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Description
While perfect type-II superconductors are expected to completely expel their internal magnetic flux after transitioning into the Meissner state, impurities and defects may cause some level magnetic flux to be trapped as vortices. Such trapped vortices can be problematic for SRF cavities, causing unwanted localized heating from vortex interaction with the applied RF field, while the trapped flux itself is impacted by cool down dynamics [1] and thermal topology.
As part of dedicated magnetic flux expulsion measurements performed on bulk, thin-film and multilayer superconductor samples using CERN’s Magnetic Flux Lens (MFL) [2], thermally driven flux dynamics have been studied, and thermo-electric current and recursive flux ratcheting effects observed. To better understand and isolate contributions of the observed flux dynamics, a detailed multiphysics simulation model of the MFL setup has been implemented, based on time-dependent Ginzburg-Landau equations coupled with thermodynamics. These numerical methods, in combination with measurement data, are used to investigate flux trapping mechanisms during the superconducting transition. Results show a clear and physical understanding of the dynamics, with connections to the order parameter and magnetic flux density distributions, while detailing the temperature-dependent critical field evolution across the closed cooling topology of the MFL sample geometry, in line with recent theoretical models [3].
[1] O. Kugeler et al., Manipulating the intrinsic quality factor by thermal cycling and magnetic fields, Proc. 14th Int. Conf. RF Supercond. (SRF'09) p. TUPPO053 (2009).
[2] D. Turner et al. Flux expulsion lens: concept and measurements, Proc. 21st Int. Conf. RF Supercond. (SRF'23) p. 56—61 (2023)
[3] T. Kubo, Flux trapping in superconducting accelerating cavities during cooling down with a spatial temperature gradient, Prog. Theor. Exp. Phys, Volume 2016, Issue 5, 053G01.