Fast and Slow Flux Dynamics in Superconductors : A Magneto-Optical Imaging Study

The thesis presents results on two different aspects of flux penetration into superconductors. It is predominantly an experimental study using magneto-optical imaging, a microscopy technique for visualising magnetic fields, but also contains some simulation results. The experimental technique is explained in some detail. The bulk of the thesis deals with dendritic avalanches in a ring- shaped thin film. These avalanches appear as beautiful, tree-like structures which form extremely fast: the advancing tip velocity has been measured to roughly 100 km/s. While they are fascinating and visually appealing objects, they are a significant problem from a practical point of view since they limit the current capacity of superconductors. The ring geometry opens up for several new phenomena because of the characteristic way flux enters a superconducting ring. While positive flux enters at the outer edge, negative flux enters at the inner edge, but in such a way that the superconductor actually screens the hole. When avalanches nucleate at the outer edge, they may span the ring width and thereby inject flux into the central hole. The flux injection is studied in some detail in the thesis, and it is shown how the temperature in the core region of the avalanche may be inferred from measurements of flux changes in the hole: it is found that the maximum temperature is 100 K in typical cases. The avalanches may also halt just before they reach the inner edge, and sometimes trigger anti-flux dendrites. Single vortices are also observed as they enter a NbSe$_2$ sample. The observations show that there is a strong magnetic field at the sample edge where vortices nucleate, but thereafter they quickly move across a certain region leaving a vortex free band near the edge. The observed features can be explained qualitatively within the geometric barrier model. Finally a surprising interaction between Bloch walls and vortices is discussed. Observations show that Bloch walls may attract vortices of opposite polarity, which seems a counter-intuitive behaviour if the Bloch wall is pictured as a simple bar magnet. However, by including the effect of the surrounding domains this conundrum is resolved. The current author only contributed interpretations of experimental results and the identification of the problem in this case

Flux Dendrites of Opposite Polarity in Superconducting MgB2_2 rings observed with magneto-optical imaging

Magneto-optical imaging was used to observe flux dendrites with opposite polarities simultaneously penetrate superconducting, ring-shaped MgB$_2$ 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.

Conformance assessment of electrical energy meters investigated by risk analysis – a case study

This paper presents a case study of more effective decision rules for the conformance assessment of electrical energy meters in private households in Norway, and proposes how to use a specific risk analysis in order to set the time for the next meter test. The MID regulation today prescribes conformance assessment of electrical energy meters based on ISO standards for attribute sampling where decision rules are purely statistical decision rules and economic consequences are not explicitly taken into account. The risk analysis we introduce calculates the risks involved for erroneous decisions, either rejecting a conforming batch of meters (the producer risk) or accepting a non-conforming batch (the consumer risk). The consumer risk is sensitive to the period until the next test which becomes a quality characteristic of each batch. This time interval can be optimized by balancing the consumer risk against the producer risk. When the quality drops, the period until the next test will need to become shorter. But at a certain level of quality, the energy net supplier would rather replace the complete batch, than continue testing at such short intervals