Effect of Inhomogeneous Disorder on the Superheating Field of SRF Cavities

19th International Conference on RF Superconductivity, Dresden, Germany, June 30 - July 5, 2019

Speaker: James A. Sauls
Center for Applied Physics & Superconducting Technologies
Department of Physics, Northwestern University, Evanston, IL 60208
July 4, 2019

Abstract: Recent advances in surface treatments of Niobium SRF cavities have led to increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for performance enhancements. I report theoretical results for the effects of inhomogeneous surface disorder on the superheating field.* We find that inhomogeneous disorder, such as that introduced by infusion of Nitrogen into the surface layers of Niobium SRF cavities, can increase the superheating field above the maximum for superconductors in the clean limit or with homogeneously distributed disorder. Disorder increases the penetration of screening current, but also suppresses the maximum supercurrent. Inhomogeneous disorder in the form of an impurity diffusion layer biases this trade-off by increasing the penetration of the screening currents into cleaner regions with larger critical currents, thus limiting the suppression of the screening current to a thin dirty region close to the surface. Our results suggest that the impurity diffusion layers play a role in enhancing the maximum accelerating gradient of Nitrogen treated Niobium SRF cavities.

*https://arxiv.org/abs/1809.04057

The research of the authors is supported by National Science Foundation Grant PHY-1734332 and the Northwestern-Fermilab Center for Applied Physics and Superconducting Technologies.

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