Computation of the field in an axial gap, trapped-flux type superconducting electric machine

The Bulk Superconductivity Group at the University of Cambridge is currently investigating the use of high temperature superconductors in wire and bulk form to increase the electrical and magnetic loading of an axial gap, trapped flux-type superconducting electric machine. The use of superconducting materials in electric machines can lead to increases in efficiency, as well as power density, which results in reductions in both the size and weight of the machine. In this paper, the authors present a method to compute the field in such an electric machine generated by an array of fully magnetized bulk superconductors. Analytical expressions are derived for the field that would exist in the coil region of the motor, which will act as a powerful tool for carrying out parametric analysis of the motor’s design and performance.This work was supported in part by a Royal Academy of Engineering Research Fellowship and by the China Scholarship Council.This is the accepted manuscript. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6945871

Numerical analysis of non-uniformities and anisotropy in high-temperature superconducting coils

—High-temperature superconducting (HTS) coils play an important role in a number of large-scale engineering applications, such as electric machines employing HTS coated conductors. Non-uniformities and anisotropy in the properties of the coated conductor along its length and width can have a large impact on the performance of the tape, which directly influences the performance of an HTS electric machine. In this paper, the specific influences of non-uniformity and anisotropy on the dc properties of coils, such as the maximum allowable dc current, and the ac properties, such as ac loss, are analyzed using a numerical model based on the H formulation. It is found that non-uniformity along the conductor width has a large effect on the ac properties (i.e., ac loss) of a coil, but a relatively small effect on the dc properties (i.e., critical current). Conversely, non-uniformity along the length has a small effect on the ac coil properties, but has a large effect on the dc properties. Index Terms—AC loss, critical current density (superconductivity), high-temperature superconductors, numerical analysis, superconducting coils, transport ac loss.This work was supported in part by a Henan International Cooperation Grant, China: 144300510014. The work of D. Hu and J. Zou was supported in part by Churchill College, by the China Scholarship Council, and by the Cambridge Commonwealth, European, and International Trust. The work of M. D. Ainslie was supported by a Royal Academy of Engineering Research Fellowship.This is the accepted manuscript. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6983573

Influence of time-varying external magnetic fields on trapped fields in bulk superconductors

Large, single-grain bulk high-temperature superconductors (HTS) can trap magnetic fields over 17 T below 30 K and up to 3 T at 77 K, and have significant potential to replace permanent magnets, the fields from which are limited to significantly less than 2 T. Therefore, bulk HTS samples are ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. In electric machines, in particular, the higher flux density improves the power density of the machine, resulting in smaller, lighter devices. However, in a real electric machine environment, bulk HTS samples can be exposed to AC magnetic field fluctuations that can affect the distribution of the supercurrent in the material and attenuate the trapped field, leading to a reduction in the magnetic loading of the machine, and in some cases, full demagnetisation. In this paper, the variation of trapped field with the frequency and magnitude of an external time-varying magnetic field is analysed numerically, and the mechanisms of the attenuation of the trapped field in HTS bulks are investigated using a two-dimensional (2D) axisymmetric finite-element model based on the H-formulation, considering both the electromagnetic and thermal behaviour of the bulk sample.This work was supported in part by a Henan International Cooperation Grant, China: 144300510014. . M. D. Ainslie would like to acknowledge financial support from a Royal Academy of Engineering Research Fellowship. D. Hu and J. Zou would like to acknowledge financial support from Churchill College, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust.This is the accepted manuscript. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=6983588&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_Publication_Number%3A77%29%26pageNumber%3D5%26rowsPerPage%3D100

Mitigation of Demagnetization of Bulk Superconductors by Time-Varying External Magnetic Fields

Large, single-grain high-temperature superconducting (HTS) bulks have significant potential to replace permanent magnets in various engineering applications. However, based on our previous research, the trapped field in a bulk superconductor can be attenuated or even erased when a bulk is subjected to a time-varying, external magnetic field. Therefore, it is important to develop a method to protect bulks from demagnetization by (a) improving the thermal conduction of the bulk and/or (b) reducing AC losses. Improvement in the thermal conduction of bulks involves modification of the material fabrication process, which may have a detrimental effect on its superconducting properties. Employing shielding materials around a bulk helps to decrease the AC losses, but also provides a durable way to maintain the original material properties. In this paper, two shielding cases are proposed and evaluated numerically: ring-shaped shielding with a copper coil, and surface shielding with a ferromagnetic material. Based on the numerical modelling results, the ring-shaped coil works well for externally applied AC fields of larger magnitude and higher frequency. However, the ferromagnetic material was preferable for surface shielding for relatively lower fields. Finally, an optimal shield design is presented.M. D. Ainslie would like to acknowledge financial support from a Royal Academy of Engineering Research Fellowship. D. Hu and J. Zou would like to acknowledge financial support from Churchill College, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust. This work was supported in part by a Henan International Cooperation Grant, China: 144300510014.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/TASC.2016.252582

Simulating the in-field AC and DC performance of high-temperature superconducting coils

In this paper, the authors investigate numerically the in-field behaviour of high-temperature superconducting (HTS) coils and a method to potentially improve their performance using ferromagnetic material as a flux diverter. The ability to accurately predict the electromagnetic behaviour of superconductors in complex geometries and electromagnetic environments is crucial to the design of commercially-viable superconductor-based electrical devices, such as power transmission cables, superconducting fault current limiters, transformers, and motors and generators. The analysis is carried out using a two-dimensional (2D) axisymmetric model of a circular pancake coil based on the H-formulation and implemented in Comsol Multiphysics 4.3a. We explore the use of flux diverters to improve an HTS coil’s performance with respect to its DC (maximum allowable/critical current) and AC (AC loss) characteristics, for various background magnetic fields. It is found that while flux diverters can improve the AC properties of coils, they can be detrimental to the DC properties in this particular configuration.Dr Mark Ainslie would like to acknowledge the support of a Royal Academy of Engineering Research Fellowship. Di Hu and Jin Zou would like to acknowledge support of Churchill College, Cambridge, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust.This is the accepted manuscript. The final published version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6965596&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A6353170%29%26rowsPerPage%3D100

Flux vortex dynamics in type-II superconductors

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 Campbell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the Campbell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBa2Cu3O 7 − δ superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.This work was supported by Siemens AG. Dr Mark Ainslie would like to acknowledge financial support from an Engineering and Physical Sciences Research Council (EPSRC) Early Career Fellowship EP/P020313/1

A portable magnetic field of >3 T generated by the flux jump assisted, pulsed field magnetization of bulk superconductors

A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa$_2$Cu$_3$O$_{7-}$$_\delta$ (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, $B_a$, exceeded a critical value (e.g., 3.78 T at 60 K). A sharp d$B_a$/d$t$ 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 sample 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.This work was supported by the Boeing Company and by the Engineering and Physical Sciences Research Council (Grant No. EP/P00962X/1

Pulsed Field Magnetization of Single-Grain Bulk YBCO Processed from Graded Precursor Powders

Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials.M. D. Ainslie would like to acknowledge financial support from a Royal Academy of Engineering Research Fellowship. This work was also supported by a Royal Society International Exchanges Scheme Grant, IE131084. J. Zou would like to acknowledge financial support from Churchill College, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/TASC.2015.250916

Pulsed Field Magnetization of Bridge-Seeded, Bulk YBCO using Solenoid and Split Coils

The multi-seeding process has the potential to enlarge the sample size of (RE)BCO (where RE = rare earth or Y) single-grain, bulk superconductors with improved fabrication speed, and in previous studies on multi-seeding, a significant improvement was made in the alignment of the seeds in such samples using a novel bridge-seeding technique. In this paper, we report the experimental measurements of the pulsed field magnetization (PFM) of a 0°-0° bridge-seeded Y-Ba-Cu-O (YBCO) sample. The PFM is carried out using a solenoid coil, as well as a split coil arrangement with an iron yoke, at temperature of 65, 40 and 20 K, and the resultant trapped fields and magnetic flux dynamics for these two PFM techniques are compared. It is shown that such bridge-seeded bulk YBCO can be fabricated that performs as a bulk magnet with trapped fields comparable to or better than standard, single-seeded high-$J_\text{c}$ samples, with the potential of enlarging the sample size. Furthermore, the split coil arrangement with an iron yoke is useful to enhance the trapped field and has a positive effect on the maximum temperature rise in the sample, which increases at lower temperatures and seriously impacts the achievable trapped field from PFM.The work of M. D. Ainslie was supported in part by the Royal Academy of Engineering Research Fellowship and in part by the Royal Society International Exchanges Scheme under Grant IE131084. The work of H. Fujishiro was supported in part by the Open Partnership Joint Projects of the Japan Society for the Promotion of Science (JSPS) Bilateral Joint Research Projects, and in part by the JSPS KAKENHI under Grants 23560002 and 15K04646

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(RE)BCO; RE = rare earth] bulk superconductors to trap magnetic field is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter 25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.This is the final published version, distributed under a Creative Commons Attribution License. This can also be found on the publisher's website at: http://iopscience.iop.org/0953-2048/27/8/08200