Progress in superconducting bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both the materials science of bulk superconducting materials and the technologies required to use them effectively in engineering applications. In particular, magnetised bulk superconductors are capable of acting as quasi-permanent magnets with the potential of providing magnetic fields of several tesla or greater from a small volume of material, they can act as magnetic shields and they can provide self-stabilised levitation. This roadmap, based on a workshop which involved the participation of a wide range of academic and industrial participants (see doi: 10.17863/CAM.586 for details of the workshop methodology), aims to explore some of the key potential domains of application of bulk superconductors. Detailed technological roadmaps are presented for four key applications that were identified as providing both good market opportunity and feasibility. These are: portable systems for bulk superconductivity; portable, high-field magnet systems for medical devices; ultra-light superconducting rotating machines for next-generation transport & power applications; and magnetic shielding applications for electric machines, equipment and other high-field devices.

Investigating and predicting the magnetization of bulk superconducting materials and developing practical magnetizing techniques is crucial to using them as trapped field magnets (TFMs) in engineering applications. The pulsed field magnetization (PFM) technique is considered to be a compact, mobile and relative inexpensive way to magnetize bulk samples, requiring shorter magnetization times (on the order of milliseconds) and a smaller and less complicated magnetization fixture; however, the trapped field produced by PFM is generally much smaller than that of slower zero field cooling (ZFC) or field cooling (FC) techniques, particularly at lower operating temperatures. In this paper, the PFM of two, standard Ag-containing Gd-Ba-Cu-O samples is carried out using two types of magnetizing coils: 1) a solenoid coil, and 2) a split coil, both of which make use of an iron yoke to enhance the trapped magnetic field. It is shown that a significantly higher trapped field can be achieved using a split coil with an iron yoke, and in order to explain these how this arrangement works in detail, numerical simulations using a 2D axisymmetric finite element method based on the H-formulation are carried to qualitatively reproduce and analyse the magnetization process from both electromagnetic and thermal points of view. It is observed that after the pulse peak significantly less flux exits the bulk when the iron core is present, resulting in a higher peak trapped field, as well as more overall trapped flux, after the magnetization process is complete. The results have important implications for practical applications of bulk superconductors as such a split coil arrangement with an iron yoke could be incorporated into the design of a portable, high magnetic field source/magnet to enhance the available magnetic field or in an axial gap-type bulk superconducting electric machine, where iron can be incorporated into the stator windings to 1) improve the trapped field from the magnetization process, and 2) increase the effective air-gap magnetic field.

Investigating, predicting and optimising practical magnetization techniques for charging bulk superconductors is a crucial prerequisite to their use as high performance 'psuedo' permanent magnets. The leading technique for such magnetization is the pulsed field magnetization (PFM) technique, in which a large magnetic field is applied via an external magnetic field pulse of duration of the order of milliseconds. Recently "giant flux leaps" have been observed during charging by PFM: this effect greatly aids magnetization as flux jumps occur in the superconductor leading to magnetic flux suddenly intruding into the centre of the superconductor. This results in a large increase in the measured trapped field at the centre of the top surface of the bulk sample and full magnetization. Due to the complex nature of the magnetic flux dynamics during the PFM process simple analytical methods, such as those based on the Bean critical state model (CSM), are not applicable. Consequently, in order to successfully model this process, a multi-physical numerical model is required, including both electromagnetic and thermal considerations over short time scales. In this paper, we show that a standard numerical modelling technique, based on a 2D axisymmetric finiteelement model implementing the H-formulation, can model this behaviour. In order to reproduce the observed behaviour in our model all that is required is the insertion of a bulk sample of high critical current density, J c. We further explore the consequences of this observation by examining the applicability of the model to a range of previously reported experimental results. Our key conclusion is that the "giant flux leaps" reported by Weinstein et al. and others need no new physical explanation in terms of the behaviour of bulk superconductors: it is clear the "giant flux leap" or flux jump-assisted magnetisation of bulk superconductors will be a key enabling technology for practical applications.

The paper contains a review of recent advancements in rotating machines with bulk high-temperature superconductors (HTS). The high critical current density of bulk HTS enables us to design rotating machines with a compact configuration in a practical scheme. The development of an axial-gap-type trapped flux synchronous rotating machine together with the systematic research works at the Tokyo University of Marine Science and Technology since 2001 are briefly introduced. Developments in bulk HTS rotating machines in other research groups are also summarized. The key issues of bulk HTS machines, including material progress of bulk HTS, in situ magnetization, and cooling together with AC loss at low-temperature operation are discussed.

A study has been made of the decay of the trapped magnetisation in superconductors when exposed to a crossed field. Numerical results have been compared with the theory of Brandt and Mikitik (4) which solves the problem for a thin strip superconductor. FlexPDE with the A formulation and COMSOL with the H formulation were both used. Simulations of a strip with a cross section aspect ratio of 20 showed good agreement with theory both for the case of a transverse field larger than the transverse penetration field and for one smaller. In the latter case the magnetisation saturates as predicted, however the simulations show a slow decay after many cycles. In the case of stacked YBCO tapes the movement of flux lines is very small and the effects of the reversible motion were investigated. This can decrease the decay initially for very thin decoupled tapes, but cause a steady decay after very large numbers of cycles. Simulations on stacked strips showed that the decay constant increased approximately linearly with the number of strips. When combined with the theory for one tape this can explain the very slow decay observed in previous experiments. Experimental results were qualitatively in agreement with theory and simulations but showed some discrepancies. However there are a number of differences between the experimental situation and theory so good agreement is not expected. IntroductionIn most applications of bulk superconductors using trapped fields the bulk will be exposed to external fields which will change in both angle and magnitude. While changes in in magnitude are easily incorporated in the Bean model, changes in angle lead to much more complex situations, in many cases involving flux cutting and force free configurations. However many experimental measurements have shown that in general trapped fields are always reduced by the application of crossed fields and this effect may limit the application of bulk superconductors in electrical machines. Most of the experiments were at relatively high fields compared with the crossed fields and directed at elucidating the physics, rather than practical applications of bulk superconductors in motors and generators. In this paper we consider geometries in which the induced currents are mostly perpendicular to the fields so that the standard Bean model can be applied. One of the first observations was that of Sychev et. al. (1), who applied an oscillating field to a wire carrying a current and observed a DC voltage. Sakamoto et. al. (2) used a thin film carrying a current in an applied field and measured the decay of the trapped magnetisation when a transverse field was applied, as in this paper. They showed that the flux lines pivoted about two points alternately and 'walked' out of the film, a bit like penguins heading for their breeding grounds in the Antarctic. They proposed an approximate theory which assumed a uniform current density in the film. The DC voltage generated by an AC field was termed the 'dynamic resistance' and the clearest explanation of t...