59 research outputs found
Diversity of flux avalanche patterns in superconducting films
The variety of morphologies in flux patterns created by thermomagnetic
dendritic avalanches in type-II superconducting films is investigated using
numerical simulations. The avalanches are triggered by introducing a hot spot
at the edge of a strip-shaped sample, which is initially prepared in a
partially penetrated Bean critical state by slowly ramping the transversely
applied magnetic field. The simulation scheme is based on a model accounting
for the nonlinear and nonlocal electrodynamics of superconductors in the
transverse geometry. By systematically varying the parameters representing the
Joule heating, heat conduction in the film, and heat transfer to the substrate,
a wide variety of avalanche patterns is formed, and quantitative
characterization of areal extension, branch width etc. is made. The results
show that branching is suppressed by the lateral heat diffusion, while large
Joule heating gives many branches, and heat removal into the substrate limits
the areal size. The morphology shows significant dependence also on the initial
flux penetration depth.Comment: 6 pages, 6 figure
Magnetostrictive behaviour of thin superconducting disks
Flux-pinning-induced stress and strain distributions in a thin disk
superconductor in a perpendicular magnetic field is analyzed. We calculate the
body forces, solve the magneto-elastic problem and derive formulas for all
stress and strain components, including the magnetostriction . The
flux and current density profiles in the disk are assumed to follow the Bean
model. During a cycle of the applied field the maximum tensile stress is found
to occur approximately midway between the maximum field and the remanent state.
An effective relationship between this overall maximum stress and the peak
field is found.Comment: 8 pages, 6 figures, submitted to Supercond. Sci. Technol., Proceed.
of MEM03 in Kyot
Flux Penetration in Superconducting Strip with Edge-Indentation
The flux penetration near a semicircular indentation at the edge of a thin
superconducting strip placed in a transverse magnetic field is investigated.
The flux front distortion due to the indentation is calculated numerically by
solving the Maxwell equations with a highly nonlinear law. We find that
the excess penetration, , can be significantly ( 50%) larger than
the indentation radius , in contrast to a bulk supercondutor in the
critical state where . It is also shown that the flux creep tends
to smoothen the flux front, i.e. reduce . The results are in very good
agreement with magneto-optical studies of flux penetration into an
YBaCuO film having an edge defect.Comment: 5 pages, 7 figure
Flux Dendrites of Opposite Polarity in Superconducting MgB rings observed with magneto-optical imaging
Magneto-optical imaging was used to observe flux dendrites with opposite
polarities simultaneously penetrate superconducting, ring-shaped MgB films.
By applying a perpendicular magnetic field, branching dendritic structures
nucleate at the outer edge and abruptly propagate deep into the rings. When
these structures reach close to the inner edge, where flux with opposite
polarity has penetrated the superconductor, they occasionally trigger anti-flux
dendrites. These anti-dendrites do not branch, but instead trace the triggering
dendrite in the backward direction. Two trigger mechanisms, a non-local
magnetic and a local thermal, are considered as possible explanations for this
unexpected behaviour. Increasing the applied field further, the rings are
perforated by dendrites which carry flux to the center hole. Repeated
perforations lead to a reversed field profile and new features of dendrite
activity when the applied field is subsequently reduced.Comment: 6 pages, 6 figures, accepted to Phys. Rev.
Symmetry of the remanent state flux distribution in superconducting thin strips: Probing the critical state
The critical-state in a thin strip of YBaCuO is studied by magneto-optical
imaging. The distribution of magnetic flux density is shown to have a specific
symmetry in the remanent state after a large applied field. The symmetry was
predicted [PRL 82, 2947 (1999)] for any Jc(B), and is therefore suggested as a
simple tool to verify the applicability of the critical-state model. At large
temperatures we find deviations from this symmetry, which demonstrates
departure from the critical-state behavior. The observed deviations can be
attributed to an explicit coordinate dependence of since both a surface
barrier, and flux creep would break the symmetry in a different way.Comment: 5 pages including 5 eps figures, submitted to PR
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