Laurent Wera

Influence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y-Ba-Cu-O superconductor

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this paper, soft ferromagnetic sections are combined with a bulk, large grain Y–Ba–Cu–O high temperature superconductor to form superconductor/ferromagnet hybrid structures. We study how the ferromagnetic sections influence the shape of the profile of the trapped magnetic induction at the surface of each structure and report the surface magnetic flux density measured by Hall probe mapping. These configurations have been modelled using a 2D axisymmetric finite element method based on the H -formulation and the results show excellent qualitative and quantitative agreement with the experimental measurements. The model has also been used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and geometries. The results show that the ferromagnetic material acts as a magnetic shield, but the flux density and its gradient are enhanced on the face opposite to the ferromagnet. The thickness and saturation magnetization of the ferromagnetic material are important and a characteristic ferromagnet thickness d* is derived: below d*, saturation of the ferromagnet occurs, and above d*, a weak thickness-dependence is observed. The influence of the ferromagnet is observed even if its saturation magnetization is lower than the trapped flux density of the superconductor. Conversely, thin ferromagnetic discs can be driven to full saturation even though the outer magnetic field is much smaller than their saturation magnetization.

Magnetic Shielding With YBCO Coated Conductors: Influence of the Geometry on Its Performances

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic shields made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in detail the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic shield is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly at a fixed sweep rate. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the applied magnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside the shield) is improved for increasing aspect ratios of the global coated conductor assembly and that the threshold magnetic induction (defined for SF = 10) increases with the number of layers. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized.

Magnetic Shielding Above 1 T at 20 K With Bulk, Large Grain YBCO Tubes Made by Buffer-Aided Top Seeded Melt Growth

YBCO tubes of ~10 mm diameter closed at one extremity were engineered by a buffer-aided top seeded melt growth fabrication process. These tubes can act as efficient “dc” magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding “dc” stray fields generated by high temperature superconductor magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K), or liquid neon (27 K).

A comparative study of triaxial and uniaxial magnetic shields made out of YBCO coated conductors

Persistent current loops of arbitrary size can be made from currently manufactured RE123 coated conductors. Our previous work has shown that an assembly of such loops is able to effectively shield quasi static axial magnetic field due to the absence of a resistive joint. The shielding effectiveness depends on the aspect ratio and the number of layers. In the present work we study experimentally the detailed magnetic response of two different configurations of the magnetic shields for various orientations of the applied field. Using a three-axis Hall probe we determine the amplitude of magnetic field generated by the induced persistent currents and its direction with respect to the applied field. The effectiveness of the uniaxial shield decreases strongly when the applied field is not collinear with its axis. In the triaxial (Polywell type) structure comprised of three pairs of coils whose axes are mutually orthogonal, the field attenuation is shown to be only weakly dependent on the direction of the magnetic field. We discuss the properties of the triaxial shield and the ways to improve its screening performance.

Magnetic shielding of an inhomogeneous magnetic field source by a bulk superconducting tube

Bulk type-II irreversible superconductors can act as excellent passive magnetic shields, with a strong attenuation of low frequency magnetic fields. Up to now, the performances of superconducting magnetic shields have mainly been studied in a homogenous magnetic field, considering only immunity problems, i.e. when the field is applied outside the tube and the inner field should ideally be zero. In this paper, we aim to investigate experimentally and numerically the magnetic response of a high-Tc bulk superconducting hollow cylinder at 77 K in an emission problem, i.e. when subjected to the non-uniform magnetic field generated by a source coil placed inside the tube. A bespoke 3D mapping system coupled with a three-axis Hall probe is used to measure the magnetic flux density distribution outside the superconducting magnetic shield. A finite element model is developed to understand how the magnetic field penetrates into the superconductor and how the induced superconducting shielding currents flow inside the shield in the case where the emitting coil is placed coaxially inside the tube. The finite element modelling is found to be in excellent agreement with the experimental data. Results show that a concentration of the magnetic flux lines occurs between the emitting coil and the superconducting screen. This effect is observed both with the modelling and the experiment. In the case of a long tube, we show that the main features of the field penetration in the superconducting walls can be reproduced with a simple analytical 1D model. This model is used to estimate the maximum flux density of the emitting coil that can be shielded by the superconductor.

A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm(3)) at cryogenic temperature. The signal is constructed by integrating the electromotive force generated by two coils wound in series-opposition that move around the sample. We show that an octupole expansion of the magnetic vector potential can be used conveniently to treat near-field effects for this geometrical configuration. The resulting expansion is tested for the case of a large, permanently magnetized, type-II superconducting sample. The dimensions of the sensing coils are determined in such a way that the measurement is influenced by the dipole magnetic moment of the sample and not by moments of higher order, within user-determined upper bounds. The device, which is able to measure magnetic moments in excess of 1 A m(2) (1000 emu), is validated by (i) a direct calibration experiment using a small coil driven by a known current and (ii) by comparison with the results of numerical calculations obtained previously using a flux measurement technique. The sensitivity of the device is demonstrated by the measurement of flux-creep relaxation of the magnetization in a large bulk superconductor sample at liquid nitrogen temperature (77 K).

Magnetic shielding performances of YBa2Cu3O7−δ -coated silver tubes obtained by electrophoretic deposition

We report a complete procedure to achieve multilayer YBa2Cu3O7−δ (YBCO) thick films by electrophoretic deposition on silver tubes using a suspension of YBCO powder in butanol. With the aim to optimize the magnetic shielding performances of the coatings, we have carried out an extensive investigation of the influence of the deposition parameters, the multilayer deposition sequence and the intermediate/final heat treatments on the coating microstructure. Using the optimized conditions, a 24-layer YBCO coating has been successfully prepared on an 80 mm long Ag tube: the melt growth processed multilayered YBCO thick film thus obtained can shield an applied magnetic field of 1.9 mT at 77 K, the highest value per thickness unit reported so far in the literature for these materials.