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.

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Pulsed field magnetization (PFM) of a high-J c MgB 2 bulk disk has been investigated at 20 K, in which flux jumps frequently occur for high pulsed fields. Using a numerical simulation of the PFM procedure, we estimated the time dependence of the local magnetic field and temperature during PFM. We analyzed the electromagnetic and thermal instability of the high-J c MgB 2 bulk to avoid flux jumps using the time dependence of the critical thickness, d c (t), which shows the upper safety thickness to stabilize the superconductor magnetically, and the minimum propagation zone (MPZ) length, l m (t), to obtain dynamical stability. The values of d c (t) and l m (t) change along the thermally-stabilized direction with increasing temperature below the critical temperature, T c. However, the flux jump can be qualitatively understood by the local temperature, T(t), which exceeds T c in the bulk. Finally, possible solutions to avoid flux jumps in high-J c MgB 2 bulks are discussed.

Large, single-grain (RE)BCO (where RE = rare earth or Y) bulk superconductors with complicated geometries are required for a variety of potential applications, such as rotating machines, magnetic bearings and magnetic separation. As a consequence, the top multiseeded melt growth process has been studied over many years in an attempt to deliver large, single grains for practical applications. Among these techniques, the so-called bridgeseeding produces the best alignment of two seeds during melt processing of multi-seed samples. In this paper, the trapped field performance and magnetic flux dynamics of two bridge-seeded, multi-seed samples magnetized by pulsed field magnetization (PFM) are analysed: one with a 45⁰-45⁰ and another with a 0⁰-0⁰ bridge seed. Based on an analysis of the flux penetration across the seeds and in-between the seeds of the 45⁰-45⁰ multi-seed sample, an estimated J c distribution over the ab-plane was determined, which provides the basis for further analysis via numerical simulation. A 3D finite-element model, developed to qualitatively reproduce and interpret the experimental results, was employed to investigate the influence of the length of the bridge seed for such multi-seed samples. The simulation results agree well with the observed experimental results, in that the multi-seed sample's particular inhomogeneous J c distribution acts to distort the trapped field profile from a traditional conical Bean's profile, which is determined by the length and direction of the bridge seed on the bulk surface.Keywords: bulk superconductors, multi-seeding, bridge-shaped seeds, trapped field magnets, pulsed field magnetization, numerical modelling, finite element method (FEM) modelling c c c J J J x y z J J J J    .

MgB 2 superconducting bulks have promising potential as trapped field magnets. We have achieved a trapped field of B z =1.1 T on a high-J c MgB 2 bulk at 13 K without flux jumps by pulsed field magnetization (PFM) using a split-type coil with a soft iron yoke, which is a recordhigh trapped field by PFM for bulk MgB 2 to date. The flux jumps, which frequently took place using a solenoid-type coil during PFM, were avoided by using the split-type coil, and the B z value was enhanced by the insertion of soft iron yoke. The flux dynamics and heat generation/ propagation were analyzed during PFM using a numerical simulation, in which the magnetic flux intruded and attenuated slowly in the bulk and tended to align along the axial direction due to the presence of soft iron yoke. The advantages of the split-type coil and the simultaneous use of a soft iron yoke are discussed.