Electromagnetic Response of Superconducting RF Cavities

Recently reported anomalies in frequency shift of order \(\delta f\sim 0.1 -10\,\mbox{kHz}\) for Niobium SRF cavities in a narrow temperature region near \(T_c\simeq 9\,\mbox{K}\) are sensitive to the surface preparation and Nitrogen doping. We developed methods for calculating the surface current r...

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Bibliographic Details
Published in:arXiv.org 2022-09
Main Authors: Ueki, Hikaru, Zarea, Mehdi, Sauls, J A
Format: Article
Language:English
Subjects:
Anomalies
Boundary conditions
Electromagnetic fields
Fermi surfaces
Frequency shift
Impurities
Niobium
Q factors
Resonant frequencies
Response functions
Superconductivity
Surface preparation
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Summary:Recently reported anomalies in frequency shift of order \(\delta f\sim 0.1 -10\,\mbox{kHz}\) for Niobium SRF cavities in a narrow temperature region near \(T_c\simeq 9\,\mbox{K}\) are sensitive to the surface preparation and Nitrogen doping. We developed methods for calculating the surface current response of Nb SRF cavities, as well as the resonant frequency shift and quality factor, as functions of temperature, frequency and disorder based on the Keldysh formulation of the theory for nonequilibrium superconductivity coupled with Maxwell's theory and boundary conditions for the response functions and electromagnetic field. We show that the anomaly is sensitive to impurity disorder. Our results for the anomaly in the frequency shift are in good agreement with experimental results for Nb with an anisotropic gap on the Fermi surface and inhomogeneous non-magnetic disorder. We also show that the quality factor as a function of the impurity scattering rate is maximum for in SRF cavities with intermediate disorder, \(\hbar/\tau\Delta\sim{\cal O}(1)\) with the maximum \(Q\) decreasing with increasing frequency.
ISSN:2331-8422
DOI:10.48550/arxiv.2209.11752