Modelling higher trapped fields by pulsed field magnetisation of composite bulk MgB 2 superconducting rings

Funder: EPSRCAbstract: The recent results of Hirano et al (2020 Supercond. Sci. Technol. 33 085002) reported a high trapped field of 1.61 T in a composite MgB2 ring comprising copper plates and and a soft iron yoke magnetised by pulsed field magnetisation (PFM). Inspired by these results, an investigation using systematic modelling methods was conducted to investigate the key parameters leading to the success of Hirano et al. Our results indicate that composite structures of MgB2 rings present a viable method of trapping high magnetic fields when magnetised with PFM. Leveraging a finite element method modelling framework with a commercial software package (COMSOL Multiphysics), we have successfully modelled the experimental data with excellent agreement. We have paid careful attention to the assumptions regarding the thermal physics, which enabled the successful and accurate modelling of the experiment. Exploiting the flexibility of computational modelling, we extend our studies to investigate the influence of the constituent elements of the composite bulk on the electromagnetic and thermal behaviour, and discuss in detail how each can enhance the trapped field performance of the bulk. Aided by the models, it is shown how the number of copper layers influences the elongation of the applied pulse, reducing the field penetration and the maximum temperature rise of the bulk. The addition of the iron yoke significantly increases the trapped field, by concentrating flux during and after the pulse

Improved pulsed field magnetisation in MgB 2 trapped-field magnets

Funder: University of Cambridge; doi: http://dx.doi.org/10.13039/501100000735Abstract: 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

Modelling and mitigating flux jumps in bulk high-temperature superconductors during quasi-static, high-field magnetisation

Abstract (RE)-Ba2Cu3O 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.</jats:p

Horizontal carbon nanotube alignment.

The production of horizontally aligned carbon nanotubes offers a rapid means of realizing a myriad of self-assembled near-atom-scale technologies - from novel photonic crystals to nanoscale transistors. The ability to reproducibly align anisotropic nanostructures has huge technological value. Here we review the present state-of-the-art in horizontal carbon nanotube alignment. For both $\textit{in}$ and $\textit{ex situ}$ approaches, we quantitatively assess the reported linear packing densities alongside the degree of alignment possible for each of these core methodologies

Modelling higher trapped fields by pulsed field magnetisation of composite bulk MgB<inf>2</inf> superconducting rings

The recent results of Hirano et al. (Supercond. Sci. Technol. 33, 085002 (2020)) reported a high trapped field of 1.61 T in a composite MgB₂ ring comprising copper plates and and a soft iron yoke magnetised by pulsed field magnetisation (PFM). Inspired by these results, an investigation using systematic modelling methods was conducted to investigate the key parameters leading to the success of Hirano et al. Our results indicate that composite structures of MgB₂ rings present a viable method of trapping high magnetic fields when magnetised with PFM. Leveraging a finite element method modelling framework with a commercial software package (COMSOL Multiphysics), we have successfully modelled the experimental data with excellent agreement. We have paid careful attention to the assumptions regarding the thermal physics, which enabled the successful and accurate modelling of the experiment. Exploiting the flexibility of computational modelling, we extend our studies to investigate the influence of the constituent elements of the composite bulk on the electromagnetic and thermal behaviour, and discuss in detail how each can enhance the trapped field performance of the bulk. Aided by the models, it is shown how the number of copper layers influences the elongation of the applied pulse, reducing the field penetration and the maximum temperature rise of the bulk. The addition of the iron yoke significantly increases the trapped field, by concentrating flux during and after the pulse.EPSRC DTP Fund; EPSRC Early Career Fellowship EP/P020313/1; JSPS KAKENHI Grant No. 19K0524

Portable, desktop high-field magnet systems using bulk, single-grain RE-Ba-Cu-O high-temperature superconductors

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 sample 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 sample for the SP and TSMP methods, respectively. The PFM process is substantially an adiabatic process so, therefore, the thermal contact between the sample and sample holder is of critical importance for cooling the bulk sample during application of the pulse. The design of the sample holder can be modified easily to enhance the thermal stability of the sample in order to achieve a higher trapped field.</jats:p

Trapped Fields &gt;1 T in a Bulk Superconducting Ring by Pulsed Field Magnetization

One potential application of magnetized RE-Ba-Cu-O (where RE = rare earth or Y) bulk superconductors is as a high-field alternative to conventional permanent magnets in desktop NMR and MRI systems. Pulsed field magnetization (PFM) is one of the most promising practical methods of magnetizing such bulks. However, the trapped fields obtained by PFM are much lower than those obtained using quasi-static methods like field-cooling magnetization (FCM) due to heating during PFM. Furthermore, bulk superconducting rings have proved more difficult to magnetize via PFM than discs. The reported trapped fields in single bulk superconducting rings magnetized by PFM are less than 0.35 T at the centre of the bore due to thermomagnetic instabilities. In this work, systematic PFM measurements on a bulk Gd-Ba-Cu-O ring were carried out and a trapped field of 1.3 T at 55 K was achieved using a multi-pulse, stepwise cooling (MPSC) method. In the MPSC method, a sequence of pulsed fields is used to magnetize the ring bulk. The pulsed field is increased in small increments and the sample temperature is decreased sequentially. Consequently, as some field is already trapped after the first pulse, the motion of the flux for subsequent pulses will be reduced, leading to less heat generated in the bulk sample. This greatly improves the thermomagnetic stability of the PFM process, enabling larger trapped fields.EPSRC (EP/P020313/1; EP/T014679/1

Waveform Control Pulsed Field Magnetization of RE-Ba-Cu-O Bulk Superconducting Rings

One of the potential applications of ring-shaped, single grain RE-Ba-Cu-O bulk superconductors is in desktop magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) systems as an alternative to conventional permanent magnets. The higher magnetic field available from magnetized bulk superconductors could significantly improve the performance of such systems, as well as reduce their size and increase portability. The pulsed field magnetization (PFM) method provides a fast, compact and cost-effective method for magnetizing these materials as trapped field magnets. However, bulk superconducting rings are very suscep-tible to thermomagnetic instabilities during the PFM process, and thus, to date, the reported trapped fields in ring bulks magnet-ized by PFM are less than 0.35 T at the centre of single rings. In this work, we demonstrate that the trapped field in a super-conducting ring bulk can be enhanced significantly by optimizing the waveform of the magnetizing pulse used in the PFM method. This optimization can be achieved easily by using an Insulated Gate Bipolar Transistor as a fast-switching device with a control-lable switching frequency in the pulse-generating electric circuit. Our findings represent a key step forward in utilizing bulk, sin-gle-grain superconducting rings magnetized by PFM in portable magnet systems

Improved pulsed field magnetisation in MgB2_{2} trapped-field magnet

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 (0.95T) for a single MgB₂ bulk sample magnetised by a solenoidal pulsed field magnet. The findings illustrate the fundamental role bulk cooling plays during PFM