Experimental evidence for electric surface resistance in niobium

TL;DR

This paper provides experimental evidence that the surface electric field causes increased surface resistance in niobium cavities, with implications for accelerator technology and surface loss mitigation.

Contribution

It demonstrates that surface electric field effects are responsible for resistance increases, supported by measurements and a model involving interface tunnel exchange in oxides.

Findings

Surface resistance increases with electric field above a threshold.

Temperature and frequency dependence align with the interface tunnel exchange model.

Electric field effects are a key loss mechanism in niobium cavities.

Abstract

Identifying the loss mechanisms of niobium cavities enables an accurate determination of applications for future accelerator projects and points to research topics required to mitigate current limitations. For several cavities an increasing surface resistance above a threshold field, saturating at higher field has been observed. Measurements on samples give evidence that this effect is caused by the surface electric field. The measured temperature and frequency dependence is consistent with a model that accounts for these losses by interface tunnel exchange between localized states in oxides formed along grain boundaries and the adjacent superconductor.