Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films

We present numerical and analytical studies of coupled nonlinear Maxwell and thermal diffusion equations which describe nonisothermal dendritic flux penetration in superconducting films. We show that spontaneous branching of propagating flux filaments occurs due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heat generation. The branching is triggered by a thermomagnetic edge instability which causes stratification of the critical state. The resulting distribution of magnetic microavalanches depends on a spatial distribution of defects. Our results are in good agreement with experiments performed on Nb films.Comment: 4 pages, 3 figures, see http://mti.msd.anl.gov/aran_h1.htm for extensive collection of movies of dendritic flux and temperature pattern

Hydrodynamic Instability of the Flux-antiflux Interface in Type-II Superconductors

The macroturbulence instability observed in fluxline systems during remagnetization of superconductors is explained. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface when the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in crystals. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins.Comment: 12 pages, 3 figures, submitted to Physical Review Letter

Plasma treatment of heat-resistant materials

Refractory lining of thermal generating units is exposed to chemical, thermal, andmechanical attacks. The degree of fracture of heat-resistant materials depends on the chemicalmedium composition, the process temperature and the material porosity. As is known, a shorttermexposure of the surface to low-temperature plasma (LTP) makes possible to createspecific coatings that can improve the properties of workpieces. The aim of this work is toproduce the protective coating on heat-resistant chamotte products using the LTP technique.Experiments have shown that plasma treatment of chamotte products modifies the surface, anda glass-ceramic coating enriched in mullite is formed providing the improvement of heatresistance. For increasing heat resistance of chamotte refractories, pastes comprising mixturesof Bacor, alumina oxide, and chamot were applied to their surfaces in different ratios. It isproved that the appropriate coating cannot be created if only one of heat-resistant componentsis used. The required coatings that can be used and recommended for practical applications areobtained only with the introduction of powder chamot. The paste composition of 50% chamot,25% Bacor, and 25% alumina oxide exposed to plasma treatment, has demonstrated the mostuniform surface fusion