Köszegi, J.-M.; Knobloch, J.; Kugeler, O.; Abou-Ras, D.; Schäfer, E.: Towards the Perfect Meissner State: A Magneto-Optical Study on Competing Pinning Centers in Niobium. In: Volker RW Schaa ... [Ed.] : SRF 2017, Proceedings of the 18th International Conference on RF SuperconductivityGeneve: JACoW, 2018. - ISBN 978-3-95450-191-5, p. THPB018/766-770
http://accelconf.web.cern.ch/AccelConf/srf2017/papers/thpb018.pdf
Open Access Version
Abstract:
Over the past years trapped magnetic flux has emerged as a main limiting factor of high quality factors in SRF cavities. Several studies investigated how the ambient magnetic field can be minimized or how the flux expulsion during the phase transition can be improved. We now present a study that targets the pinning centers which allow for the flux to remain inside the superconductor in the first place. Using magneto-optical imaging we were able to not only measure the amount of trapped flux but in addition we managed to image its distribution with a resolution in the order of 10μm and correlate it with electron backscatter diffraction maps. As a result we found that the grain boundaries did not play a major role as pinning centers nor did the crystal orientation influence the amount of trapped flux significantly. However, niobium hydrides which formed during the cooldown to cryogenic temperatures were found to enhance trapping.