Simulation of Non-linear SRF losses derived from characteristic topography of etched and electropolished niobium surfaces

TL;DR

This paper presents a numerical model to simulate non-linear SRF surface losses in niobium cavities caused by topographical magnetic field enhancements, explaining differences between BCP and EP treatments.

Contribution

The study introduces a simplified numerical model that links surface topography to non-linear SRF losses, providing insights into cavity performance variations.

Findings

Model predictions align with observed BCP vs. EP high field Q0 differences.

Surface topography significantly impacts non-linear SRF losses.

The model helps understand the effects of surface treatment on cavity performance.

Abstract

A simplified numerical model has been developed to simulate non-linear superconducting radiofrequency (SRF) losses on Nb surfaces. This study focuses exclusively on excessive surface resistance (Rs) losses due to the microscopic topographical magnetic field enhancements. When the enhanced local surface magnetic field exceeds the superconducting critical transition magnetic field Hc, small volumes of surface material may become normal conducting and increase the effective surface resistance without inducing a quench. Using topographic data from typical Buffered Chemical Polish (BCP) and Electropolish (EP) treated fine grain niobium , we have estimated the resulting field-dependent losses and extrapolated this model to the implications for cavity performance. The model predictions correspond well to the characteristic BCP versus EP high field Q0 performance differences for fine grain niobium. We describe the algorithm of the model, its limitations, and the effects of this non-linear loss contribution on SRF cavity performance.