The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate remarkable engineering current density (averaged over the full wire), JE, at very high magnetic fields, driving progress in fusion and other applications. The key challenge for HTS wires has been to offer an acceptable combination of high and consistent superconducting performance in high magnetic fields, high volume supply, and low price. Here we report a very high and reproducible JE in practical HTS wires based on a simple YBa2Cu3O7 (YBCO) superconductor formulation with Y2O3 nanoparticles, which have been delivered in just nine months to a commercial fusion customer in the largest-volume order the HTS industry has seen to date. We demonstrate a novel YBCO superconductor formulation without the c-axis correlated nano-columnar defects that are widely believed to be prerequisite for high in-field performance. The simplicity of this new formulation allows robust and scalable manufacturing, providing, for the first time, large volumes of consistently high performance wire, and the economies of scale necessary to lower HTS wire prices to a level acceptable for fusion and ultimately for the widespread commercial adoption of HTS.

Temperature and magnetic field dependences of the anomalous Hall effect have been investigated for single crystals of Mo-based ferromagnets with pyrochlore structure. The Hall resistivity of Nd 2 Mo 2 O 7 compound shows unconventional temperature dependence whereas Gd 2 Mo 2 O 7 exhibits rather normal behaviour. The Berry phase model can explain the difference well; it is attributed to the difference in nature of the anisotropy of the rare-earth moment and the resultant Mo spin state. The Hall resistivity of Nd 2 Mo 2 O 7 changes its sign with increasing field applied along the [111] direction, while it monotonically approaches zero with the field applied along the [100] or [110] direction. This behaviour is also in accord with the prediction of the Berry phase theory.

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We fabricated (Ba,Na)Fe 2 As 2 superconducting wires using the powder-in-tube method and hot isostatic pressing. By improving powder synthesis processes compared with previous studies, highly pure raw materials for the wire fabrication were obtained. The largest transport critical current density (J c ) reached 40 kA cm −2 at T = 4.2 K under a magnetic field of 100 kOe. This value exceeds not only the value of transport J c of the previous (Ba,Na)Fe 2 As 2 wire but also those of all iron-based superconducting round wires. Improvements of polycrystalline powder synthesis play a key role for the enhancement of J c . Furthermore, it was clarified that higher densification by high-pressure sintering and the texturing of grains in the core of the wire due to drawing also increased J c effectively. Advantages of (Ba,Na)Fe 2 As 2 wires compared with (Ba,K)Fe 2 As 2 wires are also discussed.

We report the fabrication of small (Ba,A)Fe 2 As 2 (A: Na, K) coils using 10 m-class long round wires, fabricated by powder-in-tube (PIT) method. Coils are sintered using hot-isostatic-press (HIP) technique after glass-fiber insulations are installed. Critical current (I c ) of the whole coil using (Ba,Na)Fe 2 As 2 and (Ba,K)Fe 2 As 2 are 60 A and 66 A under the self-field, and the generated magnetic fields at the center of the coil reach 2.6 kOe and 2.5 kOe, respectively. Furthermore, the largest transport critical current density (J c ) and I c in (Ba,Na)Fe 2 As 2 wires picked up from the coil reach 54 kAcm −2 and 51.8 A at T = 4.2 K under a magnetic field of 100 kOe, respectively. This value exceeds transport J c of all previous iron-based superconducting round wires. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of J c .