ORCID Profile
0000-0003-0466-3680
Current Organisation
King's College London
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Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2010
Publisher: IOP Publishing
Date: 06-07-2020
Publisher: IOP Publishing
Date: 28-08-2018
Publisher: IOP Publishing
Date: 06-07-2021
Publisher: IOP Publishing
Date: 22-10-2018
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: IOP Publishing
Date: 14-10-2015
Publisher: IOP Publishing
Date: 25-06-2014
Publisher: IOP Publishing
Date: 31-08-2020
Abstract: The high- T c superconducting (HTS) dynamo is a promising device that can inject large DC supercurrents into a closed superconducting circuit. This is particularly attractive to energise HTS coils in NMR/MRI magnets and superconducting rotating machines without the need for connection to a power supply via current leads. It is only very recently that quantitatively accurate, predictive models have been developed which are capable of analysing HTS dynamos and explain their underlying physical mechanism. In this work, we propose to use the HTS dynamo as a new benchmark problem for the HTS modelling community. The benchmark geometry consists of a permanent magnet rotating past a stationary HTS coated-conductor wire in the open-circuit configuration, assuming for simplicity the 2D (infinitely long) case. Despite this geometric simplicity the solution is complex, comprising time-varying spatially-inhomogeneous currents and fields throughout the superconducting volume. In this work, this benchmark problem has been implemented using several different methods, including H -formulation-based methods, coupled H - A and T - A formulations, the Minimum Electromagnetic Entropy Production method, and integral equation and volume integral equation-based equivalent circuit methods. Each of these approaches show excellent qualitative and quantitative agreement for the open-circuit equivalent instantaneous voltage and the cumulative time-averaged equivalent voltage, as well as the current density and electric field distributions within the HTS wire at key positions during the magnet transit. Finally, a critical analysis and comparison of each of the modelling frameworks is presented, based on the following key metrics: number of mesh elements in the HTS wire, total number of mesh elements in the model, number of degrees of freedom, tolerance settings and the approximate time taken per cycle for each model. This benchmark and the results contained herein provide researchers with a suitable framework to validate, compare and optimise their own methods for modelling the HTS dynamo.
Publisher: IOP Publishing
Date: 14-09-2023
Publisher: IOP Publishing
Date: 17-02-2020
Abstract: We have investigated the electromagnetic and thermal properties of a REBaCuO ring bulk with an inhomogeneous critical current density, J c , profile during pulsed field magnetization (PFM) using a numerical simulation and compared those to a bulk with a homogeneous J c profile. A notch was introduced in the bulk periphery, which was assumed as a crack existing in the actual bulk material. A sudden flux penetration (flux jump) took place through the notch area and as a result, a large temperature rise also took place around this notch. Consequently, the final trapped field profile was simulated to be a ‘C-shaped profile’, which qualitatively reproduced our previous experimental results. The size and position dependences of the notch on the flux penetration behaviour were also simulated, in which a larger and outer notch promotes the flux jump phenomenon easily. On the other hand, in the homogeneous model, under the same conditions, no flux jump phenomenon was observed. These results suggest that the imperfection in the bulk can be a possible starting point of the flux jump. The electromagnetic and thermal hoop stresses were also simulated in the ring bulk during PFM, in which the electromagnetic stress and the thermal stress were both observed to be lower than the fracture strength of the bulk material. This provides good evidence that the experimentally observed ‘C-shaped profile’ results from the flux jump rather than the fracture of the bulk.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2018
Publisher: IOP Publishing
Date: 24-04-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2011
Publisher: IOP Publishing
Date: 10-12-2019
Abstract: The flux-pinning landscape in type-II superconductors determines the response of the flux line lattice to changing magnetic fields. Typically, the flux vortex behaviour is hysteretic and well described within the framework of the Bean critical-state model and its extensions. However, if the changing magnetic field does not move the flux vortices from their pinning sites, their response remains linear and reversible. The vortex displacement, then, is characterised by the C bell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the C bell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBa 2 Cu 3 O 7 − δ superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.
Publisher: IOP Publishing
Date: 15-03-2023
Abstract: A 10 mm-period, high-temperature superconducting (HTS) undulator consisting of 20 staggered-array GdBa 2 Cu 3 O 7− x (GdBCO) bulk superconductors has been fabricated and tested successfully. Each GdBCO disk was machined into a half-moon shape with micro-meter accuracy and shrink-fitted into a slotted oxygen-free copper disk which provided pre-stress and effective conduction-cooling. The HTS undulator prototype, consisting of GdBCO disks, copper disks, and CoFe poles fitted in a long copper shell, was field-cooled magnetized in fields of up to 10 T at 10 K. An undulator field of 2.1 T in a 4 mm magnetic gap was obtained. This field is the largest reported yet for the same gap and period length and exceeds the target value of 2 T for the meter-long HTS undulator scheduled for the hard x-ray I-TOMCAT beamline in the Swiss Light Source 2.0. We have demonstrated that bulk superconductor based undulators can provide significantly improved performance over alternative technologies.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: IOP Publishing
Date: 23-06-2021
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2022
Publisher: IOP Publishing
Date: 13-07-2023
Abstract: It is necessary to fabricate (RE)BCO bulk high temperature superconductors in the form of in idual single grains in order to maximise the length scale over which current flows, and hence the trapped magnetic field. However, inherent difficulties in the grain growth process place limitations on the diameter and height of the single grain that may be achieved by existing melt processes. A practical approach to increase the height of the s le and the trapped field at its surface is by assembling in idual single grains in a stack formation with their ab planes aligned parallel, primarily to avoid the expensive process of fabricating large, in idual monoliths. The trapped fields observed at the top and bottom surfaces of a single grain s le are frequently different since both the superconducting and physical properties of single grain (RE)BCO s les are generally non-uniform. This leads to challenges in determining how to spatially arrange stacks of single grain s les to generate the largest and most uniform trapped field overall. In this study, we report the optimisation of two-stack configurations involving a total of 8 in idual GdBCO/Ag single grains. The s les were arranged in four pairs and configured with different surfaces in contact in the assembly of the stack. The primary superconducting properties for trapped field and total flux distributions were measured at 77 K and compared for each stack arrangement. The initial results indicate that surfaces with inferior flux trapping properties (measured in terms of the overall trapped field value) of a two-s le stack should be positioned at the middle of the assembly to achieve the best overall trapped field and higher total flux at the external, and therefore, usable surface of the stack s le. A numerical modelling method that incorporates different J c -B characteristics for the top and bottom layers of a single grain to take account of the variability in physical properties and spatial non-uniformity confirmed the optimised experimental arrangement of the stacked bulk s les. Furthermore, the optimisation of single grains of ring geometry to achieve a longer and wider uniform magnetic field zone inside the bore was also performed.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2022
Publisher: ANU Press
Date: 2006
Publisher: IOP Publishing
Date: 29-01-2021
Abstract: 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 GdBa 2 Cu 3 O 7- 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%.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2013
Publisher: Elsevier BV
Date: 09-2023
Publisher: IOP Publishing
Date: 30-01-2017
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: IOP Publishing
Date: 04-10-2021
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: IOP Publishing
Date: 12-02-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2020
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: IOP Publishing
Date: 04-04-2022
Abstract: The concept of a high-gradient trapped field magnet (HG-TFM), which incorporates a hybrid system of two (RE)BaCuO superconducting bulk components with different functions, was proposed in 2021 by the authors based on the results of numerical simulations. The HG-TFM as a desktop-type magnet can be a more effective way to generate a higher magnetic field gradient product of B z · d B z /d z ( −1400 T 2 m −1 , as calculated for a pure water), which can realize a quasi-microgravity space applicable for Space Environment Utilization on a laboratory scale. In this study, to validate the quasi-microgravity space in the HG-TFM, a prototype HG-TFM apparatus has been built using a slit-bulk TFM and stacked full-TFM (without slits) with inner diameters of 36 mm. After field-cooled magnetization from 8.60 T at 21 K, a trapped field of B T = 8.57 T was achieved at the center (i.e. at the bottom of a room temperature bore of 25 mm diameter outside the vacuum chamber), and consequently, a maximum B z · d B z /d z = −1930 T 2 m −1 was obtained at the intermediate position between the slit-bulk TFM and the stacked full-TFM. Magnetic levitation was demonstrated successfully for bismuth particles and a pure water drop, which validates the quasi-microgravity environment in the HG-TFM. Based on numerical simulation results of the trapped field profile, it is concluded that the reason for the instability of the levitated targets is because of the repulsive magnetic force applied along the horizontal plane. The levitating state can be controllable, for ex le, by changing the operating temperature, which would allow objects to levitate statically along the central axis.
Publisher: IOP Publishing
Date: 11-01-2021
Publisher: IOP Publishing
Date: 29-04-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2021
Publisher: Elsevier BV
Date: 2012
Publisher: Elsevier BV
Date: 03-2022
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2011
Publisher: IOP Publishing
Date: 28-08-2020
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2016
Publisher: IOP Publishing
Date: 30-03-2015
Publisher: IOP Publishing
Date: 08-04-2014
Publisher: IOP Publishing
Date: 19-07-2017
Publisher: IOP Publishing
Date: 16-06-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2011
Publisher: IOP Publishing
Date: 22-01-2019
Publisher: Elsevier BV
Date: 12-2022
Publisher: IOP Publishing
Date: 06-2020
DOI: 10.1088/1742-6596/1559/1/011001
Abstract: The 14 th European Conference on Applied Superconductivity (EUCAS 2019) was held in Glasgow, UK from the 1 st to the 5 th September 2019. The conference was attended by over 1000 delegates from around the world, drawn from both academia and industry. The conference programme covered the latest research and industrial developments in all areas of applied superconductivity including superconducting electronics, large-scale applications and superconducting materials. Selected papers presented at EUCAS 2019 have been published separately in a Special Focus Issue of the journal Superconductor Science and Technology (SUST). This present volume in the Journal of Physics: Conference Series (JPCS) collects together the remainder of the contributed papers submitted to the EUCAS 2019 proceedings. Editors: Edward Romans (Editor-in-chief) Mark Ainslie Kévin Berger Nikolay Bykovskiy Oscar Kennedy Anna Palau Colin Pegrum Loïc Quéval Guilherme Gonçalves Sotelo Wescley Tiago Batista de Sousa Min Zhang
Publisher: IOP Publishing
Date: 05-02-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2022
Publisher: American Physical Society (APS)
Date: 06-08-2020
Publisher: Emerald
Date: 08-03-2011
DOI: 10.1108/03321641111101195
Abstract: The purpose of the paper is to provide a comparison of first‐ and second‐order two dimensional finite element models for evaluating the electromagnetic properties and calculating AC loss in high‐temperature superconductor (HTS) coated conductors. The models are based on the two‐dimensional (2D) H formulation, which is based on directly solving the magnetic field components in 2D. Two models – one with a minimum symmetric triangular mesh and one with a single‐layer square mesh – are compared based on different types of mesh elements: first‐order (Lagrange – linear) and second‐order (Lagrange – quadratic) mesh elements, and edge elements. The number and type of mesh elements are critically important to obtain the minimum level of discretization to achieve accurate results. Artificially increasing the superconductor layer and choosing a minimum symmetric mesh with triangular edge elements can provide a sufficiently accurate estimation of the hysteretic superconductor loss for a transport current. This paper describes how the selection of mesh type and number of elements affects the computation speed and convergence properties of the finite element model using two different types of meshing. It offers an insight into the different factors modelers must consider when modeling HTS coated conductors and the methods that may be applied when extending the model to complex device geometries, such as wound coils.
Publisher: IOP Publishing
Date: 19-03-2021
Abstract: In this work, we have verified experimentally an all-(RE)BaCuO hybrid trapped field magnet lens (HTFML) using only one cryocooler and a special technique named the ‘loose contact method’. In the experimental setup, only the inner magnetic lens was tightly connected to the cold stage and cooled at all times, and the outer trapped field magnet (TFM) cylinder was loosely connected to the cold stage before the magnetizing process by introducing a gap between the outer TFM and cold stage of the cryocooler. As a result, the superconducting state for zero-field cooled magnetization of the inner magnetic lens and the non-superconducting (normal) state for field-cooled magnetization of the outer TFM cylinder can co-exist at the same time. A maximum concentrated field of B c = 9.8 T was achieved for the magnetizing process with an applied field of B app = 7 T in the present HTFML, consistent with the numerical estimation in our previous conceptual study. These results validate the HTFML concept as a compact and desktop-type magnet device that can provide 10 T-class magnetic field enhancement from the viewpoint of the magnetizing method. However, during magnetization with a higher B app of 10 T, thermal instability of the outer stacked TFM cylinder caused flux jumps to occur, resulting in mechanical fracture of multiple bulks. These results suggest that the further development of a practical cooling method that can realize a stable and controllable cooling process for each part of the HTFML is necessary based on fundamental studies relating to the thermal stability of the large stacked TFM cylinder.
Publisher: IOP Publishing
Date: 12-07-2010
Publisher: IOP Publishing
Date: 30-05-2023
Abstract: The practical magnetization of arrays of multiple single grain, bulk high temperature superconductors is essential for practical applications, such as trapped flux rotating machines, magnetic resonance imaging and nuclear magnetic resonance. We report a systematic investigation of the pulsed field magnetization (PFM) of a bulk assembly consisting of two rectangular Y – Ba – Cu – O bulk single grains, in close proximity, at various temperatures. The measurements of the dynamic variation of the magnetic flux density, supported by numerical analysis, reveal that the induced screening currents during the rise of a pulsed field may greatly enhance the flux density in the region of the junction leading to uneven flux penetration and to an increased likelihood of flux jumps in this region. Such coupling between field and current promotes magnetic flux penetration and improves the peak trapped field from 3.01 T for a bulk single grain to 3.11 T for the bulk assembly at 30 K, improving the magnetization efficiency from 80% to 90%. The peak trapped field was further enhanced to 3.39 T and 3.31 T for the single bulk single grain and the bulk assembly, respectively, by employing a two-step multi-pulse PFM process.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2011
Publisher: IOP Publishing
Date: 18-08-2021
Abstract: The development of a new hard x- ray beamline I-TOMCAT equipped with a 1 m long short-period bulk high-temperature superconductor undulator (BHTSU) has been scheduled for the upgrade of the Swiss Light Source at the Paul Scherrer Institute. The very hard x -ray source generated by the BHTSU will increase the brilliance at the beamline by over one order of magnitude in comparison to other state-of-the-art undulator technologies and allow experiments to be carried out with photon energies in excess of 60 keV. One of the key challenges for designing a 1 m long (100 periods) BHTSU is the large-scale simulation of the magnetization currents inside 200 staggered-array bulk superconductors. A feasible approach to simplify the electromagnetic model is to retain five periods from both ends of the 1 m long BHTSU, reducing the number of degrees of freedom to the scale of millions. In this paper, the theory of the recently-proposed 2D A -V formulation-based backward computation method is extended to calculate the critical state magnetization currents in the ten-period staggered-array BHTSU in 3D. The simulation results of the magnetization currents and the associated undulator field along the electron beam axis are compared with the well-known 3D H -formulation and the highly efficient 3D H - ϕ formulation method, all methods showing excellent agreement with each other as well as with experimental results. The mixed H - ϕ formulation avoids computing the eddy currents in the air subdomain and is significantly faster than the full H -formulation method, but is slower in comparison to the A -V formulation-based backward computation. Finally, the fastest and the most efficient A -V formulation, implemented in ANSYS 2020R1 Academic, is adopted to optimize the integrals of the undulator field along the electron beam axis by optimizing the sizes of the end bulks.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2021
Publisher: IOP Publishing
Date: 05-11-2019
Abstract: A hybrid trapped field magnet lens (HTFML) is a promising device that is able to concentrate a magnetic field higher than the applied field continuously, even after removing an external field, which was conceptually proposed by the authors in 2018. This paper presents, for the first time, the experimental realization of the HTFML using a GdBaCuO magnetic lens and MgB 2 trapped field magnet cylinder. A maximum concentrated magnetic field of B c = 3.55 T was achieved at the central bore of the HTFML after removing an applied field of B app = 2.0 T at T = 20 K. For higher B app , the B c value was not enhanced because of a weakened lens effect due to magnetic flux penetration into the bulk GdBaCuO material comprising the lens. The enhancement of the trapped field using such an HTFML for the present experimental setup is discussed in detail.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2015
Publisher: IOP Publishing
Date: 27-01-2011
Publisher: IOP Publishing
Date: 04-11-2019
Abstract: In this paper, simultaneous measurements of the electromagnetic strains along both the circumferential ( θ ) and radial ( r ) directions are reported for a large single-grain EuBaCuO ring bulk reinforced by an Al alloy ring during field-cooled magnetization (FCM) from 5 T at 50 K using several strain gauges adhered to the surface. To verify the experimental results and to understand the complex stress–strain behavior, mechanical analyses were carried out using a three-dimensional finite element model that closely represents the experimental setup. The simulation results of the electromagnetic strains along both directions showed excellent qualitative and quantitative agreement with the experimental ones. These results strongly suggest that the numerical model must include the exact same structure (size, shape and materials) of the mechanical support structure as the experimental setup in order to reproduce the experimental results both qualitatively as well as quantitatively. This also explains our previous research (SuST 2019 32 015007), where the measured circumferential strains were about 50% smaller those in the numerical simulation. Furthermore, the electromagnetic stresses along both directions during the FCM process are estimated from the obtained experimental strains. As a result, the estimated stresses were fairly consistent with those obtained by the numerical simulations, suggesting that our stress–strain simulation technique is both qualitatively and quantitatively reliable and useful to clarify the possibility of mechanical fracture of bulk superconductors.
Publisher: Springer Science and Business Media LLC
Date: 14-10-2016
Publisher: Elsevier BV
Date: 05-2017
Publisher: No publisher found
Date: 2018
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2022
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2020
Publisher: IOP Publishing
Date: 31-10-2016
Publisher: AIP Publishing
Date: 06-02-2017
DOI: 10.1063/1.4973991
Abstract: A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa2Cu3O7-δ (GdBaCuO) bulk superconductor of diameter 30 mm by employing pulsed field magnetization. The magnet system is portable and operates at temperatures between 50 K and 60 K. Flux jump behaviour was observed consistently during magnetization when the applied pulsed field, Ba, exceeded a critical value (e.g., 3.78 T at 60 K). A sharp dBa/dt is essential to this phenomenon. This flux jump behaviour enables the magnetic flux to penetrate fully to the centre of the bulk superconductor, resulting in full magnetization of the s le without requiring an applied field as large as that predicted by the Bean model. We show that this flux jump behaviour can occur over a wide range of fields and temperatures, and that it can be exploited in a practical quasi-permanent magnet system.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2022
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: Institute of Electrical Engineers of Japan (IEE Japan)
Date: 03-2020
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2018
Publisher: IOP Publishing
Date: 04-09-2017
Publisher: IOP Publishing
Date: 26-05-2015
Publisher: IOP Publishing
Date: 07-07-2017
Publisher: IOP Publishing
Date: 02-2008
Publisher: IOP Publishing
Date: 17-06-2022
Abstract: Bulk high-temperature superconducting materials can trap magnetic fields up to an order of magnitude larger than conventional permanent magnets. Recent advances in pulsed field magnetization (PFM) techniques now provide a fast and cost-effective method to magnetize bulk superconductors to fields of up to 5 T. We have developed a portable, desktop bulk high-temperature superconducting magnet system by combining advanced PFM techniques with state-of-the-art cryocooler technology and single-grain, RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] bulk superconducting materials. The base temperature of the system is 41 K and it takes about 1 h for the system to cool down to 50 K from room temperature. A capacitor bank, combined with easily-interchangeable, solenoid- or split-type copper magnetizing coils and an insulated bipolar gate transistor acting as a high-speed switch, allows magnetic pulses to be generated with different pulse profiles. The system is capable of trapping magnetic fields of up to ∼3 T. In this work, we report the results of the magnetization of a range of single-grain Y–Ba–Cu–O, Eu–Ba–Cu–O and Gd–Ba–Cu–O (GdBCO), bulk superconducting discs using this system. A higher trapped field was recorded using a split coil incorporating iron yokes at temperatures of 65 K and above, whereas at lower temperatures, a higher trapped field was obtained using the solenoid coil. The GdBCO s le achieved the highest trapped field for both single-pulse (SP) and two-stage-multi-pulse (TSMP) methods using the solenoid coil. Maximum trapped fields of 2.26 T at 55 K and 2.85 T at 49 K were recorded at the centre of the top surface of the GdBCO s le for the SP and TSMP methods, respectively. The PFM process is substantially an adiabatic process so, therefore, the thermal contact between the s le and s le holder is of critical importance for cooling the bulk s le during application of the pulse. The design of the s le holder can be modified easily to enhance the thermal stability of the s le in order to achieve a higher trapped field.
Publisher: IOP Publishing
Date: 12-05-2014
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 11-2023
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2018
Publisher: IOP Publishing
Date: 25-01-2021
Abstract: In this work, we propose a new concept of a high gradient trapped field magnet (HG-TFM). The HG-TFM is made from (RE)BaCuO bulk superconductors, in which slit ring bulks (slit-TFMs) are tightly stacked with TFM cylinders (full-TFMs), and state-of-the-art numerical simulations were used to investigate the magnetic and mechanical properties in detail during and after magnetization. A maximum value of the magnetic field gradient product of B z ⋅ d B z / d z = 6040 T 2 m −1 was obtained after conventional field cooled magnetization (FCM) with an applied field, B app , of 10 T of the HG-TFM with 60 mm in outer diameter and 10 mm in inner diameter. This value may be the highest value ever reported compared to any other magnetic sources. The B z ⋅ d B z / d z value increased with decreasing inner diameter of the HG-TFM and with increasing B app during FCM. The electromagnetic stress in the HG-TFM during the FCM process mainly results from the hoop stress along the circumferential direction. The simulations suggested that there is no fracture risk of the bulk components during FCM from 10 T in a proposed realistic configuration of the HG-TFM where both TFM parts are mounted in Al-alloy rings and the whole HG-TFM is encapsulated in a steel capsule. A quasi-zero gravity space can be realized in the HG-TFM with a high B z ⋅ d B z / d z value in an open space outside the vacuum chamber. The HG-TFM device can act as a compact and cryogen-free desktop-type magnetic source to provide a large magnetic force and could be useful in a number of life/medical science applications, such as protein crystallization and cell culture.
Publisher: IOP Publishing
Date: 07-03-2018
Publisher: IOP Publishing
Date: 06-06-2018
Publisher: Elsevier BV
Date: 11-2010
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2009
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2022
Publisher: Elsevier BV
Date: 08-2020
Publisher: IOP Publishing
Date: 11-2019
Publisher: IOP Publishing
Date: 04-03-2022
Abstract: During quasi-static magnetization of bulk superconductors using field-cooled magnetization (FCM) from high fields at low temperatures, such bulks are sometimes broken, which is believed to be mainly due to an electromagnetic force—and subsequent stress—larger than the fracture strength. However, a ring bulk can break, even during pulsed field magnetization (PFM), from relatively lower pulsed fields and at relatively higher temperatures. Previous simulation results suggest that the ring bulk should not break due to the electromagnetic force during PFM. In this paper, taking experimental and numerical results into consideration, we propose the possibility of mechanical fracture of a ring bulk during PFM due to thermal stress induced by local heat generation, which has not been considered and investigated to date. Two numerical models with different sizes of heat-generating region were constructed for the ring bulk with a relatively large inner diameter (60 mm outer diameter, 36 mm inner diameter, 17 mm height). For Model-1, with a large heat region, the bulk fracture due to the thermal stress results from the tensile stress along the radial direction in the neighboring heat region. The risk of bulk fracture is enhanced at the inner or outer edges of the bulk surface, compared with that inside the bulk. For Model-2, with a small heat region inside the bulk, the bulk fracture due to the thermal stress results from the compressive stress along the radial direction in the neighboring heat region. These results strongly suggest the possibility of mechanical fracture of an actual ring bulk due to thermal stress induced by local heat generation. This idea is also applicable more generally to the fracture mechanism during FCM of superconducting bulks.
Publisher: IOP Publishing
Date: 04-02-2020
Abstract: Dynamic resistance can be observed in a superconducting tape carrying a DC current which is exposed to an oscillating magnetic field. This effect is attributed to the interaction between the transport current and moving fluxons, and can occur in various superconducting components including high temperature superconducting (HTS) flux pumps, fast-r ing magnets and HTS rotating machines. Although conventionally expressed in terms of a DC ‘resistance,’ the phenomenon is inherently transient in nature, and the voltage drop across the superconductor follows a time-dependent periodic waveform. Here we present experimental measurements of the dynamic resistance of different REBCO tapes carrying a DC current and exposed to an oscillating perpendicular field. Measurements of both the transient voltage waveforms and the time-averaged DC resistances are compared with numerical finite element simulations obtained using the H -formulation. We observe clear variations between the voltage response from different tapes, which can be understood in terms of their differing J c ( B , θ ) dependence. In particular, a key feature of the experimentally measured waveforms is the emergence of a split ‘double peak’ at higher applied fields. Graphical visualisations of the finite element data show that this coincides with a periodic increase in J c ( B, θ ) throughout the tape. This occurs during each cycle at those times when the applied field falls below the shielding threshold of the tape (as the penetrating field within the tape then approaches zero). Our findings show that models which assume a constant J c irrespective of local field strength cannot capture the full range of behaviour observed by experiment. This emphasises the importance of employing experimentally measured J c ( B, θ ) data when simulating transient effects in HTS materials.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: Institute of Electrical Engineers of Japan (IEE Japan)
Date: 03-2020
Publisher: IOP Publishing
Date: 11-09-2018
Publisher: American Physical Society (APS)
Date: 08-04-2022
Publisher: Wiley
Date: 22-07-2021
DOI: 10.1111/JACE.18017
Abstract: The intrinsic mechanical properties of single‐grain RE‐Ba‐Cu‐O bulk high‐temperature superconductors can be improved by employing a thin‐wall geometry. This is where the s les are melt‐processed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thin‐wall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thin‐wall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thin‐wall s le should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the s le. High‐field magnetization experiments were performed by applying magnetization fields of up to 11.5 T, specifically to break the s les in order to verify the effect of increased mechanical strength (and improved cooling) on the ability of bulk (RE)BCO to trap field successfully. The standard YBCO s le failed when it was magnetized with a field of 10 T at 35 K, suffering permanent damage. As a result, the standard s le could only trap a maximum surface field of 7.6 T without failure. On the other hand, the thin‐wall YBCO s le survived all magnetization cycles, including a maximum magnetization field of 11.5 T at 35 K, demonstrating a greater intrinsic ability to withstand significantly higher electromagnetic stresses. By subsequently field‐cooling the thin‐wall s le with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2023
Publisher: IOP Publishing
Date: 30-09-2020
Abstract: Bulk high temperature superconductors based on the rare-earth copper oxides can be used effectively as trapped field magnets capable of generating large magnetic fields. The top-seeded infiltration growth (TSIG) processing technique can provide a more homogeneous microstructure and therefore more uniform superconducting properties than s les grown using conventional melt growth processes. In the present investigation, the properties of bulk, single grain superconductors processed by TSIG and magnetised by the pulsed-field magnetisation technique using a copper-wound solenoid have been studied. A trapped field of ∼3 T has been achieved in a 2-step buffer-assisted TSIG-processed Y-Ba-Cu-O (YBCO) s le at 40 K by magnetising the bulk superconductor completely via a single-pulse magnetisation process. S les were also subjected to pulsed-field magnetisation at 65 K and by conventional field-cooled magnetisation at 77 K for comparison. Good correlation was observed between the microstructures, critical current densities and trapped field performance of bulk s les fabricated by TSIG and magnetised by pulsed-field and field-cooled magnetisation. The homogeneous distribution of Y 2 BaCuO 5 inclusions within the microstructure of bulk YBCO s les fabricated by the 2-step buffer-assisted TSIG process reduces inhomogeneous flux penetration into the interior of the s le. This, in turn, results in a lower temperature rise of the bulk superconductor during the pulsed-field magnetisation process and a more effective and reliable magnetisation process.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2021
Publisher: IOP Publishing
Date: 10-10-2022
Abstract: When used as trapped field magnets (TFMs), single grain, bulk high-temperature superconducting (HTS) rings are promising candidates for the generation of strong, uniform magnetic fields for nuclear magnetic resonance. The pulsed field magnetisation (PFM) technique provides a low cost, compact and portable method to magnetise these s les as TFMs however it has proven difficult to achieve high trapped fields in HTS rings using PFM. To date, a record field of only 0.60 T has been achieved for rings magnetised by single-pulse PFM—compared with over 4 T for disc-shaped HTS—and the reasons for this discrepancy are poorly understood. In this work, we use the finite element method to model the propagation of magnetic flux into HTS rings under quasi-static zero field cooled magnetisation and PFM, and validate the results analytically and experimentally. Magnetic flux is found to penetrate finite HTS rings from both the inner and outer surfaces, inducing a negative field at the inner face of the ring. This field is reversed as the applied field increases past the point of full penetration, locally dissipating magnetic energy and heating the s le. HTS rings are therefore more susceptible to local instabilities that severely limit their ability to trap a useful magnetic field. Consequently, thermomagnetic stability of HTS rings during single-pulse PFM can only be ensured by taking careful consideration of reducing flux movement through the bulk around the point at which the field is reversed. This may require more advanced PFM techniques like waveform control or multi-pulse stepwise-cooling to reduce local heating and increase the trapped field.
Publisher: IOP Publishing
Date: 07-10-2020
Abstract: A backward computation method has been developed to accelerate modelling of the critical state magnetization current in a staggered-array bulk high-temperature superconducting (HTS) undulator. The key concept is as follows: (i) a large magnetization current is first generated on the surface of the HTS bulks after rapid field-cooling (FC) magnetization (ii) the magnetization current then relaxes inwards step-by-step obeying the critical state model (iii) after tens of backward iterations the magnetization current reaches a steady state. The simulation results show excellent agreement with the H -formulation method for both the electromagnetic and electromagnetic-mechanical coupled analyses, but with significantly faster computation speed. The simulation results using the backward computation method are further validated by the recent experimental results of a five-period Gd–Ba–Cu–O (GdBCO) bulk undulator. Solving the finite element analysis (FEA) model with 1.8 million degrees of freedom (DOFs), the backward computation method takes less than 1.4 h, an order of magnitude or higher faster than other state-of-the-art numerical methods. Finally, the models are used to investigate the influence of the mechanical stress on the distribution of the critical state magnetization current and the undulator field along the central axis.
Publisher: No publisher found
Date: 2022
DOI: 10.18742/21215801
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2023
Publisher: IOP Publishing
Date: 19-08-2022
Abstract: Bulk, single grain RE–Ba–Cu–O (where RE = rare earth or yttrium) [(RE)BCO] high temperature superconductors could potentially be used to generate stable magnetic fields for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). In these applications, however, the homogeneity of the magnetic field is of critical importance. As a result, the spatial distribution of critical current density, J c , within the bulk single grain and the effects of the magnetisation process, which are primary drivers of the uniformity of the achievable trapped magnetic field, are fundamental to assessing the performance of these technologically important materials. This paper reports the systematic measurement of the distribution of J c – B at 77 K over a vertical cross-section of a single grain along a facet line and through the seed crystal [(110)- F ] at 20 positions within a 20 mm diameter Gd–Ba–Cu–O s le in an attempt to understand and assess the distribution of J c along this microstructural feature. A comparison of the data within the whole vertical plane across the seed measured along the a or b direction within the [(100)- a ] plane shows that J c – B at 77 K at the facet line is more than 10% higher for applied fields between 0.2 T and 2.5 T. The effect of the J c – B relationship of the facet line on the overall trapped field measured in an in idual bulk s le was investigated by measuring the magnitudes of trapped fields and their contour maps for sections cut from four single grain s les of GdBCO–Ag at different sizes and shapes parallel to the ab -plane from the top to the bottom of the bulk s le. Based on the results reported here, we demonstrate a method to achieve more uniform trapped fields through an optimal arrangement of an assembly of sections of in idual GdBCO single grains.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2020
Publisher: IOP Publishing
Date: 02-02-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2009
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2009
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2022
Publisher: IOP Publishing
Date: 05-07-2021
Abstract: 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 MgB 2 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 MgB 2 bulk s le magnetised by a solenoidal pulsed field magnet. The findings illustrate the fundamental role bulk cooling plays during PFM.
Publisher: IOP Publishing
Date: 10-01-2023
Abstract: (RE)-Ba 2 Cu 3 O 7 − δ bulk superconductors acting as permanent trapped field magnet analogues have been shown to trap fields in excess of 17 T. However, the occurrence of thermomagnetic instabilities during their magnetisation process can limit their performance. In 2019, Naito et al trapped 15.1 T in a (Y)-BaCuO bulk pair under applied fields of up to 22 T (2019 J. Appl. Phys. 126 243901), whilst also documenting the mechanical failure of the bulks due to a flux jump. Here, we accurately numerically model this experiment and the observed flux jump, providing insight into the experimental results, such as the role of heating, the presence of thermal gradients, and a possible reason for the location of the bulk-pair fracturing. Extending the study with an investigation into the role of enhanced cooling of the bulk-pair, the influence of cooling power on the calculated trapped field and flux jump is explored. Finally, we propose and numerically investigate a new composite bulk configuration, consisting of stacks of (Y)-BaCuO bulk and interspaced metallic discs, to enhance the thermal stability of the bulk pair, to ultimately improve the trapped field performance.
Publisher: Elsevier BV
Date: 12-2023
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2017
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2021
Publisher: IOP Publishing
Date: 22-06-2020
Publisher: IOP Publishing
Date: 31-05-2016
Publisher: IOP Publishing
Date: 05-02-2015
Publisher: American Physical Society (APS)
Date: 10-08-2023
Publisher: AIP Publishing
Date: 11-05-2020
DOI: 10.1063/5.0005356
Abstract: A hybrid trapped field magnet lens (HTFML) is a promising device that is able to concentrate a magnetic field higher than the applied field continuously, even after removing an external field, which was conceptually proposed by the authors in 2018. In this study, we propose a new additional advantage of the HTFML, which could be applicable for magnetic levitation and separation. The HTFML device consisting of a GdBaCuO bulk cylinder and a GdBaCuO magnetic lens, after the magnetization process from an applied field, Bapp = 10 T, can generate a maximum trapped field, Bc = 11.4 T, as well as an ultra-high magnetic field gradient product, Bz⋅dBz/dz, over ±3000 T2/m at Ts = 20 K, which is higher than that of existing superconducting magnets and large-scale hybrid magnets. Through detailed numerical simulations, the HTFML device is considered for the magnetic separation of a mixture of precious metal particles (Pt, Au, Ag, and Cu) dispersed in pure water, by exploiting the magneto-Archimedes effect. The HTFML can be realized as a compact and mobile desktop-type superconducting bulk magnet system, and there are a wide range of potential industrial applications, such as in the food and medical industries.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2011
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2021
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2013
Publisher: IOP Publishing
Date: 11-12-2019
Abstract: The Insertion Device group of the Paul Scherrer Institute has started an R& D program on a high temperature superconducting undulator to reduce the period length and increase the undulator's magnetic field well beyond the present capability. Simulation results for a 10 mm period and 4 mm magnetic gap staggered array of GdBCO bulks predict peak magnetic field above 2 T. Building on the existing working principle of undulator design and simulated performance, the first experimental results of a 5 period 6.0 mm gap short undulator measured in the new test facility available at the University of Cambridge will be presented together with details of the experimental setup and s le preparation.
Publisher: IOP Publishing
Date: 14-06-2019
Abstract: 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 s les 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 s les 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 GdBa 2 Cu 3 O 7-δ 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. These post-melt-processing treatments, which are relatively straightforward to implement, were used to improve both the mechanical properties and the thermal stability of the resultant composite structure, providing what we believe is a promising route to achieving reliably fields of over 20 T.
Publisher: AIP Publishing
Date: 22-04-2019
DOI: 10.1063/1.5085226
Abstract: Despite their proven ability to output DC currents of & A, the physical mechanism which underpins the operation of a high-Tc superconducting (HTS) dynamo is still debated widely. Here, we show that the experimentally observed open-circuit DC output voltage, Vdc, is due to the action of overcritical eddy currents within the stator wire. We demonstrate close agreement between experimental results and numerical calculations, and show that large over-critical currents flow within the high-Tc stator during certain parts of the dynamo cycle. These overcritical currents experience a non-linear local resistivity which alters the output voltage waveform obtained in the superconducting state. As a result, the full-cycle integral of this altered waveform outputs a non-zero time-averaged DC voltage. We further show that the only necessary requirement for a non-zero Vdc output from any dynamo is that the stator must possess a non-linear local resistivity. Here, this is provided by the flux-flow regime of an HTS coated conductor wire, where conduction is described by the E–J power law. We also show that increased values of Vdc can be obtained by employing stator wires which exhibit a strong in-field dependence of the critical current Jc(B,θ). However, non-linear resistivity is the key requirement to realize a DC output, as linear magneto-resistance is not sufficient. Our results clarify this longstanding conundrum, and have direct implications for the optimization of future HTS dynamo devices.
Publisher: IOP Publishing
Date: 25-06-2018
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2019
End Date: 2022
Funder: Marsden Fund
View Funded ActivityStart Date: 2015
End Date: 2018
Funder: Japan Society for the Promotion of Science
View Funded ActivityStart Date: 2012
End Date: 2017
Funder: Royal Academy of Engineering
View Funded ActivityStart Date: 2014
End Date: 2016
Funder: Royal Society
View Funded ActivityStart Date: 2017
End Date: 2022
Funder: Engineering and Physical Sciences Research Council
View Funded ActivityStart Date: 2020
End Date: 2023
Funder: Engineering and Physical Sciences Research Council
View Funded ActivityStart Date: 2015
End Date: 2018
Funder: Japan Society for the Promotion of Science
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: Engineering and Physical Sciences Research Council
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