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.
Highlights
One of the outstanding scientific issues related to the performance of superconducting rf (SRF) cavities made of high-purity niobium (Nb) [having a residual resistivity ratio (RRR) exceeding 200] is the occurrence of fielddependent rf losses in the cavity walls
For the first time using the muon spin rotation technique, the superconducting properties of niobium samples, which have been cut out of 1.5 GHz niobium rf cavities, whose surface was treated via buffered chemical polishing
Results show a change in surface pinning with typical cavity treatments like mild baking and buffered chemical polishing
Summary
One of the outstanding scientific issues related to the performance of superconducting rf (SRF) cavities made of high-purity niobium (Nb) [having a residual resistivity ratio (RRR) exceeding 200] is the occurrence of fielddependent rf losses in the cavity walls. The rf losses typically increase at a gradual rate in the peak surface magnetic field range 20–80 mT, and increase sharply above $80–100 mT. Thermometry tests of SRF Nb cavities [1,2] show that high field losses (HFQS) always appear in the high magnetic peak field region. This indicates that it is the magnetic field component that is responsible for the losses in the Nb cavity walls in the HFQS regime
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