Enhancement in Quality Factor of SRF Niobium Cavities by Material Diffusion

TL;DR

This paper discusses how material diffusion, specifically titanium or nitrogen doping, enhances the quality factor of superconducting niobium cavities by reducing surface resistance components.

Contribution

It demonstrates that thermal diffusion of non-magnetic impurities like titanium or nitrogen can significantly improve the Q-factor of SRF niobium cavities.

Findings

Q-rise observed with titanium diffusion

Nitrogen diffusion also increases Q-factor

Surface resistance components are effectively reduced

Abstract

An increase in the quality factor of superconducting radiofrequency cavities is achieved by minimizing the surface resistance during processing steps. The surface resistance is the sum of temperature independent residual resistance and temperature/material dependent Bardeen-Cooper-Schrieffer (BCS) resistance. High temperature heat treatment usually reduces the impurities concentration from the bulk niobium, lowering the residual resistance. The BCS part can be reduced by selectively doping non-magnetic impurities. The increase in quality factor, termed as Q-rise, was observed in cavities when titanium or nitrogen thermally diffused in the inner cavity surface.