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

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.

Room-temperature superconductivity was considered in the EPSRC grand challenges survey (2011) as a technology that could lead to tremendous economic and societal benefits in a number of areas, such as energy and healthcare. However, until such room-temperature superconductors are discovered, superconducting technology is strongly underpinned by the need for cryogenics, and UK-wide cryogenic activities contribute an estimated £170 million total direct GVA (gross value added) with around 1,500 people employed across around 120 companies.

The development of a portable, compact and efficient pulse charging system that can readily achieve fields of 5 Tesla or more, with inexpensive, off-the-shelf cryogenic cooling technology, will bring about a step change in applications that currently use permanent magnets, as well as technology enabled by such high fields. An industry advisory board will be formed for the project, who will meet annually during the project to advise on key technological and strategic elements of the project. With their combined knowledge and experience, the board will advise on material selection, cryogenic options, and the magnetic fields required from an applications viewpoint. A portable, high magnetic field system can be directly exploited by a variety of industries and for further academic research: healthcare (portable magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) systems, and magnetic drug delivery systems (MDDS)) and electrical power (compact, energy-efficient electrical machines) are two prime examples. A compact MRI machine that would allow small-scale, targeted scanning, for example, would complement conventional whole-body scanners that are expensive and have a large footprint because of the low-temperature superconducting wire used to wind their coils.

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 MDDS. With the continued development of conventional superconducting magnets and the achievement of higher magnetic fields, even the chemical and physical processes associated with diamagnetic materials, which make up many of the materials found on earth, are significantly influenced. Portable, high-field magnets based on bulk superconductors are therefore highly attractive for magnetically-oriented growth of organic semiconductors and carbon nanotubes (CNTs). CNTs have recently emerged as one of the most important nanomaterials with the potential to drive the next industrial revolution and the UK is also well-placed at the forefront of this particular field.

The proposed market feasibility studies and the advisory board will enable accelerated diffusion of the technology by identifying the most promising applications, market sizes and the best routes to market. The portable, high magnetic field system developed during the project, aiming at providing magnetic fields > 5 Tesla at temperatures > 40 K, would be used as a proof-of-concept for bespoke designs for these applications in cooperation with an industry partner.