34 research outputs found
Dendritic flux avalanches in rectangular superconducting films -- numerical simulations
Dendritic flux avalanches is a frequently encountered instability in the
vortex matter of type II superconducting films at low temperatures. Previously,
linear stability analysis has shown that such avalanches should be nucleated
where the flux penetration is deepest. To check this prediction we do numerical
simulations on a superconducting rectangle. We find that at low substrate
temperature the first avalanches appear exactly in the middle of the long
edges, in agreement with the predictions. At higher substrate temperature,
where there are no clear predictions from the theory, we find that the location
of the first avalanche is decided by fluctuations due to the randomly
distributed disorder.Comment: 3 pages, 2 figure
Oscillatory regimes of the thermomagnetic instability in superconducting films
The stability of superconducting films with respect to oscillatory precursor
modes for thermomag- netic avalanches is investigated theoretically. The
results for the onset threshold show that previous treatments of
non-oscillatory modes have predicted much higher thresholds. Thus, in film
supercon- ductors, oscillatory modes are far more likely to cause
thermomagnetic breakdown. This explains the experimental fact that flux
avalanches in film superconductors can occur even at very small ramping rates
of the applied magnetic field. Closed expressions for the threshold magnetic
field and temperature, as well oscillation frequency, are derived for different
regimes of the oscillatory thermomagnetic instability.Comment: 5 pages, 5 figure
Modelling nonlocal electrodynamics in superconducting films: The case of a concave corner
We consider magnetic flux penetration in a superconducting film with a
concave corner. Unlike convex corners, where the current flow pattern is easily
constructed from Bean's critical state model, the current flow pattern at a
concave corner is highly nontrivial. To address the problem, we do a numerical
flux creep simulation, where particular attention is paid to efficient handling
of the non-local electrodynamics, characteristic of superconducting films in
the transverse geometry. We find that the current stream lines at the concave
corner are close to circular, but the small deviation from exact circles ensure
that the electric field is finite and continuous. Yet, the electric field is,
as expected, very high at the concave corner. At low fields, the critical state
penetration is deeper from the concave corner than from the straight edges,
which is a consequence of the electrodynamic non-locality. A magneto-optical
experiment on a YBCO displays an almost perfect match with the magnetic flux
distribution from the simulation, hence verifying the necessity of including
electrodynamic non-locality in the modelling of superconducting thin films.Comment: 7 pages, 7 figure
Diversity of flux avalanche patterns in superconducting films
The variety of morphologies in flux patterns created by thermomagnetic
dendritic avalanches in type-II superconducting films is investigated using
numerical simulations. The avalanches are triggered by introducing a hot spot
at the edge of a strip-shaped sample, which is initially prepared in a
partially penetrated Bean critical state by slowly ramping the transversely
applied magnetic field. The simulation scheme is based on a model accounting
for the nonlinear and nonlocal electrodynamics of superconductors in the
transverse geometry. By systematically varying the parameters representing the
Joule heating, heat conduction in the film, and heat transfer to the substrate,
a wide variety of avalanche patterns is formed, and quantitative
characterization of areal extension, branch width etc. is made. The results
show that branching is suppressed by the lateral heat diffusion, while large
Joule heating gives many branches, and heat removal into the substrate limits
the areal size. The morphology shows significant dependence also on the initial
flux penetration depth.Comment: 6 pages, 6 figure
Nucleation and propagation of thermomagnetic avalanches in thin-film superconductors (Review Article)
Stability of the vortex matter - magnetic flux lines penetrating into the material - in type-II superconductor films is crucially important for their application. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger an electromagnetic breakdown. In this paper, we review recent theoretical and experimental results on development of the above mentioned ther-momagnetic instability. Starting from linear stability analysis for the initial critical-state flux distribution we then discuss a numerical procedure allowing to analyze developed flux avalanches. As an example of this approach we consider ultra-fast dendritic flux avalanches in thin superconducting disks. At the initial stage the flux front corresponding to the dendrite\u27s trunk moves with velocity up to 100 km/s. At later stage the almost constant ve-locity leads to a specific propagation r egime similar to ray optics. We discuss this regime observed in supercon-ducting films coated by normal strips. Finally, we discuss dramatic enhancement of the anisotropy of the flux patterns due to specific dynamics. In this way we demonstrate that the combination of the linear stability analysis with the numerical approach provides an efficient framework for understanding the ultra-fast coupled nonlocal dynamics of electromagnetic fields and dissipation in superconductor films
The thermomagnetic instability in superconducting films with adjacent metal layer
Dendritic flux avalanches is a frequently encountered consequence of the
thermomagnetic instability in type-II superconducting films. The avalanches,
potentially harmful for superconductor-based devices, can be suppressed by an
adjacent normal metal layer, even when the two layers are not in thermal
contact. The suppression of the avalanches in this case is due to so-called
magnetic braking, caused by eddy currents generated in the metal layer by
propagating magnetic flux. We develop a theory of magnetic braking by analyzing
coupled electrodynamics and heat flow in a superconductor-normal metal bilayer.
The equations are solved by linearization and by numerical simulation of the
avalanche dynamics. We find that in an uncoated superconductor, even a uniform
thermomagnetic instability can develop into a dendritic flux avalanche. The
mechanism is that a small non-uniformity caused by the electromagnetic
non-locality induces a flux-flow hot spot at a random position. The hot spot
quickly develops into a finger, which at high speeds penetrates into the
superconductor, forming a branching structure. Magnetic braking slows the
avalanches, and if the normal metal conductivity is sufficiently high, it can
suppress the formation of the dendritic structure. During avalanches, the
braking by the normal metal layer prevents the temperature from exceeding the
transition temperature of the superconductor. Analytical criteria for the
instability threshold are developed using the linear stability analysis. The
criteria are found to match quantitatively the instability onsets obtained in
simulation.Comment: 14 pages, 9 figure