The Effect of Inhomogeneous Surface Disorder on the Superheating Field of Superconducting RF Cavities

TL;DR

This paper presents a theoretical study showing that inhomogeneous surface disorder, such as nitrogen infusion, can enhance the superheating field in superconducting RF cavities, potentially improving their maximum surface fields.

Contribution

It introduces a model demonstrating how inhomogeneous surface disorder can increase the superheating field beyond traditional limits in superconductors.

Findings

Inhomogeneous disorder can raise the superheating field above the clean limit.

Homogeneous disorder suppresses the maximum supercurrent.

Impurity diffusion layers help enhance the maximum accelerating gradient.

Abstract

Recent advances in surface treatments of Niobium superconducting radio frequency (SRF) cavities have led to substantially increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for such performance enhancements. We report theoretical results for the effects of inhomogeneous surface disorder on the superheating field --- the surface magnetic field above which the Meissner state is globally unstable. 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. Homogeneous 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.