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The Spherical Tokamak for Energy Production (STEP) requires high-field magnet designs and has therefore adopted the REBCO-based high-temperature superconductor (HTS) as its current carrier. The HTS enables the toroidal field (TF) coils to be remountable, which unlocks STEP's vertical maintenance approach; however, remountable joints, approximately 18 GJ of stored energy and limited space down the centre of a spherical tokamak, make the TF coils the most challenging. STEP has pursued a passive approach to TF coil quench protection in order to limit coil terminal voltage. Initial results suggest that a solution may rely on tuning internal coil resistance coupled with actively powered heaters. The pre-conceptual inter-coil structure demonstrates acceptable stresses and deflections under steady-state operating conditions and preliminary fault scenarios, and loads are distributed to limit the tensile force on the TF centre rod. Finally, the HTS must operate reliably in a high radiation environment and endure high neutron fluences, ensuring commercially relevant magnet lifetimes. Initial experiments indicate that instantaneous gamma irradiation of HTS has no negative impact on current carrying capacity. Experimental programmes are underway to cold irradiate HTS to fusion-relevant fluences and to develop a method of assuring tape irradiation tolerance using oxygen ions as an analogue for neutrons. This article is part of the theme issue 'Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)'. [ABSTRACT FROM AUTHOR]

Superconducting films of YNi2B2C deposited on an Ir buffered MgO(001) substrate by pulsed laser deposition have been investigated by x-ray diffraction and transmission electron microscopy (TEM) techniques. X-ray texture results show that the Ir buffer layer grows cube on cube and that the texture of YNi2B2C consists of four c-axis components. Cross-sectional TEM shows that during high temperature deposition of YNi2B2C, Ir diffuses into the nanocrystalline film and leads to the formation of an Y0.915Ni4.12B hexagonal impurity phase instead of Y2O3 as interlayer. There are clear orientation relationships between the different phases: MgO(001)[100]|Y0.915Ni4.12B(0001)[1210]|YNi2B2C(001)[100]/(001)[110]. Y0.915Ni4.12B formation leading to deviations from stoichiometry of the YNi2B2C as well as Ni substitution by Ir may be responsible for the deterioration of the superconducting properties of the YNi2B2C film. [ABSTRACT FROM AUTHOR]

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. The findings of the spontaneous flux instabilities in the additional family of low-temperature superconductors suggest that the responsible mechanism is universal to this class of materials.

In this article the author comments on the article "Dimpling in critical current density vs. magnetic field angle in YBa2Cu3O7 films irradiated with 3-MeV gold ions," that was published in a previous issue of the journal. The author says that the main point to note is that for neither angle the proposed solution best fit to the data according to a least squares analysis.

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials. [ABSTRACT FROM AUTHOR]

One of the central themes in cuprate superconductivity is the evolution of the electronic properties with the charge carrier concentration, p, expressed as the number of holes per Cuion in the CuO2 planes. In YBa2Cu3O7-δ (YBCO), p is well known to vary with oxygen content (δ) and with substitution of Ca2+ for Y3+. In this Letter, we show that p also varies with the fraction ofCu-O "chain layer" stacking faults. The resulting parabolic variation in the superconducting transition temperature (Tc) is also well known; however, the effect on the critical current density (Jc) is less well explored. We show that the self-field Jc follows a common trend for all these hole doping methods, rising to a sharp peak on the slightly overdoped side of the Tc(p) parabola. In contrast, the in-field Jc anisotropy at high temperatures and low fields is determined solely by the stacking-fault fraction, not by hole doping, such that a sample free from stacking faults tends towards an isotropic Jc. These results serve to emphasize the role of sample microstructure in determining Jc in-field anisotropy. [ABSTRACT FROM AUTHOR]

A straightforward and reproducible method has been developed for creating <100 nm wide, 400-600 nm long c-axis columnar defects in fluorine-free MOD YBCO films (700 nm thick) using ion milling through free-standing nanoporous anodic aluminum oxide membranes positioned in contact with the films. At 77.3 K, a moderate Jc increase of 50% was achieved in self-field, whereas at 1 T a more than doubling of Jc was observed along ab, part of this enhancement coming from uncorrelated pinning effective over all field directions. X-ray analysis confirmed an additional in-plane microstrain component which can be attributed to oxygen disorder induced during the ion milling process. [ABSTRACT FROM AUTHOR]