Flux Trapping Sensitivity on the Metallurgical State of Superconducting Radio Frequency Niobium

Trapped magnetic flux in superconducting radio-frequency (SRF) niobium cavities due to the incomplete Meissner effect is a significant source of rf losses. The relationship between flux expulsion and flux trapping sensitivity to the metallurgical state of 1.3 GHz and 3.0 GHz SRF niobium cavities fabricated from standard and cold-worked Nb sheets and subjected to various heat treatments has been studied. Flux expulsion increased with higher heat treatment temperature in all cavities. Nb cavities made from cold-worked sheets showed better flux expulsion after 800 ◦C/3 h compared to the standard SRF-grade Nb cavities. Similar flux expulsion was observed after annealing at 900 ◦C/3 h, and 1000 ◦C/3 h. The flux trapping sensitivity showed the same trend for all cavities, being independent of the original material, and decreasing with increasing heat treatment temperature. Stronger dependence of the trapped flux sensitivity was found with respect to the final surface preparations rather than the bulk microstructure. Samples’ analysis shows that a higher annealing temperature increases the grain size, decreases grain boundary density, reduces precipitates, and possibly pinning centers. Localized rf losses due to trapped flux have also been investigated, using temperature and magnetic-field mapping on the surface of a 3.0 GHz SRF Nb cavity for different magnitudes of residual magnetic fields. It is found that the magnitude of the hotspots increases with a higher trapped magnetic field, indicating increasing rf losses. The hot-spots and quench location were moved to the location where the magnetic field was trapped locally, providing direct evidence of vortex-induced hot-spots in SRF cavities.