K.P. Thakur

The dependence of AC loss characteristics on the spacing between strands in YBCO Roebel cables

Transport AC loss in a short length of 9/2 YBCO Roebel cable, (i.e. with 9 × 2 mm width strands), with 0.25 mm spacers between the strands is measured. The frequency varies from 59 to 354 Hz. The result is compared with the loss for a cable without spacers between the strands. Transport AC loss is decreased by the presence of the spacers. The AC loss reduction due to the extra spacing is more significant when the amplitude of the cable current is small compared to the cable Ic. The losses in the cable with spacers normalized by the square of the cable currents plotted against It/Ic approximately agree with those in a cable without spacers. Electromagnetic modelling was carried out for 9/2 and 8/2 cables, modelled as parallel stacks, to assist in understanding the above experimental results. The 8/2 configuration allows the greater use of symmetry to speed computations. Reasonable agreement between the numerical results and the measured results was obtained. This supports suggestions made in previous publications that the transport AC loss in a Roebel cable is roughly equivalent to the loss in two parallel stacks carrying the same current in each tape. The electromagnetic analysis in the 8/2 stacks shows the flux lines are more perpendicular to the strand face when the vertical space between strands is smaller, and this leads to a larger induced electrical field and larger AC loss. At small current amplitudes, the modelling shows the spacing has a strong effect on the AC loss in the surfaced part of the strands.

Numerical Computation of AC Losses and Flux Profiles in High-Aspect-Ratio Superconducting Strips in Perpendicular AC Magnetic Field

We present the results of finite-element modeling of a YBCO thin-film superconductor deposited on a nonmagnetic substrate and on a weakly ferromagnetic substrate. The model was implemented using commercial software to calculate the ac loss in the presence of a sinusoidal external magnetic field applied perpendicular to the surface of the superconducting tape. A strategy is demonstrated to overcome the difficulties in the finite-element method due to the high aspect ratio of the YBCO film by using the computed values of ac loss for thicker samples to extrapolate the results to the actual physical thickness of the superconductor ( ~ 1 mum). The effect of the width of the weekly ferromagnetic substrate upon the ac loss of the superconductor film has also been studied.

Magnetic and Transport AC Losses in HTS Roebel Cable

We present results for magnetic losses in a five strand HTS Roebel cable with 2 mm wide insulated strands. The loss as a function of field is compared to the isolated strands and an un-insulated cable. The loss as a function of the angle of the applied field to the cable normal is found to scale simply with the normal component of field. The dependence of the loss on the frequency of the applied field is presented and a small intrinsic frequency dependence of the superconductor is observed. The frequency dependence of the loss in a cable coupled by copper bridges is presented. The coupling loss is found to follow the Debye form at low fields. Transport AC losses in HTS Roebel cable with different strand insulation thicknesses are found to vary with the vertical separation of strands.

Screening of Eddy Current Loss in Metal Layers of Coated HTS Conductors

The influence of weakly-magnetic substrate materials on the AC loss of HTS coated conductors has been explored in a number of studies. In the case of magnetic AC loss in perpendicular orientation, at low field amplitude the superconductor effectively diverts magnetic flux into the parallel orientation in the substrate, concentrating the flux and increasing the substrate loss compared to that expected in perpendicular field. At intermediate field amplitudes, flux deflection in the substrate retards flux penetration of the superconductor and so reduces the loss. At higher field, with the tape fully penetrated, the flux is nearly perpendicular to the tape, substrate loss is negligible, and the loss in the superconductor is unaffected by the substrate. We present here experimental results and modeling which demonstrate that a similar effect occurs with the eddy current losses in the metallic layers of coated conductor tapes. At low field amplitude the superconducting layer of a coated conductor deflects the flux away from the perpendicular orientation so the eddy current loss in metallic layers is reduced, by almost an order of magnitude in the present case. At high field amplitude the eddy current loss is unaffected. In addition we present measurements showing the small changes in AC loss associated with the ends of sample tapes.

Transport AC Loss Characteristics of a Five Strand YBCO Roebel Cable With Magnetic Substrate

Transport AC loss in a short length of 5/2 YBCO Roebel cable (five 2 mm wide strands) with weakly magnetic substrate is measured. The AC loss data are compared with those for a single strand. All the strands composing the cable and the single strand are cut from the same stock material. Transport AC loss in the 5/2 Roebel cable is larger than that predicted by both the Norris strip and ellipse models over a large range of the cable current. The difference between normalized AC losses in the Roebel cable and the single strand is larger at low It/Ic than at high It/Ic, where It is the amplitude of transport current. These AC loss characteristics were not observed in transport AC loss results for YBCO Roebel cables composed of coated conductor with non-magnetic substrate. Results from an electromagnetic analysis of two parallel stacks of wire with magnetic substrate are presented to elucidate the results.

Analysis of remnant field detected by hall sensors above superconductor tape

This paper describes various approaches to the analysis of the remnant field arising from the current distribution in superconductor tape. This is of use as a quality control technique when it is important that the superconducting tape has a homogenous current distribution across its width. Superconducting tape is exposed to a magnetic field perpendicular to its surface, inducing currents within the tape. The magnetic field produced by these induced currents can then be measured to assess the distribution of the currents. This paper discusses three techniques to analyze the remnant field in superconducting tape viz. (i) analytic inversion of the Biot-Savart law (ii) computation of the local perpendicular field gradient across the tape and (iii) a least squares fit of the measured magnetic field data to a critical state model. Defects are detected as disruptions to the current distribution calculated for a uniform superconductor. Gradient computation offers a quick way to compare the quality of the samples.

Deducing local influence neighbourhoods with application to edge-preserving image denoising

Traditional image models enforce global smoothness, and more recently Markovian Field priors. Unfortunately global models are inadequate to represent the spatially varying nature of most images, which are much better modeled as piecewise smooth. This paper advocates the concept of local influence neighbourhoods (LINs). The influence neighbourhood of a pixel is defined as the set of neighbouring pixels which have a causal influence on it. LINs can therefore be used as a part of the prior model for Bayesian denoising, deblurring and restoration. Using LINs in prior models can be superior to pixel-based statistical models since they provide higher order information about the local image statistics. LINs are also useful as a tool for higher level tasks like image segmentation. We propose a fast graph cut based algorithm for obtaining optimal influence neighbourhoods, and show how to use them for local filtering operations. Then we present a new expectation-maximization algorithm to perform locally optimal Bayesian denoising. Our results compare favourably with existing denoising methods.