Trapped Magnetic Flux of Bulk HTS Magnets in the External AC Magnetic Field at Low Temperatures

Supercond. Sci. Technol.

Ida

et al. 2016

The present manuscript addresses key issues in the course of our study of materials processing of bulk high-temperature superconductors, trapped flux and its application to a prototype axial-gap-type rotating machine. The TUMSAT group has conducted a series of studies since 2003 on the growth of GdBa2Cu3O7−δ bulk material and its application in a compact low-speed high-torque rotating machine. In the stage of material growth, gaining the advantage of a large motive torque density requires large integrated flux in the motor/generators. A large grain surface might be required with sophisticated techniques for the melt-growth texture in the bulk with optimal flux pinning. In the second stage, the in situ magnetization procedure for bulk superconductors in the applied machine is a crucial part of the technology. Pulsed current excitation by using an armature copper winding has magnetized field pole bulks on the rotor. The axial-gap flux synchronous machine studied in the past decade is a condensed technology and indicates that further scientific development is required for a future compact machine to be superior to conventional ones in accordance with the cryogenic periphery and flux stabilization.

Section: Influence Of An Ac Magnetic Field On the Trapped Field Of Bu...supporting

confidence: 59%

Melt-growth bulk superconductors and application to an axial-gap-type rotating machine

Zhang

Supercond. Sci. Technol.

Ida

et al. 2016

“…Large, single grain (RE)-Ba-Cu-O [(RE)BCO, where RE is a rare-earth element or Y] bulk high temperature superconducting (HTS) materials can potentially be used to improve the performance of a range of engineering applications, such as superconducting bearings, flywheel energy storage systems, rotating machines and non-contact mixers [1][2][3][4][5][6]. This is due, primarily, to their ability to trap magnetic fields that are significantly greater than those produced by conventional, iron-based permanent magnets.…”

Section: Introductionmentioning

confidence: 99%

Exploiting flux jumps for pulsed field magnetisation

Supercond. Sci. Technol.

Ainslie

Srpcic

et al. 2018

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 GdBa 2 Cu 3 O 7−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 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, B j , was determined experimentally and found to increase with decreasing temperature. An extended instability criterion based on a 2D axisymmetric model was used to predict B j 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 optimised two-step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.

“…Significant technical challenges of bulk superconductors, however, remain in cooling and magnetisation technologies for practical applications. Pulsed field magnetization (PFM), which is a traditional method for activating permanent magnets, has been considered the most practical technique to magnetize HTS bulk superconductors either in situ or ex situ in portable systems [9,10]. However, the PFM technique is essentially a zero field cooling (ZFC) process, which implies that an external field of at least twice the peak field trappable in a bulk superconductor is required to achieve complete magnetization.…”

mentioning

confidence: 99%

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

Applied Physics Letters

Ainslie

Shi

et al. 2017

A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa 2 Cu 3 O 7δ (GdBaCuO) bulk superconductor of diameter 30 mm by employing pulsed field magnetisation (PFM). The magnet system is portable and operates at temperatures between 50 K and 60 K. Flux jump behaviour was observed consistently during magnetisation when the applied pulsed field, B a , exceeded a critical value (e.g. 3.78 T at 60 K). A sharp dB a /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 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.