N-doped surfaces of superconducting niobium cavities as a disordered composite

TL;DR

This paper explains the complex effects of nitrogen doping on superconducting niobium cavities' Q-factor by extending a model involving RF losses, percolation, and the proximity effect in a disordered composite.

Contribution

It introduces an extended model that accounts for the observed RF frequency and field amplitude dependence of Q-factor in N-doped niobium cavities.

Findings

Q-factor varies with RF frequency and field amplitude.

The model explains both increase and decrease in Q with doping.

Disordered composite behavior is key to understanding RF losses.

Abstract

The Q-factor of superconducting accelerating cavities can be substantially improved by a special heat treatment under N2 atmosphere (N-doping). Recent experiments at Fermi National Laboratory investigated the dependence of Q on the RF frequency and showed, unexpectedly, both an increase and a decrease with the RF field amplitude. This paper shall explain this finding by extending a previously proposed model founded on the two fluid model of RF losses, percolation and the proximity effect in a disordered composite.