Direct evidence of microstructure dependence of magnetic flux trapping in niobium

Abstract

Elemental type-II superconducting niobium is the material of choice for superconducting radiofrequency cavities used in modern particle accelerators, light sources, detectors, sensors, and quantum computing architecture. An essential challenge to increasing energy efficiency in rf applications is the power dissipation due to residual magnetic field that is trapped during the cool down process due to incomplete magnetic field expulsion. New SRF cavity processing recipes that use surface doping techniques have significantly increased their cryogenic efficiency. However, the performance of SRF Nb accelerators still shows vulnerability to a trapped magnetic field. In this manuscript, we report the observation of a direct link between flux trapping and incomplete flux expulsion with spatial variations in microstructure within the niobium. Fine-grain recrystallized microstructure with an average grain size of 10–50 µm leads to flux trapping even with a lack of dislocation structures in grain interiors. Larger grain sizes beyond 100–400 µm do not lead to preferential flux trapping, as observed directly by magneto-optical imaging. While local magnetic flux variations imaged by magneto-optics provide clarity on a microstructure level, bulk variations are also indicated by variations in pinning force curves with sequential heat treatment studies. The key results indicate that complete control of the niobium microstructure will help produce higher performance superconducting resonators with reduced rf losses1 related to the magnetic flux trapping.

Investigation of the effect of structural defects from hydride precipitation on superconducting properties of high purity SRF cavity Nb using magneto-optical and electron imaging methods

“…Without prior deformation or LAGBs, the lower number of defects that enable transport and/or concentration of subsurface hydrogen may prevent the formation of large hydrides within the bulk crystal, and favor formation in a LAGB. Recent results also confirm that the LAGB and very fine grains with limited grain growth lead to early flux penetration [55,56]. Nevertheless, the fact that bulk flux penetration occurs at lower fields following annealing suggests that residual dislocations surrounding scars and LAGBs within scars may facilitate bulk flux penetration more easily than when hydrides are present, suggesting that hydride scar dislocation substructure may facilitate trapped flux more effectively than hydrides.…”

“…However, recent systematic studies do indicate that a grain size or GB density relationship with flux penetration behavior exists. These studies show that flux penetration and expulsion is easier in large grains resulting from grain growth to a size greater than 100 µm that may sweep out LAGBs, whereas penetrating flux gets trapped when the grain sizes are of a finer dimension of ∼30-50 µm [55], in which the likelihood of GB facets being aligned with the magnetic field would be larger.…”

Effect of Successive Heat Treatment on the Performance of Superconducting Radio Frequency Niobium Cavities

“…In addition, the metallurgical state with larger grain size is expected as the annealing temperature is increased [21]. The fine-grain recrystallized microstructure with an average grain size of 10-50 µm leads to flux trapping even with a lack of dislocation structures in grain interiors and larger grain sizes beyond 100-400 µm do not lead to preferential flux trapping, as observed directly by magneto-optical imaging [22]. In order to obtain repeatable and consistent cavity performance and improve process control, it is important to understand the relationship between microstructure, flux trappig and eventual cavity performance.…”

Effect of duration of 120 oC baking on the performance on superconducting radio frequency niobium cavities

“…Even though, the final surface preparation may play some role on the initial condition of rf surface and successive baking duration, the cavity TE1-05 showed higher flux trapping sensitivity compared to cavity TE1-06. Furthermore, the cavity TE1-06 was fabricated from cold worked Nb, may have less density of dislocations and better recrystallization compared to TE1-05 cavity, leading to lower flux trapping sensitivity [14,19]. The 𝑄 0 vs. 𝐸 𝑎𝑐𝑐 measurements at 2.0 K showed that the high-field Q-slope is eliminated just after 3 hours of baking at 120 • C.…”