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A REBCO Persistent-Current Switch, Immersed in Solid Nitrogen, Operating at Temperatures near 10 K

We present design and test results for a thermally-activated persistent-current switch (PCS) applied to a double pancake (DP) coil (151 mm ID, 172 mm OD), wound, using the no-insulation (NI) technique, from a 120-m long, 76-µm thick, 6-mm wide REBCO tape. For the experiments reported in this paper, the NI DP assembly was immersed in a volume of solid nitrogen (SN2), cooled to a base temperature of 10 K by conduction to a two-stage cryocooler, and energized at up to 630 A. The DP assembly operated in quasi-persistent mode, with the conductor tails soldered together to form a close-out joint with resistance below 6 nΩ. The measurements confirm PCS activation at heating powers below our 1-W design target, and a field decay time constant in excess of 900 h (i.e 0.1%/h field decay rate), limited by the finite resistance of the close-out joint

Disorder anisotropy of layered structure in multi-band MgB2 superconducting materials with high critical current performance

Layered crystal structures of various materials form through strong in-plane covalent and weaker out-of-plane bonding. The different bonding states can lead to the appearance of anisotropies not only of electronic/electrical and magnetic properties but also of structural disorder. A deeper understanding of the disorder anisotropy is essential to carry out structural modification and to enhance the material properties. However, in the case of multi-band MgB2 superconducting materials that have layered structures, including graphene-like and six-membered rings, the nature and extent of the disorder anisotropy are not well understood. Also unknown is the influence on the transport critical current performance under magnetic fields in terms of charge-carrier scattering and vortex pinning. Herein, we have investigated the disorder anisotropy to reveal the relation with the in-field superconductivity. The MgB2 phase formed by appropriate sintering conditions with carbon doping for high transport critical current performance exhibited a small anisotropy in the strain distribution and a large anisotropy in the crystallite size. The anisotropic behavior reflects small out-of-plane domains of crystallites with the strain distribution. The disordered formation may be the reason why the π band is usually dirtier than the σ band. In contrast, although the strain distribution in the in-plane structural state can be selectively tuned by carbon doping, the in-plane crystal growth is still considerably large. Such in-plane crystallization has shortcomings in terms of scattering and pinning. We therefore argue that further selective modification of the disordered structure, especially for the in-plane size properties, is a practical approach to achieve enhancement beyond the currently attainable transport performance