Recent progresses in the second generation REBa 2 Cu 3 O 7 − x (RE123) coated conductor (CC) have paved a way for the development of superconducting solenoids capable of generating fields well above 23.5 T, i.e. the limit of NbTi−Nb 3 Sn-based magnets. However, the RE123 magnet still poses several fundamental and engineering challenges. In this work we review the state-ofthe-art of conductor and magnet technologies. The goal is to illustrate a close synergetic relationship between evolution of high-field magnets and advancement in superconductor technology. The paper is organized in three parts: (1) the basics of RE123 CC fabrication technique, including latest developments to improve conductor performance and production throughput; (2) critical issues and innovative design concepts for the RE123-based magnet; and (3) an overview of noteworthy ongoing magnet projects.

The peculiar features of magnesium diboride make it suitable for many potential applications. In the last four years of investigation of MgB 2 , the scientific community's research was mainly motivated by features such as low cost, compatibility with straightforward processing methods, relatively high critical temperature, and absence of weak links between grains. Other applications suggest the accentuation of other MgB 2 properties. In particular its very low mass density makes it attractive for space activities, where the cost per kilogram for orbiting is still a huge obstacle, e.g. the cost of transport to low Earth orbit can reach 15 k$ kg −1 . In order to promote the use of this compound for space activities, we tested titanium as a sheath material. Titanium is non-magnetic and its alloys are almost half the weight of steel but with yield stresses up to three times higher. We fabricated Ti-sheathed MgB 2 through the powder-in-tube process. These wires show similar results to those sheathed with Fe. At 4 K the critical current density J c is well above 7 × 10 5 A cm −2 . The interface between the superconducting intermetallic compound and the lightweight sheath does not show any evidence of reaction and diffusion up to 900 • C. An analysis was carried out using x-ray diffraction, scanning electron microscopy with an energy dispersive spectrometer, and electron microprobe analysis.

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The design of next-generation magnets based on 2G high-temperature superconductors requires a careful investigation of the possibilities and performance of joints between different superconducting wire pieces. In this paper, we present a study of the electrical performance of splices made with commercial Superpower ReBCO tapes at both 77 K and 4.2 K. For this study we developed a special splicing device based on the hot-bar soldering technique that allowed estimating the degree of reproducibility of the splicing method in view of future standard series production. During our study we investigated the influence of the splicing parameters (temperature, pressure, duration, flux fluid, solder composition, cooling etc.) on the final joint performance in terms of specific resistance, critical current degradation and n-value degradation. The best performing splice was characterized at 4.2 K in perpendicular magnetic field up to 11 T. The results show an excellent degree of reproducibility with no appreciable degradation of the original tape performances. The ultimate splice resistance seems determined by the intrinsic architecture of the tape rather than by the splicing method itself.

The current-carrying capability of superconducting wires is degraded by stress. Therefore electromechanical properties are one of the key feedback parameters needed for progress in conductor applications. In this work, uniaxial tensile stresses and bending stresses were applied to Fe /MgB2 wires at room temperature, followed by measurement of critical current using a transport method at 4.2 K. Basic mechanical properties were calculated from the measured stress–strain characteristics. The irreversible tensile strain at which the critical current density of MgB2 wire starts to degrade was found to be 0.5%. In addition, the degradation of Ic with decreasing bending diameters was found to be very rapid for wires that were deformed after the heat treatment that forms the MgB2 compound, while not much degradation of Ic was found for wires that were bent before being annealed. SEM observations confirmed that cracks could be healed by post-annealing.

The thermal stability of superconducting wires is one of the important issues for wire applications. We present a numerical study on the effect of the wire design parameters on the quench behavior of superconducting MgB 2 wire employed in coils. The model considers a stack of MgB 2 wires of rectangular cross section separated by insulation layers and subjected to a thermal disturbance. The problem is solved on a two-dimensional domain and employs the current sharing concept in the transition between superconducting and normal states. The effects of three design parameters in wire manufacturing are investigated. Quench behavior is compared for wires having different filling factor of superconducting filaments, different volume of copper stabilizer, and different residual resistivity ratio (RRR) values for copper. The results indicate that the quench propagation velocity (QPV) at 1.5 T is weakly affected by changes in the volume and electrical properties of copper, whereas the minimum quench energy (MQE) is strongly dependent on the RRR value of copper and can increase by a factor of nearly 2 with the RRR varying from 30 to 150. Both the MQE and QPV change remarkably by varying the MgB 2 filling factor. The MQE drops by a factor of 6 and the QPV increases by a factor of 2 with the filling factor varying from 10.5% to 25%.

The degradation of the superconducting properties of REBa 2 Cu 3 O 7−x coated conductors when heated above 150 °C is a topic of concern for any REBa 2 Cu 3 O 7−x -based applications. This study makes it clear the role of thermally activated oxygen out-diffusion processes in the modification of the superconducting properties of commercial coated conductors heat treated in air. We point out that oxygen can diffuse out of the REBa 2 Cu 3 O 7−x layer along two main channels: one associated with the grain boundaries and a second one associated with the superconducting grains. The activation energy is lower for the oxygen outdiffusion events that take place at the grain boundaries, and this is the main cause of the degradation of the critical current at temperatures lower than ∼250 °C. More specifically, through a systematic investigation of the electrical and magnetic properties, we demonstrate that the decrease of the critical current upon heating the coated conductor is initially due to a loss in the transparency of the REBa 2 Cu 3 O 7−x grain boundaries, which determines a reduction of the effective superconductor cross section available for the superconducting current. The degradation of the intragrain properties becomes important only above ∼250 °C and manifests itself through a simultaneous variation of the critical current and of the critical temperature.

Working towards developing lightweight superconducting magnets for future space and other applications, we have successfully fabricated mono-core Ti-sheathed MgB 2 wires by the powder-in-tube method. The wires were characterized by magnetization, electrical resistivity, x-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry measurements. The results indicate that the Ti sheath does not react with the magnesium and boron, and the present wire rolling process can produce MgB 2 wires with a superconducting volume fraction of at least 64% in the core. Using the Bean model, it was found that at 5 K, the magnetic critical current densities, J c , measured in magnetic fields of 0, 5, and 8 T are about 4.2 × 10 5 , 3.6 × 10 4 , and 1.4 × 10 4 A cm −2 , respectively. At 20 K and 0 T, the magnetic J c is about 2.4 × 10 5 A cm −2. These results show that at zero and low fields, the values of the magnetic J c for Ti-sheathed MgB 2 wires are comparable with the best results available for the Fe-sheathed MgB 2 wires. At high fields, however, the J c for Ti-sheathed MgB 2 wires appears higher than that for the Fe-sheathed MgB 2 wires.