Direct J c measurements and trapped field profiles using an identical superconducting bulk

2015

182

166

This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE = rare earth or Y) and MgB 2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialisation stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarised with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet-superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets (TFMs). Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

Section: Understanding Flux Dynamicsmentioning

confidence: 99%

Modelling of bulk superconductor magnetization

2015

182

166

“…where I 0 is the peak magnitude of the current flowing in each turn of the coil, N is the number turns, and τ is the rise time of the pulse (see Table 2). Table 2 also summarises the other assumed model parameters, including the thermal conductivity of the indium sheet, which is set to 0.5 W/mK to represent the finite cooling power of the refrigerator and the thermal contact between the bulk and the cold stage, as described in [41]. The rise time of the applied current, and hence resultant magnetic field, is the same value ( = 15 ms) for all models in order to adequately compare pulsed fields of equivalent duration, magnitude and shape.…”

Section: Modelling Framework and Assumptionsmentioning

confidence: 99%

Enhanced trapped field performance of bulk high-temperature superconductors using split coil, pulsed field magnetization with an iron yoke

Mochizuki

et al. 2016

111

Investigating and predicting the magnetization of bulk superconducting materials and developing practical magnetizing techniques is crucial to using them as trapped field magnets (TFMs) in engineering applications. The pulsed field magnetization (PFM) technique is considered to be a compact, mobile and relative inexpensive way to magnetize bulk samples, requiring shorter magnetization times (on the order of milliseconds) and a smaller and less complicated magnetization fixture; however, the trapped field produced by PFM is generally much smaller than that of slower zero field cooling (ZFC) or field cooling (FC) techniques, particularly at lower operating temperatures. In this paper, the PFM of two, standard Ag-containing Gd-Ba-Cu-O samples is carried out using two types of magnetizing coils: 1) a solenoid coil, and 2) a split coil, both of which make use of an iron yoke to enhance the trapped magnetic field. It is shown that a significantly higher trapped field can be achieved using a split coil with an iron yoke, and in order to explain these how this arrangement works in detail, numerical simulations using a 2D axisymmetric finite element method based on the H-formulation are carried to qualitatively reproduce and analyse the magnetization process from both electromagnetic and thermal points of view. It is observed that after the pulse peak significantly less flux exits the bulk when the iron core is present, resulting in a higher peak trapped field, as well as more overall trapped flux, after the magnetization process is complete. The results have important implications for practical applications of bulk superconductors as such a split coil arrangement with an iron yoke could be incorporated into the design of a portable, high magnetic field source/magnet to enhance the available magnetic field or in an axial gap-type bulk superconducting electric machine, where iron can be incorporated into the stator windings to 1) improve the trapped field from the magnetization process, and 2) increase the effective air-gap magnetic field.

“…The field required to trap the maximum possible field with the FC method, where the superconductor is cooled below T c in the presence of the external field, is half of the field required for ZFC. The PFM technique can be considered a type of ZFC, but the duration of the applied magnetic pulse is very short, on the order of milliseconds [26]. Compared with ZFC and FC techniques, the PFM technique provides a compact, mobile and relative inexpensive way to magnetize the bulk samples.…”

Section: Numerical Simulation Of Field Cooling (Fc) Magnetizationmentioning

confidence: 99%

Numerical modelling and comparison of MgB₂bulks fabricated by HIP and infiltration growth

Zeng

et al. 2015

MgB 2 in bulk form shows great promise as trapped field magnets (TFMs) as an alternative to bulk (RE)BCO materials to replace permanent magnets in applications such as rotating machines, magnetic bearings and magnetic separation, and the relative ease of fabrication of MgB 2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB 2 samples fabricated by the Hot Isostatic Pressing (HIP), with and without Ti-doping, and Infiltration Growth (IG) methods and the highest trapped field in an IG-processed bulk MgB 2 sample, B z = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB 2 has a more homogeneous J c distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and J c distribution can be made, and a numerical simulation technique based on the 2D axisymmetric H-formulation is introduced to model the complete process of field cooling (FC) magnetization. As input data for the model, the measured J c (B,T) characteristics of a single, small specimen taken from each bulk sample are used, in addition to measured specific heat and thermal conductivity data for the materials. The results of the simulation reproduce the experimental results extremely well: (1) indicating the samples have excellent homogeneity, and (2) validating the numerical model as a fast, accurate and powerful tool to investigate the trapped field profile of bulk MgB 2 discs of any size accurately, under any specific operating conditions. Finally, the paper is concluded with a numerical analysis of the influence of the dimensions of the bulk sample on the trapped field.