166 research outputs found
Superconductor Dynamics
Superconductors used in magnet technology could carry extreme currents
because of their ability to keep the magnetic flux motionless. The dynamics of
the magnetic flux interaction with superconductors is controlled by this
property. The cases of electrical transport in a round wire and the
magnetization of wires of various shapes (circular, elliptical, plate) in an
external magnetic field are analysed. Resistance to the magnetic field
penetration means that the field produced by the superconducting magnet is no
longer proportional to the supplied current. It also leads to a dissipation of
electromagnetic energy. In conductors with unequal transverse dimensions, such
as flat cables, the orientation with respect to the magnetic field plays an
essential role. A reduction of magnetization currents can be achieved by
splitting the core of a superconducting wire into fine filaments; however, new
kinds of electrical currents that couple the filaments consequently appear.
Basic formulas allowing qualitative analyses of various flux dynamic cases are
presented.Comment: 19 pages, contribution to the CAS-CERN Accelerator School:
Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013,
edited by R. Baile
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 flux penetration and AC loss in a composite superconducting wire with ferromagnetic parts
The current distribution and the AC loss in a composite superconducting tape
containing a layer from magnetic material is calculated and compared with
experiments, showing a very good agreement. The situations of an alternating
uniform applied field or a transport current are studied. The newly developed
numerical model is an approximation to the critical state model, adapted for
the applicability to commercial finite elements codes that solve the vector
potential. Substantial feature of this procedure is that it can be carried out
in the case when the critical current density in superconductor depends on the
magnetic field and the magnetic layer material is non- linear. Additionally,
the hysteresis loss in the magnetic material is estimated, based on its
measured magnetization loops. Measurements on Bi-2223 multifilamentary tapes
covered on edges by nickel confirmed our predictions, showing a substantial ac
loss reduction in both the investigated regimes.Comment: 33 pages, 12 figure
Electromagnetic response of LaO_0.94F_0.06FeAs: AC susceptibility and microwave surface resistance
We discuss on the electromagnetic response of a polycrystalline sample of
LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and
high-frequency responses have been investigated by measuring the AC
susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At
low as well as high DC magnetic fields, the susceptibility strongly depends on
the amplitude of the AC driving field, highlighting enhanced nonlinear effects.
The field dependence of the AC susceptibility exhibits a magnetic hysteresis
that can be justified considering the intragrain-field-penetration effects on
the intergrain critical current density. The microwave surface resistance
exhibits a clockwise magnetic hysteresis, which cannot be justified in the
framework of the critical-state models of the Abrikosov-fluxon lattice; it may
have the same origin as that detected in the susceptibility.Comment: 8 pages, 4 embedded eps figures; Proceedings of the 9th EUCAS
Conference (Dresden, Germany, September 13-17, 2009
Theoretical and experimental study of AC loss in HTS single pancake coils
The electromagnetic properties of a pancake coil in AC regime as a function
of the number of turns is studied theoretically and experimentally.
Specifically, the AC loss, the coil critical current and the voltage signal are
discussed. The coils are made of Bi2Sr2Ca2Cu3O10/Ag (BiSCCO) tape, although the
main qualitative results are also applicable to other kinds of superconducting
tapes, such as coated conductors. The AC loss and the voltage signal are
electrically measured using different pick up coils with the help of a
transformer. One of them avoids dealing with the huge coil inductance. Besides,
the critical current of the coils is experimentally determined by conventional
DC measurements. Furthermore, the critical current, the AC loss and the voltage
signal are simulated, showing a good agreement with the experiments. For all
simulations, the field dependent critical current density inferred from DC
measurements on a short tape sample is taken into account.Comment: 22 pages, 15 figures; contents extended (sections 3.2 and 4); one new
figure (figure 5) and two figures replaced (figures 3 and 8); typos
corrected; title change
Evidence for a vortex-glass transition in superconducting Ba(FeCo)As
Measurements of magneto-resistivity and magnetic susceptibility were
performed on single crystals of superconducting
Ba(FeCo)As close to the conditions of optimal
doping. The high quality of the investigated samples allows us to reveal a
dynamic scaling behaviour associated with a vortex-glass phase transition in
the limit of weak degree of quenched disorder. Accordingly, the dissipative
component of the ac susceptibility is well reproduced within the framework of
Havriliak-Negami relaxation, assuming a critical power-law divergence for the
characteristic correlation time of the vortex dynamics. Remarkably, the
random disorder introduced by the FeCo chemical substitution is
found to act on the vortices as a much weaker quenched disorder than previously
reported for cuprate superconductors such as, e.g.,
YPrBaCuO.Comment: 10 pages, 8 figure
An AC susceptometer for the characterization of large, bulk superconducting samples
The main purpose of this work was to design, develop and construct a simple,
low-cost AC susceptometer to measure large, bulk superconducting samples (up to
32 mm in diameter) in the temperature range 78-120 K. The design incorporates a
double heating system that enables a high heating rate (25 K/hour) while
maintaining a small temperature gradient (< 0.2 K) across the sample. The
apparatus can be calibrated precisely using a copper coil connected in series
with the primary coil. The system has been used successfully to measure the
temperature dependence of the AC magnetic properties of entire RE-Ba-Cu-O
[(RE)BCO] bulk superconducting domains. A typical AC susceptibility measurement
run from 78 K to 95 K takes about 2 hours, with excellent temperature
resolution (temperature step ~ 4 mK) around the critical temperature, in
particular.Comment: 25 pages, 7 figures. Accepted for publication in Measurement Science
and Technolog
Vortex dynamics and pinning properties analysis of MgB2 bulk samples by ac susceptibility measurements
The flux lines dynamics have been investigated on MgB2 bulk superconductors
obtained by reactive liquid infiltration by measuring the ac magnetic
susceptibility. The fundamental and third harmonics have been studied as a
function of temperature, dc magnetic field, ac field amplitude and frequency.
In order to determine the dynamical regimes governing the vortex motion, the
experimental results have been compared with susceptibility curves obtained by
numerical calculations of the non-linear diffusion equation for the magnetic
field. The frequency behaviour of the third harmonic response, that cannot be
explained by frequency dependent critical state models, has been related to the
current dependence of the flux creep activation energy U(J) in the diffusion
coefficient. In this way we have shown that the measured curves are correctly
interpreted within the framework of a vortex glass description.Comment: 12 pages, 4 figures, Proceedings of Boromag worksho
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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