Portable, high magnetic field charging of bulk superconductors for practical engineering applications

当冷却到低温时，块体超导体可以用作产生几特斯拉的场的超强度、稳定的永磁体，相比之下，常规永磁体例如钕磁体（Nd-Fe-B）的1.5-2特斯拉极限。这使得它们对许多依赖高磁场的工程应用具有吸引力，包括具有前所未有的功率密度的紧凑型节能电机/发电机以及紧凑型便携式磁共振成像（MRI）和核磁共振（NMR）系统。现在科学家也可以使用高磁场来利用材料的磁性来控制化学和物理过程，例如，这对于磁性分离和磁性药物输送系统（MDDS）来说是有吸引力的。大块超导体磁体的主要优点是其有效磁场可以比传统的永磁体高一个数量级（已经证明大块高温超导体能够捕获大于17特斯拉的磁场）并且不需要电源和直接连接来供应产生磁场的电流，大块超导体的磁化过程基本上包括施加和去除大磁场，该大磁场在材料中感应出循环的超导电流，该超导电流在没有阻力的情况下流动。然而，目前面临的一个重大挑战性问题是实现简单、可靠和便携的充电技术来磁化这种超导体，这对于为高场、捕获磁通型超导应用产生有竞争力的紧凑设计至关重要。目前，用于磁化大块超导体的最知名的方法实际上是脉冲场磁化（PFM）技术，由此经由毫秒量级的脉冲施加大磁场。然而，使用PFM的世界纪录在29 K时仅为5.2特斯拉，这远远低于这些材料的真实能力。PFM技术有许多设计考虑：脉冲的幅度和持续时间，施加脉冲的数量，磁化线圈/夹具的类型和形状，块状超导体如何冷却，以及施加脉冲的温度。所有这些考虑因素将通过数值建模进行分析，以彻底优化PFM设置，考虑到便携式，高场磁体系统。通过实验结果验证的数值建模是解释磁化过程中实验结果和材料物理机制的一种特别重要且具有成本效益的方法。这种建模工具也可以用来预测和提出新的磁化技术，这是更难以实现的实验。该研究计划的主要目标是开发用于实际工程应用的散装超导体的便携式高磁场充电，最终目标是生产便携式和商业可行的高磁场磁体系统。这将通过定制大块超导体的材料加工和性能以及高磁场应用的磁体几何形状来支撑，开发用于完整电磁-热-机械分析的数值模型，以避免涉及高磁场时潜在的机械断裂（> 6-7特斯拉）并进行实验以验证这种模型，以及考虑到上述所有设计考虑因素的优化PFM技术的开发。将围绕螺线管和分裂式磁化线圈开发两种类型的脉冲充电系统，这些系统将用于在高于40 K的温度下实现超过5特斯拉的捕获场，这是目前的记录，并作为特定应用定制设计的概念验证。

Bulk superconductors can be used, when cooled to cryogenic temperatures, as super-strength, stable permanent magnets generating fields of several Tesla, compared to the 1.5-2 Tesla limit for conventional permanent magnets, such as neodymium magnets (Nd-Fe-B). This makes them attractive for a number of engineering applications that rely on high magnetic fields, including compact and energy-efficient motors/generators with unprecedented power densities and compact and portable magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) systems. It is now also possible for scientists to use high magnetic fields to exploit the magnetism of a material for controlling chemical and physical processes, which is attractive for magnetic separation and magnetic drug delivery systems (MDDS), for example. The chief advantage of a bulk superconductor magnet is that the available field can be up to an order of magnitude higher than conventional permanent magnets (bulk high-temperature superconductors have been shown to be capable of trapping magnetic fields greater than 17 Tesla) and no power supply and direct connection is necessary to supply the current producing the magnetic field, as in electromagnets.The magnetisation process of a bulk superconductor essentially involves the application and removal of a large magnetic field that induces a circulating supercurrent in the material that flows without resistance. However, one significantly challenging problem currently faced is achieving a simple, reliable and portable charging technique to magnetise such superconductors, and this is crucial to producing competitive and compact designs for high-field, trapped flux-type superconducting applications. The current, best-known method for magnetising bulk superconductors practically is the pulsed field magnetisation (PFM) technique, whereby a large magnetic field is applied via a pulse on the order of milliseconds. However, the world record using PFM is only 5.2 Tesla at 29 K, which is much less than the true capability of these materials. The PFM technique has many design considerations: the magnitude and duration of the pulse(s), the number of applied pulses, the type and shape of the magnetising coil/fixture, how the bulk superconductor is cooled, and the temperature(s) at which the pulse(s) are applied. All of these considerations will be analysed through numerical modelling in order to thoroughly optimise the PFM setup in view of a portable, high-field magnet system. Numerical modelling, validated by experimental results, is a particularly important and cost-effective method to interpret experimental results and the physical mechanisms of the material during the magnetisation process. Such modelling tools can also be used to predict and propose new magnetising techniques, which is more difficult to achieve experimentally. The primary objective of this research programme is to develop portable, high magnetic field charging of bulk superconductors for practical engineering applications, with an end goal of producing portable and commercially-viable high-field magnet systems. This will be underpinned by the tailoring the material processing and properties of bulk superconductors and magnet geometry for high field applications, developing numerical models for complete electromagnetic-thermal-mechanical analysis to avoid potential mechanical fracture when high magnetic fields are involved (> 6-7 Tesla) and carrying out experiments to validate such models, and the development of an optimised PFM technique that takes into account all of the design considerations above. Two types of pulsed charging systems will be developed around solenoid- and split-type magnetising coils, which will be used to achieve trapped fields in excess of 5 Tesla, the current record, at temperatures greater than 40 K and as a proof-of-concept for bespoke designs for specific applications.