Dendritic flux avalanches in a superconducting MgB2 tape

MgB2 tapes with high critical current have a significant technological potential, but can experience operational breakdown due to thermomagnetic instability. Using magneto-optical imaging the spatial structure of the thermomagnetic avalanches has been resolved, and the reproducibility and thresholds for their appearance have been determined. By combining magneto-optical imaging with magnetic moment measurements, it is found that avalanches appear in a range between 1.7 mT and 2.5 T. Avalanches appearing at low fields are small intrusions at the tape's edge and non-detectable in measurements of magnetic moment. Larger avalanches have dendritic structures

Dendritic flux avalanches in a superconducting MgB2 tape

MgB2 tapes with high critical current have a significant technological potential, but can experience operational breakdown due to thermomagnetic instability. Using magneto-optical imaging the spatial structure of the thermomagnetic avalanches has been resolved, and the reproducibility and thresholds for their appearance have been determined. By combining magneto-optical imaging with magnetic moment measurements, it is found that avalanches appear in a range between 1.7 mT and 2.5 T. Avalanches appearing at low fields are small intrusions at the tape's edge and non-detectable in measurements of magnetic moment. Larger avalanches have dendritic structures

Hydrogen Concentration in Photovoltaic a-Si:H Annealed at Different Temperatures Measured by Neutron Reflectometry

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Nanosecond voltage pulses from dendritic flux avalanches in superconducting NbN films

Combined voltage and magneto-optical study of magnetic flux flow in superconducting NbN films is reported. The nanosecond-scale voltage pulses appearing during thermomagnetic avalanches have been recorded in films partially coated by a metal layer. Simultaneous magneto-optical imaging and voltage measurements allowed the pulses to be associated with individual flux branches penetrating the superconductor below the metal coating. From detailed characteristics of pulse and flux branches, the electrical field in the superconductor is found to be in the range of 5-50 kV/m, while the propagation speed of the avalanche during its final stage is found to be close to 5 km/s. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing

Magneto-optical imaging of columnar YBCO films

The columnar growth of superconducting films is a new approach that provides high density of evenly distributed extended defects on the nanometer scale. This growth can be activated by tightly-packed nanoparticles deposited on substrate prior to the pulse deposition of superconducting film. The magnetization measurements demonstrate that critical current density (Jc) in columnar films is higher than in common epitaxial films. However, in magnetization measurements calculation of Jc is model sensitive. An assumption of ideal critical state frequently proves to be wrong giving incorrect Jc averaged over the entire sample. Missing details of current distribution can be obtained by magneto-optical imaging (MOI) that also allows obtaining local Jc in the samples. In this paper we report MOI study of previously not imaged relatively thick (about one micrometer) columnar YBa2Cu3Ox films in order to clarify if they fit a critical state model and to what extent their properties are modified by the columnar growth. MOI demonstrates a homogeneous penetration of magnetic flux that fits a critical state model. The long-length influence of defects, especially those residing on the edges is less pronounced in columnar than in common epitaxial films. The measurement of the depth of flux front at partial flux penetration allows estimating average low-field Jc, which is in good agreement with extrapolated magnetization measurements. The spatial details of the distribution of magnetic flux provide valuable information for tailoring deposition parameters in order to obtain highest possible value of Jc. MOI could be an ideal tool for measuring Jc when other methods are not available or not appropriate. Using MOI and magnetometry, an important effect of the increase in Jc in aged columnar films has been observed