Two different origins of the Q -slope problem in superconducting niobium film cavities for a heavy ion accelerator at CERN

Superconducting niobium film cavities deposited on copper substrates (Nb/Cu) have suffered from strong field-dependent surface resistance, often referred to as theQ-slope problem, since their invention. We argue that theQ-slope may not be an intrinsic problem, but rather originates from a combinatio...

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Bibliographic Details
Published in:Physical review. Accelerators and beams 2019-07, Vol.22 (7), p.073101, Article 073101
Main Authors: Miyazaki, A., Delsolaro, W. Venturini
Format: Article
Language:English
Subjects:
Copper
Heavy ions
Holes
Magnetic flux
Magnetic shielding
Niobium
Substrates
Superconductivity
Surface resistance
Temperature dependence
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Summary:Superconducting niobium film cavities deposited on copper substrates (Nb/Cu) have suffered from strong field-dependent surface resistance, often referred to as theQ-slope problem, since their invention. We argue that theQ-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions. In this study, extrinsic effects were carefully minimized in a series of experiments on a seamless cavity. The origin of theQ-slope in low frequency cavities is traced back to two contributions with different temperature and magnetic field dependences. The first component ofQ-slope, affecting the residual resistance, is caused by trapped magnetic flux which is normally suppressed by a magnetic shield for bulk niobium cavities. The second, temperature dependent component ofQ-slope, is similar to the medium-fieldQ-slope which is well known in bulk niobium cavities. These results are compared with theoretical models and possible future studies are proposed.
ISSN:2469-9888
2469-9888
DOI:10.1103/PhysRevAccelBeams.22.073101