Eduard Demencik

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.

Influence of laser striations on the properties of coated conductors

Due to their high current carrying capability, coated conductors are regarded as the most promising high-temperature superconductor tapes for power applications. However, their high aspect ratio causes too high magnetization losses. To reduce the ac loss, one way is to striate the wide tapes into filaments. We used a picosecond laser for the structuring of (RE)BCO coated conductors. The laser allows to burn 18 μm to 21 μm wide grooves (Ag-cap) with a depth between 0.5 μm to more than 100 μm into the coated conductors, with negligible heat effects at the edges of the structures. Different numbers of filament were structured in Cu-and Ag-cap coated conductors. Patterns with up to 120 parallel filaments in 12 mm wide conductor were made. The critical current and the total ac-magnetization loss were measured as a function of the filament count. With an increasing number of filaments Ic degradation occurs. This current reduction has two contributions, the removed HTS material and current inhomogeneities within the superconductor for instance defects along the tape causing secondary phases. For 120 filaments Ag-cap tapes the hysteresis loss reduction is about two orders of magnitude, as expected. The observation of some remaining filament coupling was investigated.

Superconductor/ferromagnet heterostructures exhibit potential for significant reduction of hysteretic losses

First experimental observations of the ferromagnetic shielding effect in high-Tc superconducting coated conductors were carried out. Experimental results were compared to simulations calling upon finite-element calculations based on the H-formulation of Maxwell equations to model superconducting strips with ferromagnetic shields. Samples of copper-stabilized coated conductors were electroplated with nickel shields and afterwards characterized. Both externally applied oscillating transverse magnetic fields as well as transport currents were studied. Having observed promising gains with respect to the reduction of ac losses in both cases, we further investigated the potential of ferromagnetic shielding. The numerical model was able to reproduce and also predict experimental results very well and will serve as an indispensable tool to determine the potential of soft ferromagnetic materials to significantly reduce hysteretic losses.

AC Magnetization Loss and Transverse Resistivity of Striated YBCO Coated Conductors

It is well known that the separation of thin (RE)BCO superconducting films into electrically isolated stripes (striation process) leads to significant reduction of the magnetization losses. However, in practice, achieving the theoretically predicted loss reduction is quite complicated, due to imperfect separation of the stripes: techniques used for striation leave resistive bridges between the stripes, and coupling currents are free to flow. Very little is known about the precise paths of the coupling currents, other than the fact that the transverse resistivity may play a major role. In this paper, we investigate the magnetization ac loss and the transverse resistivity profile on samples with different numbers of filaments and with different thicknesses of the stabilization layer. The reduction of stabilization layer thickness leads to better control of the laser grooves and substantially suppresses coupling loss. The total loss in those tapes was reduced significantly and is very close to the theoretical expectation.

AC Loss and Coupling Currents in YBCO Coated Conductors With Varying Number of Filaments

Striation of high-temperature superconductor coated conductors as a way to reduce their magnetization ac losses has been the subject of intense worldwide research in the past years by several groups. While the principle of this approach is well understood, its practical application on commercial materials to be used in power applications is still far to be implemented due to manufacturing and technological constraints. Recent advances in tape quality and striation technology are now enabling systematic investigations of the influence of the number of filaments on ac loss reduction with a consistency that was not available in the past. In the present work, we demonstrate the technological feasibility of importantly reducing the magnetization losses of commercially available coated conductors by striating them into a high number of filaments (up to 120). The loss reduction exceeds one order of magnitude and does not come at the expense of current-carrying capability: samples with 10 and 20 filaments are unaffected by the striation process, whereas samples with 80 and 120 filaments still retain 80% and 70% of the current-carrying potential, respectively. We also investigate the transverse resistivity between the filaments in order to understand the paths followed by the coupling currents: we found that the coupling current prevalently flows in the metallic substrate, rather than in and out of the filaments. Finally, we use oxidation as a method to reduce the coupling currents and the corresponding losses. The contribution of this work is threefold: 1) it describes the know-how to produce a large number of high-quality striations in commercially available coated conductors, greatly reducing their losses without extensively degrading their transport properties; 2) it provides a comprehensive characterization of the said samples (e.g., measurements in a wide frequency range, transverse resistance profiles, influence of oxidation on dc and ac behavior of the sample); and 3) it provides new insight on the patterns of the coupling currents.

Magnesium Diboride Wires With Nonmagnetic Matrices—AC Loss Measurements and Numerical Calculations

In the superconducting applications, the wires are exposed to time-varying magnetic field when the current changes. This generates losses which can be minimized by reducing filament size, twisting the wire, and increasing the transverse resistivity. However, the high losses of magnesium diboride wires often arise from magnetic sheath materials, and therefore, this work presents new type of wires with nonmagnetic matrix and multi-filamentary structure. The results of AC loss measurements, in external sinusoidal magnetic field, are presented. Two MgB2 samples were measured both in two temperature ranges, as two different set-ups were used, one with fixed LHe bath temperature 4.2 K. Second one enabled operation temperatures from 23 K up to the critical temperature of 39 K. Amplitude of magnetic field of the former set-up was up to 0.8 T and frequency range was from 0.1 to 1.4 Hz. In the latter one, the maximum amplitude was 28 mT, and the frequencies were 72 and 144 Hz. The results evidenced that the superconducting filaments were uncoupled and the measurements agreed with theoretical models based on this assumption. In practice, the uncoupling was modeled so that the net current in each filament was set to zero.

Coupling losses and transverse resistivity of multifilament YBCO coated superconductors

We studied the magnetization losses of four different types of filamentary YBCO coated conductors. A 10 mm wide YBCO coated conductor was subdivided into 20 filaments by laser ablation. We measured the frequency dependence of the total losses in the frequency range 0.1 Hz < f < 608 Hz. The coupling loss was obtained from the total by subtracting the hysteresis loss. The latter was measured at low frequencies since only hysteresis loss is nonnegligible at frequencies below 1 Hz. The transverse resistivity, ρtr, was determined from the coupling losses; it was assumed that the sample length is equal to half of the twist pitch. The values of ρtr deduced from the loss data were compared with those obtained by the four-point measurements with current flowing perpendicular to the filaments. Preliminary results indicate the existence of electrical contacts between the superconducting filaments and the substrate in some areas of the samples. This was also confirmed by the Hall probe mapping of the magnetic field in the vicinity of the tape. The measured transverse resistivity was close to the resistivity of the substrate (Hastelloy).

Measurement of AC profiles of magnetic field above HTSc tape using Hall probe technique with help of DAQ cards and triggering

Power applications of superconductors, such as transformers, power cables, motors, and generators require the study of the influence of AC magnetic fields on properties of high-temperature superconductors. Our contribution comprises a convenient method for a measurement of components of magnetic fields generated by induced currents circulating in a HTSc sample located in an AC magnetic field. Also, a data acquisition system is presented that allows for the measurement of signal proportional to the position and to the component of magnetic flux density by a Hall probe at a defined level of an applied magnetic field in each period simultaneously with the help of triggering. This differential Hall measurement enables us to acquire magnetic field profiles or maps in the vicinity of the sample corresponding to a certain value of the magnetic field applied. It is possible to obtain results for any value of the magnetic field applied in the course of the period, i.e. from zero up to a maximum value of the magnetic field at the rising or falling slopes according to trigger conditions. In particular, it can be used to investigate the current distribution in the sample during the AC cycle or defect detection in HTSc tapes.

Low field critical current density of titanium sheathed magnesium diboride wires

Magnesium diboride (MgB2) is replacing some of the conventional superconductors due to its low cost and availability in kilometer lengths. MgB2 has also been considered for AC applications. In order to model the AC losses and the critical currents of the applications, intrinsic Jc(B)-dependence is an important factor also at low fields. In this work Jc(B)-dependence of an MgB2 sample is extracted from the standard in field voltage-current measurements. The proposed method is applied to a non magnetic titanium sheathed sample at 16 and 20 K and a simple formula for Jc(B) aligns with the measurements. In the fitting process, the critical current distribution inside the wire is numerically simulated in order to take the self field of the sample into account. Moreover, the same formula aligns with measurements of a different sample. These critical current measurements, performed at 4.2 K, were based on magnetization. In the self field computations, the superconducting cross section must be determined accurately. Therefore, we tailored an image processing tool for MgB2 wires to obtain the geometry from a photograph.

Visualization of coupling current paths in striated YBCO-coated conductors at frequencies up to 400 Hz

The magnetic flux density component perpendicular to the broad tape face was mapped by miniature Hall probes in the vicinity of a striated YBCO-coated tape at frequencies of external magnetic field from 21 to 400 Hz, applied perpendicularly to the tape surface. For reasons of modelling the coupling current behaviour in tapes with a conductive substrate, we amplified the coupling current–amplitude by soldering 25 µm thick copper foil on the top of the filaments. The aim of this procedure was to decrease the transverse resistivity of the tape. The longitudinal components of the total currents flowing in the tape were calculated by an inverse method from the field map corresponding to the zero phase of the applied field. The diffusion lengths, characterizing the flux penetration into the tape, were determined for the respective frequencies. The experimentally determined diffusion length is in good accordance with theoretical models. While at 21 Hz both weak coupling currents and distinctive hysteretic currents of individual filaments are observed, at 400 Hz the coupling currents are predominant in this YBCO tape.