Flux pinning characteristics in cylindrical ingot niobium used in superconducting radio frequency cavity fabrication

TL;DR

This study compares flux pinning and critical current densities in large-grain and fine-grain niobium used for superconducting RF cavities, revealing differences that influence residual resistance.

Contribution

It provides new insights into how grain size affects flux pinning and residual resistance in niobium SRF cavities through magnetization and penetration depth measurements.

Findings

Large-grain niobium has lower critical current density than fine-grain.

Flux trapping efficiency is lower in large-grain niobium.

Lower residual resistance observed in large-grain cavities.

Abstract

We present the results of from DC magnetization and penetration depth measurements of cylindrical bulk large-grain (LG) and fine-grain (FG) niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities. The surface treatment consisted of electropolishing and low temperature baking as they are typically applied to SRF cavities. The magnetization data were fitted using a modified critical state model. The critical current density Jc and pinning force Fp are calculated from the magnetization data and their temperature dependence and field dependence are presented. The LG samples have lower critical current density and pinning force density compared to FG samples which implies a lower flux trapping efficiency. This effect may explain the lower values of residual resistance often observed in LG cavities than FG cavities.