Anthony R. Dennis

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.

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.

A number of benefits result from reducing the period length of an undulator without sacrificing magnetic field strength, such as an increased photon flux and higher energy synchrotron radiation. Replacing permanent magnets with bulk high-temperature superconductors (HTSs) has proved to be an effective path towards this goal. We present a concept for a bulk HTS undulator that is magnetized via pulsed field magnetization, which is a much more compact and economical form of magnetizing HTS bulks compared to the field-cooling or zero-field cooling methods that are typically employed. The results from an experiment tailored to demonstrate the feasibility of this concept are also presented. With these results, we estimate that an undulator field of 2.3 T could be generated using this design with a magnetic gap of 4 mm and a 16 mm period length, operating at 40 K. Published by the American Physical Society 2024

Drug delivery into the brain and spinal cord is fundamentally limited by the blood‐brain barrier which impedes the use of the vast majority of drugs. Implants based on iontophoresis use an applied voltage to deliver charged drug molecules, allowing solvent‐free delivery directly into the site of interest and overcoming issues associated with systemic exposure to the drug. However, during continuous delivery over long periods, electrochemical reactions occur at the electrodes leading to corrosive gas formation. Here, the concept of redox flow iontophoresis is presented, where a redox mediator solution is used to control electrode reactions and sustain continuous delivery for theoretically unlimited duration. As a proof‐of‐concept, a redox flow iontophoresis‐based brain implant that can continuously deliver the cancer drug doxorubicin at stable rates exceeding 2 nmol min⁻¹ is demonstrated. This new concept enables the continuous delivery of various potent drugs into the brain and spinal cord and therefore has the potential to improve treatment options for various diseases.

The mechanical and flux trapping properties of 19 EuBCO(Ag) and 20 thin‐wall EuBCO(Ag) bulk superconductors were investigated statistically and compared to those observed for YBCO using Brazilian testing and high‐field magnetization. The average trapped fields of EuBCO(Ag) and thin‐wall EuBCO(Ag) bulk superconductors were significantly higher than those of YBCO bulk samples at 77 K measured under similar experimental conditions. However, due to the wide range of trapped field data for both materials, a statistical overlap was observed between the highest trapped fields observed for YBCO and the lowest values observed for EuBCO(Ag) bulk superconductors. All EuBCO(Ag) bulk materials measured in this study exhibit a superior tensile strength compared to YBCO bulk single grains. Unlike the case of YBCO, however, the introduction of artificial holes to EuBCO(Ag) bulk superconductors prior to melt processing was observed to decrease the average tensile strength of the single grain. Surprisingly, despite their superior mechanical and flux pinning properties, EuBCO(Ag) bulk superconductors do not exhibit enhanced performance when exposed to high fields at low temperatures.

Assembling trapped-field superconducting magnets with mutually orthogonal magnetizations directions in a Halbach array configuration offers the prospect of generating both high fields and large field gradients. A major issue when assembling bulk superconductors in a Halbach array, however, consists in the alteration of the initial current density distribution during the assembly process. This reorganization of supercurrent loops limits the field generated by the system. We investigate two methods for reducing this demagnetization effect. The first method consists of using stacked tapes instead of bulk superconductors. For the second method, we propose a procedure leading to a re-magnetizing the superconductors of the array after the assembly. The procedure consists in putting two superconductors on top of one another, magnetizing them along the vertical direction, and then keeping the pair in place while two other superconductors, magnetized in an horizontal direction, are approached from left and right. The top central sample is then removed from the array, thereby providing the desired re-magnetization of the bottom one. The benefits of this procedure was investigated by finite element modelling and experiments carried out at 77 K both with bulk YBa₂Cu₃O 7 − x superconductors ( ∼ 14 × 14 × 14 mm³) and with stacks of second generation YBa₂Cu₃O 7 − x tapes from Superpower ( ∼ 12 × 12 × 12 mm³). The flux density measured above the array is compared to analytical results and finite element simulations. The results show that a re-magnetization of the central sample occurs, which allows the maximum field generated with Halbach arrays made of three bulk superconductors or three stacked tapes to be increased by 5% and 11% respectively. Numerical modelling shows that using a taller top sample with this method allows to recover almost the full potential of the array.