8 research outputs found
Modification of the trapped field in bulk high-temperature superconductors as a result of the drilling of a pattern of artificial columnar holes
The trapped magnetic field is examined in bulk high-temperature
superconductors that are artificially drilled along their c-axis. The influence
of the hole pattern on the magnetization is studied and compared by means of
numerical models and Hall probe mapping techniques. To this aim, we consider
two bulk YBCO samples with a rectangular cross-section that are drilled each by
six holes arranged either on a rectangular lattice (sample I) or on a centered
rectangular lattice (sample II). For the numerical analysis, three different
models are considered for calculating the trapped flux: (i), a two-dimensional
(2D) Bean model neglecting demagnetizing effects and flux creep, (ii), a 2D
finite-element model neglecting demagnetizing effects but incorporating
magnetic relaxation in the form of an E-J power law, and, (iii), a 3D finite
element analysis that takes into account both the finite height of the sample
and flux creep effects. For the experimental analysis, the trapped magnetic
flux density is measured above the sample surface by Hall probe mapping
performed before and after the drilling process. The maximum trapped flux
density in the drilled samples is found to be smaller than that in the plain
samples. The smallest magnetization drop is found for sample II, with the
centered rectangular lattice. This result is confirmed by the numerical models.
In each sample, the relative drops that are calculated independently with the
three different models are in good agreement. As observed experimentally, the
magnetization drop calculated in the sample II is the smallest one and its
relative value is comparable to the measured one. By contrast, the measured
magnetization drop in sample (1) is much larger than that predicted by the
simulations, most likely because of a change of the microstructure during the
drilling process.Comment: Proceedings of EUCAS 09 conferenc
Numerical simulation of the magnetization of high-temperature superconductors: 3D finite element method using a single time-step iteration
We make progress towards a 3D finite-element model for the magnetization of a
high temperature superconductor (HTS): We suggest a method that takes into
account demagnetisation effects and flux creep, while it neglects the effects
associated with currents that are not perpendicular to the local magnetic
induction. We consider samples that are subjected to a uniform magnetic field
varying linearly with time. Their magnetization is calculated by means of a
weak formulation in the magnetostatic approximation of the Maxwell equations
(A-phi formulation). An implicit method is used for the temporal resolution
(Backward Euler scheme) and is solved in the open source solver GetDP. Picard
iterations are used to deal with the power law conductivity of HTS. The finite
element formulation is validated for an HTS tube with large pinning strength
through the comparison with results obtained with other well-established
methods. We show that carrying the calculations with a single time-step (as
opposed to many small time-steps) produce results with excellent accuracy in a
drastically reduced simulation time. The numerical method is extended to the
study of the trapped magnetization of cylinders that are drilled with different
arrays of columnar holes arranged parallel to the cylinder axis
Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
We present a method for characterizing the propagation of the magnetic flux
in an artificially drilled bulk high-temperature superconductor (HTS) during a
pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical
sample, the magnetic flux density is measured simultaneously in 16 holes by
means of microcoils that are placed across the median plane, i.e. at an equal
distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes
during a pulse and measure the time taken by the external magnetic flux to
reach each hole. Our data show that the flux front moves faster in the median
plane than on the surface when penetrating the sample edge; it then proceeds
faster along the surface than in the bulk as it penetrates the sample further.
Once the pulse is over, the trapped flux density inside the central hole is
found to be about twice as large in the median plane than on the surface. This
ratio is confirmed by modelling
Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux ?
Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing
and the heat exchange with the cooling liquid. However, drilling holes also
reduces the amount of magnetic flux that can be trapped in the sample. In this
paper, we use the Bean model to study the magnetization and the current line
distribution in drilled samples, as a function of the hole positions. A single
hole perturbs the critical current flow over an extended region that is bounded
by a discontinuity line, where the direction of the current density changes
abruptly. We demonstrate that the trapped magnetic flux is maximized if the
center of each hole is positioned on one of the discontinuity lines produced by
the neighbouring holes. For a cylindrical sample, we construct a polar
triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ~20% higher than in a squared lattice, for which the holes do
not lie on discontinuity lines. This result indicates that one can
simultaneously enhance the oxygen annealing, the heat transfer, and maximize
the trapped field
Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor
We use macroscopic holes drilled in a bulk YBCO superconductor to probe its
magnetic properties in the volume of the sample. The sample is subjected to an
AC magnetic flux with a density ranging from 30mT to 130mT and the flux in the
superconductor is probed by miniature coils inserted in the holes. In a given
hole, three different penetration regimes can be observed: (i) the shielded
regime, where no magnetic flux threads the hole; (ii) the gradual penetration
regime, where the waveform of the magnetic field has a clipped sine shape whose
fundamental component scales with the applied field; and (iii) the flux
concentration regime, where the waveform of the magnetic field is nearly a sine
wave, with an amplitude exceeding that of the applied field by up to a factor
of two. The distribution of the penetration regimes in the holes is compared
with that of the magnetic flux density at the top and bottom surfaces of the
sample, and is interpreted with the help of optical polarized light micrographs
of these surfaces. We show that the measurement of the magnetic field inside
the holes can be used as a local characterization of the bulk magnetic
properties of the sample