George A. Levin

AC losses in striated YBCO coated conductors

We measured AC losses in RABiTS nonstriated as well as striated coated conductors provided by the US Air Force Research Laboratory. In addition, samples with different kinds of well defined inter-filament bridging have been measured and their losses compared with those of samples with unbridged filaments and with mono-layer samples. This bridging is designed to aid current sharing between filaments, which is advantageous, but it is necessary to determine if the effect on AC losses is detrimental. It was found that AC losses of striated samples with multiple bridges are higher than those of the samples with no bridges due to significant filament coupling but even so the losses are still substantially lower than those of a monolayer sample.

YBCO/Ag boundary resistivity in YBCO tapes with metallic substrates

The quality of the contact between a YBCO layer and the protective silver coating is an important parameter affecting the current transfer between YBCO and the normal metal. We studied experimentally the quality of this contact in 6 mm wide YBCO-coated conductor with a critical current of ~60 A at 77 K. The measured current transfer length for the original sample of a YBCO-coated conductor covered by ~3 µm thick silver and for the same sample additionally laminated by a 25 µm thin copper layer was 0.19 and 0.47 mm, respectively. The contact resistivity determined from these experiments was between 3.7 × 10−11 and 7.0 × 10−12 Ω m2. A direct measurement of the resistance between two overlapped tapes soldered by indium yielded values between 2.5 × 10−12 and 5 × 10−12 Ω m2. The boundary resistance and current transfer length are important parameters for the design of the optimal tape architecture for coils and windings, especially for stability issues.

AC loss characteristics of multifilamentary YBCO coated conductors

One of the methods to reduce magnetization loss of YBCO coated conductors in a perpendicular magnetic field is subdividing the YBCO layer into filaments and twisting them as a whole. A 10 mm wide multifilamentary YBCO coated conductor with 200 /spl mu/m wide filaments was prepared by striation using the laser ablation technique. The number of filaments was 40. The sample length was varied from 100 mm to 25 mm, and their magnetization loss was measured at various frequencies. The measured magnetization losses were scaled using sample length, frequency, and field amplitude. This clarifies the magnetization loss characteristics of multifilamentary YBCO coated conductors and obtains empirical expressions for the magnetization loss. The measured loss was compared with the loss calculated numerically using a two dimensional FEM model. The experimentally confirmed effect of striation to reduce the magnetization loss was compared with theoretical predictions.

The effects of superconductor–stabilizer interfacial resistance on the quench of a current-carrying coated conductor

We present the results of numerical analysis of a model of normal zone propagation in coated conductors. The main emphasis is on the effects of increased contact resistance between the superconducting film and the stabilizer on the speed of normal zone propagation, the maximum temperature rise inside the normal zone, and the stability margins. We show that with increasing contact resistance the speed of normal zone propagation increases, the maximum temperature inside the normal zone decreases, and stability margins shrink. This may have an overall beneficial effect on quench protection quality of coated conductors. We also briefly discuss the propagation of solitons and development of the temperature modulation along the wire.

Magnetization losses in multifilament coated superconductors

We report the results of a study of the magnetization losses in experimental multifilament, as well as control (uniform) coated superconductors exposed to time-varying magnetic fields of various frequencies. Both the hysteresis loss, proportional to the sweep rate of the applied magnetic field, and the coupling loss, proportional to the square of the sweep rate, have been observed. A scaling is found that allows us to quantify each of these contributions and extrapolate the results of the experiment beyond the envelope of accessible field amplitude and frequency. The combined loss in the multifilament conductor is reduced by about 90% in comparison with the uniform conductor at full field penetration at a sweep rate as high as 3 T/s.

Multifilament YBa2Cu3O6+x-coated conductors with minimized coupling losses

We report an experimental approach to making multifilament coated conductors with low losses in applied time-varying magnetic field. Previously, the multifilament conductors obtained for that purpose by laser ablation suffered from high coupling losses. Here we report how this problem can be solved. When the substrate metal in the grooves segregating the filaments is exposed to oxygen, it forms high resistivity oxides that electrically insulate the stripes from each other and from the substrate. As the result, the coupling loss has become negligible over the entire range of tested parameters (magnetic field amplitudes B and frequencies f).

ac losses in a YBa2Cu3O7−x coil

The properties of a small pancake coil made with a 10mm wide copper-stabilized YBa2Cu3O7−x (YBCO) coated conductor were investigated. The radial component of the magnetic field was mapped at the coil edge in both the dc and ac regimes and differs significantly from that calculated assuming a uniform current distribution. The observed hysteresis indicates the strong influence of the ferromagnetic properties of the substrate. The ac losses of the coil were measured for ac frequencies between 60 and 1000Hz. The differences in properties of the YBCO coil and a similarly prepared copper coil are discussed.

Low AC loss structures in YBCO coated conductors with filamentary current sharing

Architectural design improvements, such as filamentation, to YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) coated conductors can result in a more ac-tolerant version of the conductor. However, finely made striations in the conductor make filament breakage more probable. In this case, weakly linking the filaments can enable current sharing among the filaments of striated coated conductors while maintaining reduced hysteretic losses. Data is presented for a YBCO sample divided into superconducting filaments separated such that the transverse critical current density of the striation is significantly less than the longitudinal critical current density along the filaments. A LAO substrate was physically scribed with parallel incisions to adversely affect the subsequent epitaxial growth of the YBCO layer between the striations. Vibrating sample magnetometry measurements verified a reduction in hysteretic loss compared to a control sample of epitaxially grown YBCO on an unscribed LAO substrate. Since filamentation requires a twist in the conductor for practical applications, a discussion is also given outlining an alternate means of accomplishing this by placing a twist in the coated conductor architecture itself.

Multifilament, copper-stabilized superconductor tapes with low alternating current loss

Multifilament fully stabilized second generation superconductor tapes have been fabricated with filaments widths as small as 180 μm. Each superconducting filament is copper stabilized, with copper thickness up to 30 μm, without compromising the beneficial effect of striations on magnetization AC losses. This has been accomplished by a combination of laser scribing (ablation), post-ablation oxidation, and subsequent selective electroplating of copper. Twelve millimeters wide superconductor tapes divided into 48 filaments with 10 μm thick copper stabilizer is found to meet an AC loss target of 1 W/kA/m in applied alternating magnetic field of 0.075 T at 100 Hz. This technique has a potential to be transferred to a large scale manufacturing of stabilized, low loss multifilament coated conductors.

Magnetization losses in multiply connected YBa2Cu3O6+x-coated conductors

Abstract : We report the results of a magnetization loss study in experimental multifilament, multiply connected coated superconductors exposed to time-varying magnetic field. In these samples, the superconducting layer is divided into parallel stripes segregated by nonsuperconducting grooves. In order to facilitate the current sharing between the stripes and thus increase the reliability of the striated conductors, a sparse network of superconducting bridges is superimposed on the striated film. We find that the presence of the bridges does not substantially increase the magnetization losses, both hysteresis and coupling, as long as the number of bridges per length of the sample is not large. These results indicate that it is possible to find a reasonable compromise between the competing requirements of connectivity and loss reduction in an ac-tolerant version of the high-temperature coated conductors specifically designed for ac power applications.