63 research outputs found
Electric field formulation for thin film magnetization problems
We derive a variational formulation for thin film magnetization problems in
type-II superconductors written in terms of two variables, the electric field
and the magnetization function. A numerical method, based on this formulation,
makes it possible to accurately compute all variables of interest, including
the electric field, for any value of the power in the power law current-voltage
relation characterizing the superconducting material. For high power values we
obtain a good approximation to the critical state model solution. Numerical
simulation results are presented for simply and multiply connected films, and
also for an inhomogeneous film.Comment: 15 p., submitte
3D modeling of magnetic atom traps on type-II superconductor chips
Magnetic traps for cold atoms have become a powerful tool of cold atom
physics and condense matter research. The traps on superconducting chips allow
one to increase the trapped atom life- and coherence time by decreasing the
thermal noise by several orders of magnitude compared to that of the typical
normal-metal conductors. A thin superconducting film in the mixed state is,
usually, the main element of such a chip.
Using a finite element method to analyze thin film magnetization and
transport current in type-II superconductivity, we study magnetic traps
recently employed in experiments. The proposed approach allows us to predict
important characteristics of the magnetic traps (their depth, shape, distance
from the chip surface, etc.) necessary when designing magnetic traps in cold
atom experiments.Comment: Submitted to Superconductor Science and Technolog
On the energy-based variational model for vector magnetic hysteresis
We consider the quasi-static magnetic hysteresis model based on a dry-friction like representation of magnetization. The model has a consistent energy interpretation, is intrinsically vectorial, and ensures a direct calculation of the stored and dissipated energies at any moment in time, and hence not only on the completion of a closed hysteresis loop. We discuss the variational formulation of this model and derive an efficient numerical scheme, avoiding the usually employed approximation which can be inaccurate in the vectorial case. The parameters of this model for a nonoriented steel are identified using a set of first order reversal curves. Finally, the model is incorporated as a local constitutive relation into a 2D finite element simulation accounting for both the magnetic hysteresis and the eddy current
Dynamic response of HTS composite tapes to pulsed currents
Dynamic voltage-current characteristics of an HTS Ag/BiSCCO composite tape
are studied both experimentally and theoretically. The tape is subjected by
pulsed currents with different shapes and magnitude and voltage traces are
measured using the four-point method with different location of potential taps
on the sample surface. Clockwise and anticlockwise hysteresis loops are
obtained for the same sample depending on location of the potential taps. The
dynamic characteristics deviate substantially from the DC characteristic,
especially in the range of low voltages where a criterion for the critical
current value is usually chosen (1-10 mkV/cm). The critical current determined
from dynamic characteristics and its change with the pulse magnitude depend on
location of the potential taps and on the curve branch chosen for the critical
current determination (ascending or descending). The theoretical analysis is
based on a model of the magnetic flux diffusion into a composite tape for a
superconductor described by the flux creep characteristic. Numerical simulation
based on this model gives the results in good agreement with the experimental
ones and explains the observed peculiarities of the dynamic characteristics of
HTS composite tapes. The difference between the magnetic diffusion into a tape
and a slab is discussed.Comment: 18 pages, 13 figure
Critical State in Thin Anisotropic Superconductors of Arbitrary Shape
A thin flat superconductor of arbitrary shape and with arbitrary in-plane and
out-of-plane anisotropy of flux-line pinning is considered, in an external
magnetic field normal to its plane.
It is shown that the general three-dimensional critical state problem for
this superconductor reduces to the two-dimensional problem of an infinitely
thin sample of the same shape but with a modified induction dependence of the
critical sheet current. The methods of solving the latter problem are well
known. This finding thus enables one to study the critical states in realistic
samples of high-Tc superconductors with various types of anisotropic flux-line
pinning. As examples, we investigate the critical states of long strips and
rectangular platelets of high-Tc superconductors with pinning either by the
ab-planes or by extended defects aligned with the c-axis.Comment: 13 pages including 13 figure files in the tex
Continuum theory of partially fluidized granular flows
A continuum theory of partially fluidized granular flows is developed. The
theory is based on a combination of the equations for the flow velocity and
shear stresses coupled with the order parameter equation which describes the
transition between flowing and static components of the granular system. We
apply this theory to several important granular problems: avalanche flow in
deep and shallow inclined layers, rotating drums and shear granular flows
between two plates. We carry out quantitative comparisons between the theory
and experiment.Comment: 28 pages, 23 figures, submitted to Phys. Rev.
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
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