Applications of Pulse-Magnetised Bulk Superconductors

Abstract

Magnetic fields are used in a wide variety of scientific disciplines, especially those concerned with the macroscopic and microscopic examination of matter. Superconductors have proved to be the most economical form of generating magnetic fields when strong magnetic fields are required. In recent times, the (RE)BaCu3O(7-δ) [(RE)BCO] class of high-temperature superconductors in bulk form have been shown to be capable of sustaining very high magnetic fields while maintaining a small form factor. Therefore, they have the potential to replace permanent magnets in a number of applications where there are significant advantages to increasing the strength of the magnetic field. Despite being able to reach magnetic field strengths an order of magnitude higher than permanent magnets, because their magnetisation typically requires a much larger superconducting magnet, their adoption in commercial technology has been slow. Pulsed-field magnetisation (PFM) is the most compact and economical method of magnetising bulk (RE)BCO. Unlike field cooling, PFM is typically unable to magnetise a sample to its full capability, though fields significantly higher than those of permanent magnets have still been reached. Research has shown that ring-shaped bulk (RE)BCO can be stacked to form solenoids which can then be field-cooled and used for nuclear magnetic resonance (NMR) spectroscopy. In this dissertation, the feasibility of using PFM to magnetise such assemblies is evaluated against the primary requirements of an NMR magnet: field strength and homogeneity. While the current distribution, and hence the field, from a conventional wound solenoid can be precisely controlled with the winding geometry, the current distribution in a bulk superconductor is strongly determined by its material properties. However, this dissertation presents a PFM technique which allows the current distribution along the axial direction to be iteratively modified, and thus homogenise the field generated by the stack. The limitations of the hardware used to demonstrate this technique are also addressed, and a more practical pulse coil design to carry out this technique is presented. The trapped field following PFM is often lower than what can be achieved with field cooling due to the large amount of heat generated within the bulk during the rapid magnetic field change to which it is subjected. While this heating effect has been exploited to improve the magnetisation efficiency (the ratio of the trapped field to the applied field) of disc-shaped bulks, it has proven detrimental when magnetising ring-shaped bulks. Presented in this dissertation is a modification to the PFM procedure of ring-shaped bulks that can increase the trapped field. By inserting a magnetised HTS disc into the bore of the ring during PFM and removing it afterwards, a significantly higher field can be trapped in the ring. The experimental results validating the method are presented, along with one potential conceptual design for a practical implementation of the technique. Undulators, used to generate high-energy X-rays in synchrotrons, make use of a strong spatially alternating magnetic field. This is another area of application in which bulk superconductors have demonstrated higher performance compared to permanent magnets. Several 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. To date, all bulk superconducting undulators have required a large secondary superconducting magnet for field cooling. This dissertation presents a concept for a bulk HTS undulator that is magnetised using PFM. The results from an experiment tailored to demonstrate the feasibility of this concept are also discussed. The work presented, investigating the applications of PFM, demonstrates that PFM has additional benefits besides low-cost and portability in some applications. However, the biggest challenge which remains to be overcome in order to see widespread adoption, and is unique to PFM, is the relatively low fields that are trapped in bulk (RE)BCO following PFM. Overcoming this barrier should be a primary focus of future research.