DC characterization of advanced fine-filamentary MgB2 superconducting wires.

MgB2 wires with 114, 192 and 342 filaments of size 14-19 µm manufactured by HyperTech Research, Inc. have been subjected to low temperature DC measurements. R(T), I-V characteristics, critical currents and stress and strain tolerances of these wires differing by filament architecture and filament size sheathed by resistive CuNi alloys were measured and compared with the literature data. It was found that these fine-filamentary wires have high engineering current densities not reduced by twisting up to 10 mm, sufficient strain tolerances and therefore are promising for future applications where minimised AC losses are required due resistive sheaths, thin MgB2 filaments and short twist pitches.

Degradation mechanism of the superconducting performance of a Bi2212 cable under magnetic fields

In magnetic confinement fusion reactors, superconducting magnet systems are essential for generating and controlling high magnetic fields. To increase the magnetic field, new superconducting materials such as Bi2212 (Bi2Sr2CaCu2O8+x) have been selected in the design of magnet systems. However, the stability of the Bi2212 superconductor under magnetic fields must be studied for the routine and safe operation of magnet systems. In this work, the stability and degradation mechanism of a Bi2212 cable under magnetic fields were investigated. With a magnetic field of 5.8 T, the cable carrying 29 kA was exerted with a force of ~168.2 kN per meter. In the core area of the cable, moved wires were detected by computed tomography (CT). The macroscopic movement of the wires would vary with the axial position, which could be related to the twist structure. Then, the cable was decomposed, and the acquired wires were tested under 12 T at 4.2 K by four-probe method. The results indicated that the inner wires had relatively lower critical currents, which should be the reason for the degradation of cable performance. Scanning electron microscope (SEM) images of the superconducting phase within the wires confirmed that cracks existed in the superconducting phase of the inner wires, while intact crystals were found in that of the outer wires. The variation in microstructures gave rise to changes in the wire performance.

Design, fabrication and testing of a coated conductor magnet for electrodynamic suspension

Coated conductor magnet, as the onboard magnet of the electrodynamic suspension (EDS) train, is deemed promising due to its relatively high operating temperature, low cooling cost, and good mechanical tolerance, making the liquid-helium-free high-temperature superconducting (HTS) EDS train possible. In order to promote the progress of the HTS EDS train, this work aims at designing, fabricating and testing a coated conductor magnet as the onboard magnet of EDS train. The HTS magnet is designed with the comprehensive considerations of the electromagnetic calculation, thermal-mechanical coupling analysis, as well as the heat load estimation. The magnet is conduction-cooled without any coolant. A radiation shield was used to reduce the heat leakage, enabling the cryogenic system to provide a better low-temperature environment for the magnet. Through a deliberate design, the magnet was fabricated, including two HTS coils and the tailored cryogenic system. Afterwards, the electromagnetic and thermal performances of this magnet were tested and analysed in detail. It was proven that the magnet can be cooled to below 15 K; besides, the magnet has been successfully charged to 240 A. Further increase in the current is possible because of the high safe margin of the critical currents for both the HTS magnet and its current lead, although a slight performance degradation was observed on two double-pancake coils inside the magnet. The present study will provide useful implications for the design and application of onboard HTS magnets in EDS train.

Superconducting Order Parameter Nucleation and Critical Currents in the Presence of Weak Stray Fields in Superconductor/Insulator/Ferromagnet Hybrids

The stray fields produced by ferromagnetic layers in Superconductor/Insulator/Ferromagnet (S/I/F) heterostructures may strongly influence their superconducting properties. Suitable magnetic configurations can be exploited to manipulate the main parameters of the hybrids. Here, the nucleation of the superconducting phase in an external magnetic field that periodically oscillates along the film width is studied on the base of the numerical solution of the linearized system of Usadel equations. In addition, the effect of the magnetic configuration of the F-layer on the temperature dependence of the critical current density, Jc(T), is investigated in the framework of the Ginzburg–Landau phenomenological theory on the base of the oscillating model of a stray field. By following this approach, the Jc(T) dependence of a Nb/SiO2/PdNi trilayer is reproduced for different magnetic configurations of the PdNi layer.