Influence of the inhomogeneous properties of a superconducting film on the penetration of the magnetic flux

We consider the penetration of magnetic flux perpendicular to a Nb superconducting film containing edge indentations and/or defects. The presence of such defects and indented edges forces the induced currents to change direction abruptly, which gives rise to discontinuity lines (d-lines). It has been recently shown, by magneto-optical means, that the shape of the d-lines contain information about the size and the shape of the indentations [1]. Moreover, in contrast with what is commonly assumed, thermomagnetic instabilities are not preferentially triggered near the manufactured indentations, but rather along smooth edges [1]. Such phenomena can lead to unexpected quenching of superconducting samples and considerably damage them, which must be avoided in applications. In this poster, we investigate whether inhomogeneous superconducting properties can be responsible for triggering magnetic flux avalanches. To this aim, we numerically model the flux penetration and determine the distribution of current lines and electric fields for a situation where the superconducting properties vary locally over a small region of the film. We consider the variation of the thickness of the film, the critical current density and the size of the discontinuity. The resulting shapes of the d-lines are deduced and compared to analytical critical state approximations. We also determine the location and the amplitude of the maximal electric field in the film and compare it to typical values which must be reached in order to trigger magnetic flux avalanches. We conclude that the investigated variations lead to levels of electric field which are of the same order of magnitude as those generated by artificial indentations.