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.

Magnetisation is one of the main barriers to practical use of bulk superconductors as high field magnets. Recently several authors have reported a flux jump effect that allows penetration of magnetic flux into a bulk superconductor during pulsed field magnetisation (PFM) at lower fields than that would be predicted on the basis of the Bean model. We have systematically investigated macroscopic flux jumps in single grain GdBa 2 Cu 3 O 7−δ -Ag (GdBCO-Ag) bulk superconductors with diameters of up to 30 mm when subjected to pulsed magnetic fields. Flux jumps were observed at temperatures between 30 and 77 K and in applied magnetic fields of up to 7 T. The applied pulsed field required to trigger the instability or flux jump field, B j , was determined experimentally and found to increase with decreasing temperature. An extended instability criterion based on a 2D axisymmetric model was used to predict B j at various temperatures and the results are in good agreement with experiments. A significant temperature rise has been measured experimentally during the magnetisation process which indicates that local heat generation due to the sharp rise of the applied field in the PFM process is the primary cause of the flux jumps. The experimental results suggest further that the critical current density reduces to almost zero in the warm part of the sample during the short period of non-equilibrium. A peak trapped field of 4.1 T at the surface and 5.3 T between a stack of two GdBCO-Ag bulk superconductors was achieved at 30 K by means of an optimised two-step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.

Currently, hybrid-electric aircraft are under investigation as one possible solution to reduce the emissions of the aviation industry according to Flightpath 2050 of the European Union. To meet the drive train's requirements on low mass while aiming for highest efficiency, superconducting technologies are regarded as a key enabling technology for drive train powers of several tens of megawatts. Within the German nationally funded project TELOS an exemplary mission profile and the physical measures of a 220-passenger aircraft are used to derive the requirements for a cryogenic-cooled serial hybrid-electric propulsion system. To optimize the total system performance, we subsequently evaluated the superconducting and cryogenic-cooled components by using computationally fast, analytical models. This approach allows quantifying the system performance by using component technologies being available today. In particular, the system performance of geared drive to direct drive propulsion units are compared and the influence of the DC bus voltage and the electric frequencies of the AC circuits on the mass and the efficiency of the drive trains are analysed.

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ~ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-δ bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique. These post-meltprocessing treatments, which are relatively straightforward to implement, were used to improve both the mechanical properties and the thermal stability of the resultant composite structure, providing what we believe is a promising route to achieving reliably fields of over 20 T.