Magneto-optical imaging of dendritic flux avalanches in a superconducting MgB2 tape

Abstract

Resumen del trabajo presentado al Congreso Magnetism, celebrado en Manchester (UK) del 9 al 10 de abril de 2018.We report first direct imaging of dendritic avalanches in a superconducting MgB2 tape. The avalanches are caused by thermomagnetic instabilities and are known to harm the operation of superconducting devices. They are often detected in magnetisation measurements. However, their lower threshold magnetic field, Hthr, in bulk superconductors is far above the operational range of common visualisation techniques like magneto-optical imaging (MOI). In the present work, we have imaged six different MgB2 tapes and observed dendritic avalanches in one of them. Fig. 1 shows a superposition of three colour-coded MOI images of the sample after cooling to 3.7 K and applying magnetic fields within the operating range of the technique. The imaged abrupt flux jumps are of dendritic nature like those appearing in MgB2 superconducting films. The red, green and blue dendrites are irreproducible. They occur only once in three experiments. However, unlike thin films, the branches of few dendrites are partially reproducible, as shown by yellow, cyan and magenta colours for fragments overlapped twice. The temperature dependence of Hthr will be discussed. When comparing with thin films, it was found that the observed Hthr is much lower than expected, highlighting the importance of edge defects.Peer Reviewe