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Magnetic lens, exploiting the induced screening current, may concentrate the spatial magnetic flux. This concept has been realized by several research groups using GdBCO and/or MgB₂ bulk superconductors. The limitation of the magnitude of concentrated flux density lies on the mechanical brittleness of the materials and the flux instability. High-temperature superconducting (HTS) tape possesses excellent mechanical and flux pinning properties and hence is a good candidate for magnetic lens. In this study, we implemented numerical simulations on the design of magnetic lenses using HTS stacked tapes. The models were constructed based on H-ϕ formulations. We investigated and compared the concentration effect of various magnetic lenses with different topologies. The results show that a central field of 22.69 T and 25.62 T can be achieved respectively with rectangular-shaped stacks and X-shaped stacks in an applied magnetic field of 20 T. An optimized design of the magnetic lens has been proposed and correspondingly the mechanism for a better concentration-effect has been explained which provides a good reference for future experiments and applications.

Funder: University of Cambridge; doi: http://dx.doi.org/10.13039/501100000735, Bulk superconductors can act as trapped-field magnets with the potential to be used for many applications such as portable medical magnet systems and rotating machines. Maximising the trapped field, particularly for practical magnetisation techniques such as pulsed field magnetisation (PFM), still remains a challenge. PFM is a dynamic process in which the magnetic field is driven into a superconducting bulk over milliseconds. This flux motion causes heating and a complex interplay between the magnetic and thermal properties. In this work, the local flux density during PFM in a MgB2 bulk superconductor has been studied. We find that improving the cooling architecture increases the flux trapping capabilities and alters the flux motion during PFM. These improvements lead to the largest trapped field (0.95 T) for a single MgB2 bulk sample magnetised by a solenoidal pulsed field magnet. The findings illustrate the fundamental role bulk cooling plays during PFM.

The concept of a hybrid trapped field magnet lens (HTFML) was recently proposed by the authors, which consists of a trapped field magnet (TFM) cylinder exploiting the “vortex pinning effect,” combined with a superconducting bulk magnetic lens exploiting the “diamagnetic shielding effect.” This HTFML can generate, within its bore, a magnetic field higher than the applied magnetic field, even after external field decreases to zero. In this paper, a design optimization of the inner GdBaCuO magnetic lens within the GdBaCuO TFM cylinder was carried out using numerical simulations based on the finite-element method, in order to maximize the concentrated magnetic field. The HTFML with an optimized shape and size achieved a concentrated magnetic field of B c = 5.6 and 12.8 T at the center of the lens for applied magnetic fields of B app = 3 and 10 T, respectively. A maximum tensile stress of +135 MPa exists in the outer GdBaCuO TFM cylinder during the magnetizing process for B app = 10 T, which exceeds the fracture strength of the bulk. This result suggests that mechanical reinforcement is necessary to avoid mechanical fracture under such high magnetic field conditions.

A robust and reliable in-situ magnetization method is essential for exploiting the outstanding magnetic flux trapping ability of bulk superconductors in practical applications. We report a 4.8 T peak trapped magnetic field, B T, achieved at 30 K in a 36 mm diameter GdBa2Cu3O7-δ –Ag bulk superconductor using pulsed field magnetization (PFM). To realize this, we have developed a reliable two-step multi-pulse PFM process based on understanding and exploiting the avalanche-like flux jump phenomenon observed in these materials. The magnitude of the applied pulsed magnetic field (B a) necessary to trap 4.8 T was merely 5.29 T, corresponding to a remarkable magnetization efficiency (B T/B a) of 90%.

Bulk (RE)BCO, where RE is a rare-earth element or yttrium, superconductors fabricated in the form of rings are potentially useful for a variety of solenoidal-type applications, such as small, high field nuclear magnetic resonance and electromagnetic undulators. It is anticipated that the practical exploitation of these technologically important materials will involve pulse field magnetization (PFM) and, consequently, it is important to understand the behavior of ring-shaped samples subjected to the PFM process. Macroscopic flux jumps were observed in PFM experiments on ring-shaped bulk samples when the peak applied field reaches a threshold magnitude, similar to behavior reported previously in cylindrical samples. Magnetic flux jumps inward when the thermal instability is triggered, however it subsequently flows outwards from the sample, resulting in a relatively low trapped field. This behavior is attributed to a variety of effects, including the inhomogeneity of the material, which may lead to the formation of localized hot spots during the PFM process. In order to further elucidate this phenomena, the properties of a structure consisting of a bulk superconducting ring with a cylindrical superconductor core were studied. We observe that, although a flux jump occurs consistently in the ring, a critical state is established at the boundary of the ring-shaped sample and the core. We provide a detailed account of these experimental observations and provide an explanation in terms of the current understanding of the PFM process.

GdBa 2 Cu 3 O 7-δ bulk superconductors are a route to higher magnetic fields in rotating machines. Here, we examine the resistance of pulse-field magnetized bulks to the demagnetization fields they may experience in such a system. The bulks were magnetized at 77 K, after which several thousand cycles of AC field were applied. Subsequently, the decay of the trapped field was characterized. We found that the decay per cycle decreases with frequency and is, normalized to the initial trapped field, largest at the edge of the bulk. At 77 K, the reduction in trapped field proved significant (25% in the center for 150 mT (peak) AC field at 6 Hz), however, reducing below 1% when lowering the temperature to 60 K. We explain this observation as being due to increased flux pinning strength at low temperatures. When applying an AC field, we found a temperature rise that increased with applied field amplitude and frequency. However, when applying an AC field of amplitude 45 mT with a frequency of 48 Hz, we found an increase of the bulk temperature of only 100 mK. Therefore, we conclude that the temperature rise within the analyzed AC field frequency and amplitude range does not contribute significantly to the decay of trapped field.

Progress in bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both the materials science of bulk superconducting materials and the technologies required to use them effectively in engineering applications. In particular magnetised bulk superconductors are capable of acting as quasi-permanent magnets with the potential of providing magnetic fields of several tesla or greater from a small volume of material, they can act as magnetic shields and they can provide self-stabilised levitation. This Roadmap, based on a workshop which involved the participation of a wide range of academic and industrial participants [see doi:10.17863/CAM.586 for details of the workshop methodology], aims to explore some of the key potential domains of applications of bulk superconductors. Detailed technological roadmaps are presented for four key applications that were identified as providing both good market opportunity and feasibility and selected for further exploration during the workshop. These are: Portable Systems for Bulk Superconductivity, Portable, High-Field Magnet Systems for Medical Devices, Ultra-Light Superconducting Rotating Machines for Next-Generation Transport & Power Applications and Magnetic Shielding Applications for Electric Machines, Equipment and Other High-Field Devices

Large, single grain (RE)Ba2Cu3O7 (where RE is a rare-earth element or yttrium) high temperature superconductors (HTS) are technologically important materials due to their ability to trap large magnetic fields and to provide stable magnetic levitation for a number of potential high field applications. The fabrication of samples in the form of large singe grain is a challenge, however, due to the characteristic slow growth rate of these materials and the need to produce samples that are electrically well-connected in order to generate trapped magnetic fields that are significantly greater than those produced by conventional permanent magnets (PM). In this work, we investigate whether large, single grain samples are optimum for the generation of high levitation forces for engineering applications. Three large bar-shaped Y-Ba-Cu-O (YBCO) samples of dimensions 60 x 20 x 12 mm3 were prepared for this investigation, including one single-seeded, one multi-seeded and one consisting of three square samples packed together closely in an array. The processing of these samples is described and their trapped field and levitation performance at 77 K measured using different permanent magnet arrays. We find that the multi-seeded samples and those assembled from smaller, individual bulks are able to achieve a higher levitation force than an equivalent single seed sample arrangement, at least in some geometries. This result is significant in that it suggests clearly that it is not always necessary to fabricate bulk (RE)BCO superconductors in the form of very large single grains for levitation applications, although the specific configuration of the system does need to be considered on a case-by-case basis.

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ~ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-{\delta} bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique., Comment: Accepted revised version for Superconductor Science and Technology

In order to determine the spatial distribution of the flexural strength, three-point bend tests have been performed on bar specimens cut from four YBa2Cu3O7-δ single-grain bulk superconductors. A relatively large spread of the room temperature flexural strength, average and standard deviation of 49.3 MPa and 12.7 MPa respectively, was observed across the four bulk samples. This is attributed to the systemic microstructural variation introduced by the seeded melt growth. In particular, the strength was shown to be related to the local porosity and Y2BaCuO5 content. To further examine bulk-to-bulk variability, indirect tensile (Brazilian) tests were conducted on twelve 16 mm diameter bulk superconductors from three production batches. The Weibull modulus for these was calculated to be 8.76, suggesting that, despite the large spread in strength with position within a bulk, the batch processing of bulk superconductors can consistently yield samples with comparable overall mechanical strengths.

Magnetisation is one of the main barriers to practical use of bulk superconductors as high field magnets. Recently several authors have reported a flux jump effect that allows penetration of magnetic flux into a bulk superconductor during pulsed field magnetisation (PFM) at lower fields than that would be predicted on the basis of the Bean model. We have systematically investigated macroscopic flux jumps in single grain GdBa2Cu3O7-δ-Ag (GdBCO-Ag) bulk superconductors with diameters of up to 30 mm when subjected to pulsed magnetic fields. Flux jumps were observed at temperatures between 30 K and 77 K and in applied magnetic fields of up to 7 T. The applied pulsed field required to trigger the instability or flux jump field, Bj, was determined experimentally and found to increase with decreasing temperature. An extended instability criterion based on a 2D axisymmetric model was used to predict Bj at various temperatures and the results are in good agreement with experiments. A significant temperature rise has been measured experimentally during the magnetisation process which indicates that local heat generation due to the sharp rise of the applied field in the PFM process is the primary cause of the flux jumps. The experimental results suggest further that the critical current density reduces to almost zero in the warm part of the sample during the short period of non-equilibrium. A peak trapped field of 4.1 T at the surface and 5.3 T between a stack of two GdBCO-Ag bulk superconductors was achieved at 30 K by means of an optimized two- step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.

Pulsed field magnetisation (PFM) is the most practical method for magnetising bulk superconducting materials as trapped field magnets (TFMs), but the record trapped field achieved by PFM to date is still significantly less than the true trapped field capability of these materials. In this paper, a flexible numerical modelling technique based on the finite element method is used to provide a comprehensive and realistic picture of multi-pulse PFM, which has been shown to be effective in increasing the trapped field/flux over a single pulse. Firstly, the maximum trapped field capability of a representative sample is determined using two types of numerical model simulating field-cooling (FC) and zero-field-cooling (ZFC) magnetisation. Next, various sets of magnetic field pulses are applied to the bulk to analyse multi-pulse PFM. An increase in the trapped field can be achieved after a 2nd pulse and to do so an increased amplitude of applied field is required to maximize the trapped field fully. The numerical analysis shows that this occurs in subsequent pulses because it is more difficult for the magnetic flux to penetrate the sample and there is a lower temperature rise.