Dresden 2003 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 17: Anwendungen der Supraleitung: Tieftemperatur-Teilchendetektoren und -Experimentiertechniken
TT 17.8: Vortrag
Donnerstag, 27. März 2003, 11:30–11:45, HSZ301
Effects of low temperature baking on surface superconductivity of Niobium cavity materials — •S. Casalbuoni, L. von Sawilski, and J. Kötzler — Institut f. Angewandte Physik, Univ. Hamburg, D-20355 Hamburg
In-situ baking of evacuated Nb cavities at low temperatures (l.t.) of 100−150 ∘C is necessary to increase the accelerating fields at 1.3 GHz from 23 MV/m up to ≃ 30 MV/m without a strong degradation of the quality factor ≈ 1010. The underlying mechanism is not understood. A study on the superconducting behaviour of Nb samples from materials intended for the production of cavities for the novel TESLA facility, by DC magnetization and AC susceptibility between 1 Hz and 106 Hz, is presented. While the bulk superconducting properties are not affected by the low temperature baking, evidence of an enhancement of superconductivity in the surface is observed. The l.t. baking shifts the onset of AC screening in the linear regime from ≃ 1.7 HC2 to fields ≈ 10% higher.We associate this behaviour with a decrease of the coherence length of the superconducting surface sheath, implying a generation of impurities by baking. Removal of 10 µm by chemical etching after baking brings back the onset of AC screening to ≃ 1.7 HC2. Then these results are consistent with the observation of a reduced BCS surface resistance at 1.5 GHz after baking of Nb cavities [1]. This was attributed to oxygen atoms diffusing either from the dielectric layer of Nb2O5 or from intergrain niobium oxides/suboxides deep to 200−300 nm. [1] P. Kneisel, Proceedings of the 9th Workshop on RF Superconductivity, Santa Fe USA, edited by F. Krawcyk, LANL, p.TUP044 (1999)