StefanR wrote:This page has some nice resources and ideas:
Dendritic flux avalanches in superconductors
When a superconducting film is placed in a perpendicular magnetic field, the flux penetration sometimes occurs via abrupt avalanches that result in remarkable dendritic flux patterns that can be observed using magneto-optical imagingMagneto-optical images of flux dendrites
Magneto-opitcal studies of a c-oriented MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. This behavior contrasts the gradual uniform penetration usually found in superconducting films.
Figure shows magneto-optical images of flux penetration (image brightness represents flux density) into the virgin state at 5 K. The respective images were taken at applied fields (perpendicular to the film) of 3.4, 8.5, 17, 60, 21, and 0 mT.Simultaneous Penetration of Flux and Antiflux Dendrites in MgB2 rings
Flux dendrites with opposite polarities simultaneously penetrate superconducting, ring-shaped MgB2 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.Dendrites avoid crossing
MO images taken before and after invasion of a dendrite. The new dendrite had to turn the growth direction several times (indicated by arrows) to avoid crossing the existing dendrites. The last image is obtained by subtraction of the first two. The grown dendrite is seen white, while the black regions indicate branches of existing dendrites affected by appearing the new one.Simulations
Flux and temperature distribution produced by vortex dynamics simulations
White dots: individual vortices;
Red: regions of enhanced temperature due to vortex motion
Green: traces of recently moving vortices
Good god man, why didn't somebody point me to this stuff sooner? Ya' been holding out on me?StefanR wrote:Men do not beat drums before they hunt for tigers.
At the bottom of this page (for the link to the original link which has a bigger .gif and some movies): http://mti.msd.anl.gov/highlights.htmlThe formation of a macroscopic current-carrying critical state in type II superconductors occurs via penetration of the magnetic flux front of pinned vortices from the surface of the sample. Recent advances in the magneto-optical imaging have revealed puzzling instabilities of the critical state, including magnetic macro-turbulence, kinetic front roughening, magnetic avalanches and dendritic-type instabilities. These phenomena display remarkable similarities with other dendritic structures in crystal growth, nonequilibrium chemical and biological systems, and crack propagation.
Here we study numerically and analytically the system of coupled nonlinear Maxwell and thermal diffusion equations which describes 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 thermomagnetic edge instability which causes stratification of the critical state. The resulting distribution of magnetic microavalanches depends on spatial distribution of defects. Our results are in good agreement with experiments performed on Nb films.
I'm gobsmacked! I guess this is what happens why I don't browse one of the sub-forums for a while... Egah!
This is golden! Thanks StefanR! "Flux Dendrites" and "Dendritic Flux Avalanches."
*Wanders off muttering...*
Guess we've got our mechasnism, eh...
Gonna' have to go look that up...