Measurements of AC losses in different former materials

A high temperature superconducting cable may be based on a centrally located cylindrical support, a so-called former. If electrically conductive, the former can contribute to the AC losses through eddy current losses caused by unbalanced axial and tangential magnetic fields. With these measurements we aim at investigating the eddy current losses of commonly used former materials. A one layer cable conductor was wound on a glass fibre reinforced polymer (GFRP) former. By inserting a variety of materials into this, it was possible to measure the eddy current losses of each of the former candidates separately; for example copper tubes, stainless steel braid, copper braid, corrugated stainless steel tubes, etc. The measured data are compared with the predictions of a theoretical model. Our results show that in most cases, the losses induced by eddy currents in the former are negligible. However, for materials with a low resistivity the eddy current losses may become significant, e.g., for high purity Cu or Al.     

Specimen geometry effects on the irreversible magnetization in the low field regime for specimens of bulk Nb3Sn

Irreversible magnetization curves in particular in self field conditions were investigated for specimens of bulk Nb₃Sn as an isotropic and non-granular alternative to high T _c materials. The specimens were cut into squares with side length to thickness ratios of about 0.8 to almost 20, leading to diamagnetic slopes |χ _d | ranging from slightly more than 1 (slab in parallel field) to 13 (plate in perpendicular field). The peak in the low field magnetization (caused by the field dependence of j _c ) moves from negative field for specimens with small |χ _d | (where it should be according to the standard Bean model) to zero or even positive fields for specimens with large |χ _d | . Finite element self consistent calculations with field dependent and thus spacially varying j _c were used to fit the measured hysteresis curves. A strong current peak in the center plane of the specimens — caused by the demagnetization field — probably causes the shift of the magnetization peak to positive field. Best agreement between the experimental and calculated magnetizations have been obtained for the extreme geometries: either thin disk in perpendicular (χ _d < ?4) or thin slab in parallel magnetic field (χ _d > ?1.2).     

AC power loss for superconducting strips of arbitrary thickness carrying a transport current

The magnetic field and current distributions are computed numerically for a superconducting strip in the critical state with infinite length, but finite thickness and width. The width to thickness ratio is varied from 1 to 100, covering the range from square bars to thick films. For large aspect ratios (>10) the current and field distributions differ significantly from the 2D solution, because the Norris solution does not take into account (and thus does not shield) the transverse field parallel to the strip. Power loss exponents are calculated and found to vary from roughly 3 in a square bar to 4 in a highly aspected thin strip. In particular in the thin strip case the power loss exponents are current dependent due to the contribution of the loss caused by the transverse magnetic field. The loss in a multifilamentary conductor (or cable) is also estimated, taking into account the loss in the filaments. It is found that the overall loss (without coupling) is enhanced by a factor 1+1/n, where n is the number of filaments.     

AC losses in circular arrangements of parallel superconducting tapes

The DC and AC properties of superconducting tapes connected in parallel and arranged in a single closed layer on two tubes (corresponding to power cable conductor models with infinite pitch) with different diameters are compared. We find that the DC properties, i.e., the critical currents of the two arrangements, scale with the number of tapes and hence appear to be independent of the diameter. However, the AC loss per tape (for a given current per tape) appears to decrease with increasing diameter of the circular arrangement. Compared to a model for the AC loss in a continuous superconducting layer (Monoblock model) the measured values are about half an order of magnitude higher than expected for the small diameter arrangement. When compared to the AC loss calculated for N individual superconducting tapes using a well known model (Norris elliptical) the difference is slightly smaller.