In superconducting radio frequency (SRF) accelerating cavities fabricated with niobium, the behavior of the magnetic flux quantum (vortex) is closely related to the performance of the cavity. Recently, it has become clear that the contribution of the remanent vortices during cooling is one of the factors limiting the Q-value at sufficiently low temperatures. Therefore, in order to further improve the cavity performance, it is necessary to clarify the vortex state of cavity-grade high purity niobium and the behavior of the vortices during cooling. Previously, we observed the vortex state of pure niobium during cooling in a magnetic field of 100-400 Oe by magneto-optical imaging (MOI) and reported the appearance of an intermediate mixed state (IMS) .
To study the existence of vortex bundles, that is a characteristic feature of the IMS, in much lower magnetic fields, we conducted observations by MOI during a field cooling in a few Oe. Vortex bundles with quite smaller sizes could be observed even in the low fields, suggesting that the vortices remaining in niobium may have coalesced to some extent into a bundle during the cooling process of the SRF cavity. In my presentation, I would like to introduce the results of this observation and discuss the phase diagram of magnetic flux in highly pure niobium.
 S. Ooi et al., “Observation of intermediate mixed state in high-purity cavity-grade Nb by magneto-optical imaging”, Phys. Rev. B 104, 064504 (2021).
Keywords: Vortex Bundle, Niobium, Magneto-optical imaging