Internal Friction Measures to Study Precipitates Formation in EP and N-Doped Bulk Nb for SRF Applications

TL;DR

This study uses internal friction measurements to investigate the thermodynamic phenomena, specifically precipitate formation and impurity effects, responsible for high field Q slope in superconducting RF cavities made of niobium.

Contribution

It demonstrates the application of mechanical spectroscopy to identify impurity and vacancy interactions in Nb, correlating internal friction peaks with precipitate and impurity effects relevant for SRF performance.

Findings

Internal friction spectra reveal N trapping mechanisms in Nb.

Results corroborate impurity effects on Q-disease and HFQS.

Impurity-vacancy interactions influence superconducting cavity performance.

Abstract

Main focus of this study is the investigation of thermodynamics phenomena responsible for the High Field Q Slope (HFQS) in SRF cavities by Internal Friction (IF) measurement. Mechanical spectroscopy is, indeed, a well-established technique to study precipitate formations in BCC materials and several works on the effects of impurities as N and O on the Snoek peak have been published so far and will be taken as reference to explain the mechanisms behind the observed dissipation effects. Internal Friction measurements were performed in Belgium at IMCE on Nb rectangular shape samples with different RRR values prepared at Fermilab by using Electro Polishing (EP), N-doping and heat treatments in order to reproduce the same conditions during the standard treatments applied on bulk Nb SRF cavities. From IF spectra, the H trapping mechanism by interstitial atoms (N and O and/or vacancies, depending on the purity level, RRR) can be easily recognized leading to results that perfectly corroborate previous findings on Q-disease, HFQS and RRR phenomena.