John Hay Durrell

In operational settings, superconducting electromagnets frequently cycle their transport current, presenting challenges due to the strong magnetic hysteresis observed in many type-II superconductors. This paper examines the hysteretic behaviour of Bitter-like electromagnets constructed from bulk rare-earth barium cuprate ((RE)BCO) high-temperature superconducting ceramics under both unipolar and bipolar current cycling modes. A 2D-axisymmetric H -formulation finite-element (FE) model is used to simulate the current and field distributions within a 20-layer coil during current cycling, revealing significant hysteresis effects. In particular, a trapped field opposite to the direction of the transport-induced field is predicted in the bore upon current removal, attributable to induced shielding currents at the inner radius. To validate the modelling, experiments were performed on a 4-layer GdBCO-Ag coil comprising only slit wafers. Under unipolar current cycling, the predicted history-dependent central field and negative trapped field were clearly observed. A second 4-layer coil incorporating additional unslit terminal layers was then tested under bipolar current cycling and was found to trap a field in the same direction as the transport-induced field upon current removal. The FE model was adapted to account for the unslit ends, and the positive trapped fields were attributed to circulating currents within the unslit discs. Overall, the results demonstrate that bulk Bitter-like high-temperature superconducting (HTS) electromagnets exhibit a history-dependent B – I characteristic arising from persistent screening currents, analogous to screening-current-induced field effects reported in (RE)BCO tape-based magnets. These findings highlight important considerations for practical magnet calibration and operation in bulk HTS Bitter-like electromagnets.

Superconductivity was discovered more than a century ago, and it has achieved full commercialization for MRI and NMR applications. Superconducting technology has got on spotlight recent years for transportation, power network, and fusion energy, due to the significant advantages offered against its counterpart technologies, including lighter weight, compacter size, lower losses, higher efficiency, and higher power density. Therefore, many superconducting applications are moving towards higher technology readiness levels, with a fast pace. The accelerated research around superconducting applications for modern transportation is due to unique features of this technology towards decarbonisation via electrified systems. Meeting the Net Zero targets to decelerate global warming issue is the main driver of implementing the superconducting technology for aerospace, marine, and railway transport. However, many challenges still remain to be addressed for superconducting devices and applications, which will in turn pave the way for the commercialization of superconducting technology. In this article, a roadmap on electrification of transportation systems for aerospace, marine, and railway application is presented, covering challenges and solutions in design analysis, modelling, monitoring, and operation. A series of short articles are presented to outline the potential applications and solutions. These potential futuristic routes and their materials/technologies are considered/suggested for a 10-20 years time-frame.

This paper describes the recent advancements of the meter-long bulk high temperature superconducting undulator (BHTSU) at the Paul Scherrer Institute. We present the complete design of the BHTSU, comprised of a cryostat with three cryocoolers, a 12 T Nb₃Sn solenoid and an GdBa₂Cu₃O_7−x insert. The 200 half-moon shaped GdBa₂Cu₃O_7−x bulks have been procured and characterized with field mapping at 77 K and 1 T. The GdBa₂Cu₃O_7−x disks are grown with the new single-direction melt growth technique for better magnetic and structural properties and more streamlined industrial growth. We then show the development steps for the 12 T Nb₃Sn solenoid used to magnetize the GdBa₂Cu₃O_7−x bulks. Finite element method simulations were used to estimate the field uniformity and structural analysis of the solenoid. The manufacturing steps for the first coil are also shown.

The Bitter magnet architecture is used to generate the strongest sustained dc magnetic fields in the world. However, resistive Bitter magnets require significant power and cooling, limiting their use to dedicated specialist facilities. Recent work has demonstrated the concept of a Bitter-like magnet comprised of stacked, slit annular discs of rare-earth barium cuprate ((RE)BCO) bulk superconductor, which enables a dramatic reduction in both power requirements and system size. One of the main technical challenges for realising such devices is the ability to form robust, low-resistance joints between individual bulk discs, which is essential to the fabrication of practical high-temperature superconductor Bitter coils that can operate at high currents. In this work, partially insulated c -axis joints between slit annular wafers of bulk (RE)BCO were developed in order to form a Bitter-like magnet. Silver-diffusion and peritectic-based interfacial fusion jointing methods were found to form either resistive or superconducting-like joints dependent on the heating parameters used. Two-layer test joints were formed using each method, and found to be robust to thermal cycling between 77 K and 300 K. However, as the joint I c in all superconducting joints formed was significantly lower than that of the main (RE)BCO, resistive silver diffusion joints were found to better suit applications requiring high currents. Four GdBCO-Ag prototype coils comprising a 4-turn Bitter-like stack were constructed using silver diffusion joints, with a maximum field of 0.41 T achieved at 1.17 kA. Two types of coil limiting behaviour were observed, with heat-induced failure from Cu terminal contacts being the most common. Coils limited by terminal contact heating were found to be repairable. In one case, repairing the terminal connections resulted in the achievable transport current through the coil increasing by a factor of ≈5.7.

Solenoids constructed by assembling ring-shaped bulk high-temperature superconductors (HTSs) have been demonstrated to be capable of trapping a sufficiently strong and uniform magnetic field for nuclear magnetic resonance and magnetic resonance imaging. These systems, however, were magnetised using field cooling, requiring a large and expensive secondary superconducting magnet to generate the external field. Pulsed-field magnetisation (PFM), on the other hand, is a much more economical and practical method to magnetise bulk HTSs, as it can be carried out using a normal conductor (such as copper) to generate a brief pulsed magnetic field. The trapped field following PFM is found to be significantly lower than that achieved with field-cooling. This is due to the large quantity of heat generated within the bulk during the rapid change in the applied field. While this heating effect has been exploited to improve the pulse magnetisation efficiency of disc-shaped bulks, it has proven detrimental when magnetising ring-shaped bulks. Here, we present a refinement of the PFM procedure of ring-shaped bulks that can assist with increasing the trapped field. By inserting bulk HTS discs into the bore of a ring and pulse magnetising the discs before the rings, and subsequently removing the discs, we show that a higher field can be trapped in the ring.

Bulk high-temperature superconductors (HTSs) are capable of generating very strong magnetic fields while maintaining a compact form factor. Solenoids constructed using stacks of ring-shaped bulk HTSs have been demonstrated to be suitable for low-resolution nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). However, these stacks were magnetised via field cooling, which typically requires a secondary superconducting charging magnet capable of sustaining a high magnetic field for a long period. A more economical alternative to field cooling is pulsed-field magnetisation (PFM), which can be carried out with an electromagnet wound from a normal conductor, such as copper. In this work, we present an additional advantage of PFM, where the trapped field in a stack of ring-shaped bulk HTSs is iteratively homogenised by manipulating the spatial profile of the applied pulsed field. Compared to field cooling with a uniform applied field, the variation in field along the central 10 mm of the solenoid is halved using this technique. If PFM of HTS rings could be advanced further to trap higher fields, this technique could be instrumental in magnetising HTS solenoids for NMR/MRI applications.

A wide range of technologies rely on permanent magnets, including levitation devices, motors, generators and magnetic separators. Replacing permanent magnets with bulk superconductors will enable a step change in performance by providing an order of magnitude increase in the achievable magnetic field. However, the reliable fabrication of large single grained superconducting materials with a high, homogeneously distributed magnetic trapped field remains a barrier to the widespread application of these materials. Limits to the size and geometry of RE–Ba–Cu–O single grain bulk superconductors could be overcome by developing a reliable process to assemble larger components by fabricating superconducting joints between smaller samples. In this work we propose a mechanism of joint formation in GdBCO–Ag bulk superconductors using a YBCO–Ag intermediate that is based on detailed analysis of the joint interfaces. This improved understanding of the joint formation process provides the knowledge required to fully optimise the fabrication parameters, and to produce joints with improved superconducting and mechanical properties.

Shorter period undulators typically require a higher on-axis magnetic field in order to achieve a practical deflection parameter, K. Recent simulations and experiments have demonstrated that high-temperature superconducting (HTS) undulators, constructed from staggered-array bulk superconductors, can generate high undulator fields with period length as short as 10 mm. This advanced HTS technology has the potential to significantly enhance the photon energy range of synchrotron radiation light sources and free electron laser facilities. This paper reports on the progress made in developing of a 50-period bulk HTS undulator with period length of 12 mm for Shanghai soft x-ray free electron laser facility. It details the engineering design of the undulator prototype, thermal and mechanical analysis of the HTS variable temperature insert, and the current status of the system.

The magnetic field optimization of RE-Ba-Cu-O (REBCO, RE = rare earth) bulk superconducting undulators is a fundamental step toward their implementation in an accelerator driven photon source, like a synchrotron or a free electron laser. In this article, we propose a sorting algorithm to reduce the undulator’s phase error based on the reconstruction of the trapped current inside the bulks of a staggered array undulator. The results obtained with a YBCO short prototype field-cooled down to 10 K in a 10 T magnetic field are reported. Finally, its performance is critically discussed in light of 2D magnetic field maps of its individual components, obtained at LN 2 after the magnetization tests. Published by the American Physical Society 2024

The ability of bulk high-temperature superconductors to trap magnetic flux densities up to one order of magnitude larger than the saturation magnetization of conventional ferromagnetic materials offers the prospect of generating large magnetic flux density gradients. Combining multiple superconductors, akin to assembling a Halbach array of permanent magnets, may increase the generated gradient even further. The associated challenge is that superconductors are prone to demagnetization when exposed to field components perpendicular to their main magnetization direction. In the present work, we investigate the magnetic flux density gradient achieved with a pair of cubic, bulk, large-grain melt-textured superconductors in the presence of a background DC magnetic field at 77 K. We investigate the increase of the performance when decreasing the temperature down to 59 K. The studied configuration consists in two facing cubic YBa₂Cu₃O 7 − x superconductors of 6 mm side with anti-parallel magnetization directions. It is obtained after the simultaneous magnetization of the samples followed by a rotation of 180^∘ of the top superconductor. Although the background field reduces the trapped field ability of individual samples, it is shown that this phenomenon is significantly mitigated at 65 K and at 59 K compared to 77 K. The results reveal that a sample-to-sample distance ( ∼ 16 mm) of the order of their size is sufficient to avoid any mutual demagnetization effect during the rotational motion. Furthermore, it is shown that decreasing the temperature is not only beneficial in increasing the field and field gradient achieved but also in extending the range of background fields in which the superconductor can be rotated without demagnetization. This superconducting assembly yields a magnetic flux density gradient exceeding that of an isolated superconductor and has the potential to surpass the capabilities of permanent magnets.