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.
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