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We discuss production of lengths of up to 27 m of YBCO Roebel cable. Results for 5/2 (5 strands, 2 mm width), 9/2 and 15/5 cables produced from standard 12 mm commercial YBCO wire are presented. We discuss specifications for the input wire and suggest using a statistical correlation function, using data from magnetic field scanning, that is shown to produce high performance strands. We discuss advances in manufacturing techniques including cable insulation processes. Transport and magnetic AC loss data are presented for 5/2 cable which demonstrates the effectiveness of decreased strand width and the transposition of strands. Both losses are predominantly hysteretic in nature. Finally, the cable DC transport is presented and we discuss the possibilities for high current cables in high field applications.

Transport AC loss in a short length of 9/2 YBCO Roebel cable (nine 2 mm wide strands) is measured. The AC loss data are compared with those in a 5/2 YBCO Roebel cable (five 2 mm wide strands) as well as that in a single strand. All the strands composing the cables and the single strand are insulated and cut from the same stock material. The validity of the measurement method was reconfirmed by results at a range of frequencies. At a wide range of I t /I c , the normalized AC losses in the Roebel cable were around 6.2-6.7 times of those in the single strand. This is less than the nine times predicted for a tight bundle of nine conductors. The normalized transport AC losses in the 5/2 Roebel cable are much smaller than those in the 9/2 Roebel. This should be due to larger superposition of magnetic field in the 9/2 Roebel. The I c of the 9/2 and 5/2 Roebel cables is determined by serial connection of the strands. This eliminates the effect where differing resistances in the current terminations cause uneven current sharing between strands when the strands are connected in parallel.

We have measured the frequency dependent magnetic ac loss in a five strand Roebel cable with 2 mm wide non-insulated strands and a transposition length of 90 mm. The cable is made from 12 mm wide SuperPower YBCO (yttrium barium copper oxide) coated conductor tape with a 20 μm thick surrounding copper stabilizer layer. The ac loss of the Roebel cable is measured in a sinusoidally varying external magnetic field in perpendicular orientation, with amplitudes up to 150 mT, in the frequency range of 30-200 Hz, and at a temperature of 77 K. No measurable loss is observed in the parallel orientation. In the perpendicular orientation, the cable loss is predominantly hysteretic in nature. The loss either increases or decreases with frequency, in different field ranges. To resolve the origin of the frequency dependence, we measure the loss in a single strand and in standard tape with similar copper stabilization. The cable and sum of strand losses converge in the high field limit, indicating that strands are not electrically coupled. We attribute the observed frequency dependent increase in loss to a sum of the eddy current loss in the copper stabilizer and the intrinsic frequency dependence of the hysteresis loss. Experimental results and finite element method calculated values for the frequency dependent contributions to the magnetic ac loss agree well with each other.

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Abstract:X-ray diffraction, resistivity, ac susceptibility and magnetization studies on La 0.67 Ca 0.33 Mn 1-x Ru x O 3 (0 ≤ x < 0.1) were carried out. A significant increase in the lattice parameters indicated the presence of mixed valance state of Ru: Ru 3+ and Ru 4+. The resistivity of the doped compounds exhibited two features: a broad maximum and a relatively sharp peak. While a para to ferromagnetic transition could be observed for the latter peak, no magnetic signal either in ac susceptibility or in magnetization measurements could be observed for the broad maximum. The magnetic moment decreases non linearly from 3.55 to 3 µ B over the Ru composition from 0 to 8.5 at.%. Based on the results of the present studies and on existing literature on the Mn-site substituted systems, we argue that a magnetic phase separation occurs in the Ru doped system. While the sharp peak in the resistivity corresponds to Ru 4+ enriched region with a ferromagnetic coupling with neighboring Mn ions, the broad peak corresponds to a Ru 3+ rich regions, with an antiferromagnetic coupling with neighboring Mn ions.

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 μm). The effect of the width of the weekly ferromagnetic substrate upon the ac loss of the superconductor film has also been studied.

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