55 research outputs found
Finite-element simulations of hysteretic ac losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field
Numerical simulations of hysteretic ac losses in a tubular
superconductor/paramagnet heterostructure subject to an oscillating transverse
magnetic field are performed within the quasistatic approach, calling upon the
COMSOL finite-element software package and exploiting
magnetostatic-electrostatic analogues. It is shown that one-sided magnetic
shielding of a thin, type-II superconducting tube by a coaxial paramagnetic
support results in a slight increase of hysteretic ac losses as compared to
those for a vacuum environment, when the support is placed inside; a
spectacular shielding effect with a possible reduction of hysteretic ac losses
by orders of magnitude, however, ensues, depending on the magnetic permeability
and the amplitude of the applied magnetic field, when the support is placed
outside.Comment: 7 pages, 4 figure
Hysteretic ac losses in a superconductor strip between flat magnetic shields
Hysteretic ac losses in a thin, current-carrying superconductor strip located
between two flat magnetic shields of infinite permeability are calculated using
Bean's model of the critical state. For the shields oriented parallel to the
plane of the strip, penetration of the self-induced magnetic field is enhanced,
and the current dependence of the ac loss resembles that in an isolated
superconductor slab, whereas for the shields oriented perpendicular to the
plane of the strip, penetration of the self-induced magnetic field is impaired,
and the current dependence of the ac loss is similar to that in a
superconductor strip flanked by two parallel superconducting shields. Thus,
hysteretic ac losses can strongly augment or, respectively, wane when the
shields approach the strip.Comment: 9 pages, 5 figures, submitted to PR
Magnetic detectability of a finite size paramagnet/superconductor cylindrical cloak
Cloaking of static magnetic fields by a finite thickness type-II
superconductor tube surrounded by a coaxial paramagnet shell is studied. On the
basis of exact solutions to the London and Maxwell equations, it is shown that
perfect cloaking is realizable for arbitrary geometrical parameters including
the thin film case for both constituents. In contrast to previous approximate
studies assuming perfect diamagnetism of the superconductor constituent, it is
proven that cloaking provides simultaneously full undetectability, that is the
magnetic moment of the structure completely vanishes as well as all high-order
multipole moments as soon as the uniform field outside remains unaffected.Comment: 6 pages, 2 figures, to be published in Applied Physics Letter
Magnetic cloaking by a paramagnet/superconductor cylindrical tube in the critical state
Cloaking of static magnetic fields by a finite thickness type-II
superconductor tube being in the full critical state and surrounded by a
coaxial paramagnet shell is studied. On the basis of exact solutions to the
Maxwell equations, it is shown that, additionally to previous studies assuming
the Meissner state of the superconductor constituent, perfect cloaking is still
realizable at fields higher than the field of full flux penetration into the
superconductor and for arbitrary geometrical parameters of both constituents.
It is also proven that simultaneously the structure is fully undetectable under
the cloaking conditions. Differently from the case of the Meissner state the
cloaking properties in the application relevant critical state are realized,
however, only at a certain field magnitude.Comment: 5 pages, 4 figures; to be published in Applied Physics Letters. arXiv
admin note: substantial text overlap with arXiv:1401.356
The Bean-Livingston barrier at a superconductor/magnet interface
The Bean-Livingston barrier at the interface of type-II
superconductor/soft-magnet heterostructures is studied on the basis of the
classical London approach. This shows a characteristic dependence on the
geometry of the particular structure and its interface as well as on the
relative permeability of the involved magnetic constituent. The modification of
the barrier by the presence of the magnet can be significant, as demonstrated
for a cylindrical superconducting filament covered with a coaxial magnetic
sheath. Using typical values of the relative permeability, the critical field
of first penetration of magnetic flux is predicted to be strongly enhanced,
whereas the variation of the average critical current density with the external
field is strongly depressed, in accord with the observations of recent
experiments.Comment: RevTeX 4; revised version; accepted in Journal of Physics: Condensed
Matte
Virgin magnetization of a magnetically shielded superconductor wire: Theory and experiment
On the basis of exact solutions to the London equation, the magnetic moment of a type II superconductor filament surrounded by a soft-magnet environment is calculated and the procedure of extracting the superconductor contribution from magnetic measurements is suggested. A comparison of theoretical results with experiments on MgB2/Fe wires allows the estimation of the value of critical current for the first magnetic flux penetration
Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder
A self-consistent stochastic model of domain structure formation in a
uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to
a low-temperature ferroelectric phase, is developed with an account of the
applied electric field and the feedback effect via local depolarization fields.
Both polarization and field components are considered as Gauss random
variables. A system of integro-differential equations for correlation functions
of all involved variables is derived and solved analytically and numerically.
Phase diagram in terms of the average value and dispersion of polarization
reveals different possible equilibrium states and available final single-domain
and multi-domain states. The time-dependent evolution of the average
polarization and dispersion discloses a bifurcation behavior and the
temperature-dependent value of the electric field, deciding between the
single-domain and multi-domain final states, which can be interpreted as the
coercive field. Analytical and numerical results for the time-dependent
correlation length and correlation functions exhibit plausible agreement with
available experimental data.Comment: 33 pages, 10 figure
Theory of flux cutting and flux transport at the critical current of a type-II superconducting cylindrical wire
I introduce a critical-state theory incorporating both flux cutting and flux
transport to calculate the magnetic-field and current-density distributions
inside a type-II superconducting cylinder at its critical current in a
longitudinal applied magnetic field. The theory is an extension of the elliptic
critical-state model introduced by Romero-Salazar and Perez-Rodriguez. The
vortex dynamics depend in detail upon two nonlinear effective resistivities for
flux cutting (\rho_\parallel) and flux flow (\rho_\perp), and their ratio r =
\rho_\parallel/\rho_\perp. When r < 1, the low relative efficiency of flux
cutting in reducing the magnitude of the internal magnetic-flux density leads
to a paramagnetic longitudinal magnetic moment. As a model for understanding
the experimentally observed interrelationship between the critical currents for
flux cutting and depinning, I calculate the forces on a helical vortex arc
stretched between two pinning centers when the vortex is subjected to a current
density of arbitrary angle \phi. Simultaneous initiation of flux cutting and
flux transport occurs at the critical current density J_c(\phi) that makes the
vortex arc unstable.Comment: 14 pages, 13 figure
The Bean model of the critical state in a magnetically shielded superconductor filament
We study the magnetization of a cylindrical type-II superconductor filament
covered by a coaxial soft-magnet sheath and exposed to an applied transverse
magnetic field. Examining penetration of magnetic flux into the superconductor
core of the filament on the basis of the Bean model of the critical state, we
find that the presence of a non-hysteretic magnetic sheath can strongly enhance
the field of full penetration of magnetic flux. The average magnetization of
the superconductor/magnet heterostructure under consideration and hysteresis AC
losses in the core of the filament are calculated as well.Comment: 4 pages, 3 figures; Proceedings of the 7th European Conference on
Applied Superconductivity, Vienna, Austria, September 11-15, 200
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