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Creating Portable, High-Field Magnet Systems Using Bulk Superconductors 2018-05-16
A comparison of the magnetic field available from a conventional permanent magnet, a bulk superconductor and an electromagnet, and the mechanism by which each magnet generates its magnetic field.
Superconductors are materials that carry large electrical currents with little or no resistance when cooled below a certain cryogenic temperature. The current carried by a superconductor also generates a magnetic field and a magnetized bulk superconductor— which is similar in appearance to an ice hockey puck— can be used as a super-strength, quasi-permanent magnet, generating fields of several Tesla.
In 2014, the Bulk Superconductivity Group in the Department of Engineering, University of Cambridge, broke a world record by trapping 17.6 tesla in a stack of two bulk, high-temperature superconductors at 26 K, leapfrogging the previous record of 17.24 tesla at 29 K that stood for over a decade. This was recognized by the Guinness World Records in 2016. This is an order of magnitude higher than the 1.5-2 tesla limit for applications using conventional permanent magnets (PMs), such as neodymium magnets (Nd-Fe-B), making these materials extremely 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 portable magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) systems.
Furthermore, compared with electromagnets (copper-wound or superconducting), no direct, continuous connection to a power supply is necessary and the size of the magnet to provide the same field is considerably smaller. Figure 1 shows a comparison of the magnetic field available from a conventional PM, a bulk superconductor and an electromagnet, and the mechanism by which each magnet generates its magnetic field.
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. Such applications rely on the force exerted on a magnetic particle that is proportional to both the magnetic field, B, as well as the gradient of the magnetic field, dB/dr, which is naturally large in a magnetised bulk superconductor. By using a high enough magnetic field, even the chemical and physical processes associated with diamagnetic materials, which make up many of the materials found on earth, are significantly influenced. This has led to observations of the Moses effect (where the free surface of water is deformed by a magnetic field of several Tesla), magnetic levitation of diamagnetic materials, and magnetic orientation of organic polymers and gels and carbon nanotubes, which is particularly attractive for improving crystal growth of organic semiconductors and other materials.
One significantly challenging problem currently faced by researchers in the field is achieving a simple, reliable and portable charging technique to magnetise such superconductors. This is crucial to producing competitive and compact designs for practical commercial applications. 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, ‘trapping’ magnetic flux within the material (hence why they are also known as ‘trapped field magnets’).
This can be done in a number of ways:
Field-cooling magnetisation (FCM), where a large magnetic field is applied to a bulk superconductor above its superconducting transition temperature, Tc, which is subsequently cooled to a particular cryogenic operating temperature, Top, then the field is removed;
Zero-field-cooling magnetisation (ZFCM), where the bulk superconductor is firstly cooled to Top without any background magnetic field, then such a field is applied and removed; and
Pulsed-field magnetisation (PFM), which is similar to ZFCM, but the magnetic field is applied via a pulsed current on the order of milliseconds.
The latter, PFM, is currently recognised as the most practical method for magnetising bulk superconductors in engineering applications because FCM and ZFCM techniques require long magnetising times (typically on the order of hours) and bulk and expensive equipment (usually a large superconducting coil). In contrast, PFM has a much more compact and less complicated magnetisation fixture (usually involving a capacitor bank discharging its stored energy through a copper coil). However, the trapped field using PFM is much less than that achieved by the FCM and ZFCM techniques, particularly at lower operating temperatures, because of the large temperature rise, ΔT, associated with the rapid dynamic movement of magnetic flux within the material during the pulse. The world record using PFM, using a modified multi-pulse, stepwise-cooling (MMPSC) technique, is only 5.2 Tesla at 29 K, which is much less than the true capability of these materials as indicated earlier. Due to the large number of variables for PFM, including the pulse magnitude and duration, number of pulses applied, operating temperature(s), type and shape of magnetising coils, use of ferromagnetic materials to shape/enhance the applied/trapped field and so on, much of the research landscape is still to be explored and optimised to obtain the highest trapped magnetic fields possible.
Dr Mark Ainslie, who leads the modelling and applications research team in the Bulk Superconductivity Group, has just begun a five-year, £1.1 million research Early Career Fellowship from the Engineering and Physical Sciences Research Council (EPSRC), UK, to develop portable, high magnetic field charging of bulk superconductors for practical applications. He will be combining his state-of-the-art numerical modelling expertise with his extensive knowledge of the PFM technique to produce portable and commercially-viable high field magnet systems. Dr Ainslie will be working with project partners from Adelwitz Technology Centre, Cryox Limited, Oxford Instruments and Siemens Magnet Technology to accelerate the development of the technology, as well as his close academic collaborator in Japan, Professor Hiroyuki Fujishiro from Iwate University, the world-leading expert in PFM who holds all PFM magnetic field records to date.
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