HTS flux pumps enable superconducting currents to be directly injected into a magnet coil without the requirement for thermally inefficient current leads. Here, we present results from an experimental mechanically rotating HTS flux pump employing a coated-conductor stator and operated at 77 K. We show the effect of varying the size of the flux gap between the rotor magnets and coated conductor stator from 1 to 7.5 mm. This leads to a corresponding change in the peak applied perpendicular magnetic field at the stator from approximately 350 to 50 mT. We observe that our experimental device ceases to maintain a measurable output at flux gaps above 7.5 mm, which we attribute to the presence of screening currents in the stator wire. We show that our mechanically rotating flux pump is well described by a simple circuit model which enables the output performance to be described using two simple parameters, the open-circuit voltage V _oc and the internal resistance, R _d . Both of these parameters are found to be directly proportional to magnet-crossing frequency and decrease with increasing flux gap. We show that the trend in R _d can be understood by considering the dynamic resistance experienced at the stator due to the oscillating amplitude of the applied rotor field. We adopt a literature model for the dynamic resistance within our coated-conductor stator and show that this gives good agreement with the experimentally measured internal resistance of our flux pump.

We have manufactured lengths of up to 10 m of Roebel cable from wide strips of second generation YBCO wire. The wide strips are first screened using magnetic scanning to check for superconductor defects to avoid incorporating poor quality wire into the cable. Manufacturing issues such as the long length registration of the strand crossovers and the maximum yield from wide strip is discussed. Differences in cutting and winding due to different tape substrate materials are shown to be minimal. We report electrical measurements on samples of HTS Roebel cable. Insulation which protects the assembled conductors from environmental degradation is reported.

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 Cu ion in the CuO₂ planes. In YBa₂Cu₃O_7−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 of Cu–O “chain layer” stacking faults. The resulting parabolic variation in the superconducting transition temperature (T_c) is also well known; however, the effect on the critical current density (J_c) is less well explored. We show that the self-field J_c follows a common trend for all these hole doping methods, rising to a sharp peak on the slightly overdoped side of the T_c(p) parabola. In contrast, the in-field J_c 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 J_c. These results serve to emphasize the role of sample microstructure in determining J_c in-field anisotropy.

We have constructed test lengths of Roebel cable from wide strips of second generation YBCO wire. The strand width is 2mm to allow for lowered AC losses in comparison with standard HTS wires. Up to 10 strands can be cut from the 40mm wide strip and assembled into a 10 strand cable with a transposition length of 90mm. Electrical measurements show good retention of critical current through the cutting and cabling processes. Initial AC loss measurements confirm the reduction expected from full width wire. Results from mechanical modeling are presented which have been used to optimise strand geometry to reduce stress concentrations. Manufacturing capability to produce up to 100m lengths has been demonstrated.