Mingmin Wang¹, Shreyas Balachandran², Santosh Chetri², Anatolii Polyanskii², Peter Lee², Thomas Bieler¹; ¹Michigan State University, ²National High Magnetic Field Laboratory

High-purity niobium is used for superconducting radio-frequency (SRF) cavities for particle accelerators with potential applications from water treatment to materials science. Trapped magnetic flux due to dislocations and grain boundaries in niobium can degrade the performance of SRF cavities, and hence is a scientific topic of growing interest. However, it is still unclear what defects actually trap magnetic flux, and which processing paths can minimize defects and ensure consistently high cavity performance. To investigate the mechanism of flux trapping by dislocations and grain boundaries, bi-crystal niobium samples with strategically chosen tensile axes were strained to about 5% to introduce dislocations. Electron channeling contrast imaging (ECCI) was used to characterize dislocation structures before and after the deformation, followed by magnetic flux observations at superconducting temperatures using magneto-optical (MO) imaging. The effect of dislocation density on flux trapping and conditions that increase the tendency to trap flux are discussed.