INHOMOGENEOUS FLUX PENETRATION IN NIOBIUM SHEET SAMPLED ACROSS THE CAVITY PRODUCTION ROUTE

Our initial studies of cavity-quality Nb prepared by buffered chemical polish (BCP) demonstrated that magneto-optical (MO) imaging could be used to show local flux penetration, some of which occurred preferentially at grain boundaries. In this new work, we have prepared small samples at each stage of processing associated with cavity production. The samples were machined from 2.8 mm thick sheet (RRR ~450). Samples from weld regions were compared to the regular samples. In addition to MO imaging, bulk magnetization was measured. Although large machining grooves were seen in all samples, topology was only intermittently seen in the MO images. Magnetization hysteresis associated both with surface and bulk pinning diminished through process but flux penetration became more inhomogeneous in the vicinity of Hc1 for the more highly treated samples, especially the large-grain weld samples. Our results suggest that optimum treatments for the surface of Nb grains are not optimum for at least some Nb grain boundaries.