Muon spin rotation studies of niobium for superconducting RF applications

TL;DR

This study uses muon spin rotation to analyze niobium samples from RF cavities, revealing how surface treatments influence magnetic properties and RF losses, with implications for superconducting applications.

Contribution

First application of muSR to study niobium cavity samples, linking surface treatments to magnetic behavior and RF loss mechanisms.

Findings

Surface treatments affect surface pinning strength.

Correlation between RF losses and magnetic volume fraction.

Impurity doping influences muSR line width and magnetic properties.

Abstract

In this work we investigate superconducting properties of niobium samples via application of the muon spin rotation/relaxation (muSR) technique. We employ for the first time the muSR technique to study samples that are cutout from large and small grain 1.5 GHz radio frequency (RF) single cell niobium cavities. The RF test of these cavities was accompanied by full temperature mapping to characterize the RF losses in each of the samples. Results of the muSR measurements show that standard cavity surface treatments like mild baking and buffered chemical polishing (BCP) performed on the studied samples affect their surface pinning strength. We find an interesting correlation between high field RF losses and field dependence of the sample magnetic volume fraction measured via muSR. The muSR line width observed in ZF-muSR measurements matches the behavior of Nb samples doped with minute amounts of Ta or N impurities. An upper bound for the upper critical field Hc2 of these cutouts is found.