: (i) polishing and LTB induce chemical impurities in a layer in the order of the magnetic penetration depth, which enhance the nucleation fields, and (ii) the roughness reduces the stability of the SSC. Both results demonstrate the close relation between the characteristics of SSC and RF cavity performance.- As a rather surprising feature, we detected rather large concentrations of localized paramagnetic moments of unknown origin, which even increase upon LTB.

List of authors...

Principle author first.

Last (Family) Name First Name (Initials only) Affiliation or Organization (abbreviations if possible)
Casalbuoni S Inst. of Appl. Phys., Univ. Hamburg
von Sawiliski L Inst. of Appl. Phys., Univ. Hamburg
Koetzler J Inst. of Appl. Phys., Univ. Hamburg

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