Table of contents

This focus section of Superconductor Science and Technology looks at the properties, technology and applications of (RE)BCO and MgB₂ based superconductors for power engineering systems. Both bulk and conductor forms of material are addressed, including elements of materials fabrication and processing, and the measurement of their applied properties for various levels of system application. The areas of research include ac losses in type II materials in power devices, cables and coated conductors, the development of high current dc cables and the application of superconductors in levitation devices, motors and fault current limiters. This focus section presents a broad cross-section of contemporary issues, that represent state-of-the-art for power applications of superconductors, and highlights the areas that require further development if commercial applications of these rapidly emerging materials are to be realised. It contains papers from some of the major groups in the field, including contributions from Europe, the USA and Japan, and describes devices that are relatively close to market.

In this paper, we investigate the effectiveness of alternative designs to reduce the AC losses of high temperature superconducting (HTS) power transmission cables. The idea behind these designs is to undermine the edge effect, which is one of the main factors contributing to AC losses in HTS power cables made of coated tapes. The edge effect, which arises from the presence of gaps between the tapes, results in large normal components of magnetic field near the edges of the tapes and in turns leads to a current distribution with higher density near the edges. To perform our investigation we use a numerical technique developed in our previous work which allows us to consider the helical configuration of the tapes. Through numerical simulations we assess the effectiveness of two overlapped designs, i.e. a cyclic overlapped design and an anticyclic overlapped design, in reduction of AC losses in single layer HTS power cables made of coated tapes. Simulation results show that AC losses can be reduced by about 70% as compared with a typical single layer cable.

This paper describes recent experimental results in a continuing program to develop a 1 MVA demonstration transformer that employs HTS Roebel cable for the high current windings. The electrical design parameters of the transformer are presented, including the configuration of the specially developed Roebel cable. This paper discusses estimation of the cable I_c from measured I_c(B) data for samples of the strands, AC loss minimization using flux diverters, proving the suitability of the polyimide insulation scheme to withstand the effects of a high voltage impulse and modelling and experimental verification of the fault current limiting behaviour expected from the HTS conductor.

Transmission cables made from high-temperature superconductors have been successfully demonstrated within the electric power grid. These cables carry an ac current of up to 3000 A in a much smaller cross-sectional area than conventional transmission lines, but they are not flexible enough for certain applications that involve very tight cable bends. Certain on-board Air Force applications require 5 MW of dc power transmission at 270 V and current of 18 500 A and would benefit from superconducting transmission in lightweight, flexible cables that would be cooled with helium gas down to about 55 K. To address these needs, we have constructed a 10 mm diameter RE–Ba₂Cu₃O_{7 − δ} (RE = rare earth) coated conductor cable that is lighter and more flexible than the current generation of superconducting cables, and that has a critical current of 7561 A at 76 K. The cable is expected to have a critical current of more than 20 000 A at 55 K and therefore will likely exceed the requirements for 5 MW on-board power transmission. The cable consists of two electrically insulated phases that can be operated in different modes, which allows us to study the effect of self-field on the cable performance.

In this paper, a fundamental study of the rotating characteristics of a induction/synchronous motor by use of superconducting MgB₂ cage windings is carried out based on analysis and experiment. Current transport properties of the produced monofilamentary MgB₂ wires are firstly characterized, and then utilized for the determination of the current carrying capacity of the rotor bars. Then, the motor model is designed and fabricated with the aid of conventional (copper) stator windings. We successfully observe the synchronous rotation of the fabricated motor at a rotation speed range from 300 to 1800 rpm. We can also realize an almost constant torque versus speed curve, and this characteristic is explained from the steep take-off of the electric field versus the current density curve, based on the nonlinear electrical equivalent circuit. These results are promising for the practical applications of a high efficiency motor for a liquid hydrogen circulation pump.

The basic element of a resistive superconducting fault current limiter (FCL) can consist of coated conductor tape exceeding a few meters in length and compacted into a cryogenic envelope. This paper is focused on optimizing the arrangement of coated conductors with a non-magnetic substrate for a resistive superconducting FCL. Several configurations have been tested experimentally and theoretically. Two low-loss arrangements have been identified, both utilizing the bifilar configuration, i.e. the currents in two adjacent tapes are identical in amplitude but opposite in direction. The separation between two adjacent tapes s varied from 0.07 up to 2.10 mm. For the lowest examined separation s the AC transport loss of the straight bifilar model decreased by more than one order with respect to the AC transport loss in the single-tape configuration. Further AC loss decrease is achieved when the pair of tapes carrying opposite currents forms a flat pancake coil. We developed a numerical model in order to analyze the influence of distance between adjacent tapes. To achieve agreement between experimental and numerical results it was necessary to incorporate a lateral distribution of critical current density in the tape. The remaining differences between the results of experiment and calculation can be explained by analysis of experimental imperfections. Finally we suggest an empirical fit for the prediction of AC loss of a practical superconducting fault current limiter.

Electromagnetic rail launchers (EMRLs) require very high currents, from hundreds of kA to several MA. They are usually powered by capacitors. The use of superconducting magnetic energy storage (SMES) in the supply chain of an EMRL is investigated, as an energy buffer and as direct powering source. Simulations of direct powering are conducted to quantify the benefits of this method in terms of required primary energy. In order to enhance further the benefits of SMES powering, a novel integration concept is proposed, the superconducting self-supplied electromagnetic launcher (S³EL). In the S³EL, the SMES is used as a power supply for the EMRL but its coil serves also as an additional source of magnetic flux density, in order to increase the thrust (or reduce the required current for a given thrust). Optimization principles for this new concept are presented. Simulations based on the characteristics of an existing launcher demonstrate that the required current could be reduced by a factor of seven. Realizing such devices with HTS cables should be possible in the near future, especially if the S³EL concept is used in combination with the XRAM principle, allowing current multiplication.

This paper describes the present status of high temperature superconductors (HTS) and of bulk superconducting magnet devices, their use in bearings, in flywheel energy storage systems (FESS) and linear transport magnetic levitation (Maglev) systems. We report and review the concepts of multi-seeded REBCO bulk superconductor fabrication. The multi-grain bulks increase the averaged trapped magnetic flux density up to 40% compared to single-grain assembly in large-scale applications. HTS magnetic bearings with permanent magnet (PM) excitation were studied and scaled up to maximum forces of 10 kN axially and 4.5 kN radially. We examine the technology of the high-gradient magnetic bearing concept and verify it experimentally. A large HTS bearing is tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system. The flywheel rotor tests show the requirement for additional damping. Our compact flywheel system is compared with similar HTS–FESS projects. A small-scale compact YBCO bearing with in situ Stirling cryocooler is constructed and investigated for mobile applications. Next we show a successfully developed modular linear Maglev system for magnetic train operation. Each module levitates 0.25t at 10 mm distance during one-day operation without refilling LN₂. More than 30 vacuum cryostats containing multi-seeded YBCO blocks are fabricated and are tested now in Germany, China and Brazil.

The AC losses in ReBCO coated conductors are large in situations when the conductors are subjected to a considerable magnetic field, as in rotating machines, transformers and high-field magnets. Roebel cables can reduce the AC losses in these cases. However, computer simulations are needed to interpret the experiments, understand the loss mechanisms, reduce the AC losses by optimizing the Roebel cable and design the cryogenic system. In this paper, we simulate and discuss the AC losses due to an applied magnetic field with an arbitrary angle with respect to the cable and taking into account a realistic anisotropic field dependence of the critical current density. We study the AC losses in the superconductor parts for the limits of very high coupling currents and completely uncoupled strands. The simulations for the uncoupled case also describe a double pancake coil with no transport current. For the simulations, we use two different numerical methods with complementary strengths. This serves as a mutual check of the correctness of the simulation results, which agree with each other. As opposed to what was expected, we found that the AC losses do not only depend on the perpendicular component of the applied magnetic field. We also found that the AC losses for applied fields with an orientation below 7° with the strand surface are reduced by more than one order of magnitude as compared with an untransposed cable. Therefore, we recommend to use Roebel cables for windings with important parallel components, such as transformers and high-field magnets.

The superconducting (SC) fault current limiter (FCL) improves the security and the power quality of electric networks; these are the two essential demands of today. The device must fulfil several requirements in normal and fault operations, and must operate under a variety of conditions. Low prospective current faults represent the most severe conditions. Proper design of the ReBaCuO-coated conductor is essential for safe and optimized operations. The design of the superconducting conductor is mainly based on thermal criteria. The minimum superconducting conductor volume is given by its enthalpy and the limited current through the SC conductor. The superconducting quantities play only a small role in the design. A high resistivity conductor reduces the ReBaCuO volume. Of other considerations to be taken into account, the quench homogeneity is one of the most important for resistive FCLs. The coated conductor architecture and design can help to reduce the consequences of quench inhomogeneity along the conductor. The presented arguments, for a high limited current forcing the conductor to quench uniformly and for a moderate conductor resistivity to reduce the temperature differences, are supported by experiments carried out utilizing two rather different coated conductors in various limiting conditions.

The field amplitude dependent complex ac susceptibility χ(H_m) of disc and square-planar samples cut from a Gd–Ba–Cu–O–Ag single grain fabricated by top-seeded melt growth (TSMG) has been measured at 77 K with the ac field applied along the crystallographic c axis of the single grain. The critical-current density J_c has been extracted from the measured χ(H_m) data based on the critical-state model assuming constant J_c. It is shown that J_c increases continuously with decreasing H_m, which is attributed to strong intrinsic pinning occurring at the edges of the sample.

The feasibility of high field magnet applications of the twisted stacked-tape cabling method with 2G YBCO tapes has been investigated. An analysis of torsional twist strains of a thin HTS tape has been carried out taking into account the internal shortening compressive strains accompanied with the lengthening tensile strains due to the torsional twist. The model is benchmarked against experimental tests using YBCO tapes. The critical current degradation and current distribution of a four-tape conductor was evaluated by taking account of the twist strain, the self-field and the termination resistances. The critical current degradation for the tested YBCO cables can be explained by the perpendicular self-field effect. It is shown that the critical current of a twisted stacked-tape conductor with a four-tape cable does not degrade with a twist pitch length as short as 120 mm. Current distribution among tapes and hysteresis losses are also investigated. A compact joint termination method for a 2G YBCO tape cable has been developed. The twisted stacked-tape conductor method may be an attractive means for the fabrication of highly compact, high current cables from multiple flat HTS tapes.

We used scanning tunnelling microscopy to study the morphology of superconducting FeSe_0.5Te_0.5 thin films epitaxially grown by pulsed laser deposition. Samples with critical temperature T_c above the bulk value (>16 K) show large atomic terraces, and a square lattice of periodicity 3.8 Å associated with the Se/Te surface termination. Differences in the height coordinate of the chalcogenide atoms are clearly visible at the atomic level. On the contrary, samples with lower T_c (11 K) show hillocks generated by a spiral surface growth driven by threading dislocations of screw character. A comparative x-ray diffraction analysis reveals differences of compressive strain for the two classes of specimens. Variations in the deposition rate are found to affect film growth and inner strain, which ultimately tune T_c.

A reversible strain effect on transport critical current I_c was found in Bi₂Sr₂CaCu₂O_8 + x (Bi-2212) high-temperature superconducting round wires. I_c showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, I_c was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain ε. I_c(ε) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve I_c if buckling of Bi-2212 grains does not occur, and should result in a wider I_c(ε) plateau in the applied tensile regime without degradation of the initial I_c. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

The electronic structure of non-stoichiometric superconducting NbB_2 + x has been investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The analysis of the valence band using XPS and UPS reveals that the boron doping induces a systematic decrease in the density of states at Fermi level N(E_F) similar to that observed in the Nb_{1 − x}B₂ system. In particular, N(E_F) is lower for the superconducting samples than for the non-superconducting ones. In the superconducting samples, we confirm that the T_c is inversely proportional to N(E_F). Therefore, the presence of superconductivity in these samples cannot be explained only as a function of the N(E_F). Thus, the presence of superconductivity in our samples might be explained by increases in the number of holes in-plane conduction, due to an increase in the number of niobium vacancies as boron content is increased.

We have studied distinctive features of magnetic field distributions induced by vortices in thin films of anisotropic superconductors for an arbitrary orientation of external tilted magnetic fields. The magnetic field profiles are found to be strongly modified for rather small film thickness and large field tilting angle. The single-vortex field profile reveals two maxima instead of one peak observed in relatively thick films or in the films placed in magnetic fields perpendicular to the film plane. Our calculations are based on the study of the energetically favorable shape of an isolated vortex line in the presence of the inhomogeneous supercurrents which screen the field component parallel to the film plane. Starting from the London theory with anisotropic mass tensor we justify the elastic string approximation for the vortex lineshape affected by the screening currents. The exotic magnetic field profiles typical for curved vortices can be observed by modern vortex imaging techniques.

Superconducting (SC) MgB₂ thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature (T_s ∼ 110–150 °C) and at very low Mg deposition rate, far off from the MgB₂ stability regime of the thermodynamic phase diagram. As-grown films are nanocrystalline and exhibit (002) texture, moderate surface roughness and SC onset up to T_c^on = 18 K. This is the highest critical temperature ever observed in low-T_s as-grown MgB₂ films. Our preparation method at very low T_s is favorable for the fabrication of layered SC hybrid electronic devices, where deposition and post-annealing at high temperature are detrimental to device functioning. Simple post-annealing of our films in ultrahigh vacuum enhances T_c^on up to 22 K due to the improvement in structural properties.

We report on a two-stage system consisting of a dc SQUID magnetometer inductively coupled to an amplifying second stage realized by a SQIF. The SQIF is a series or parallel connection of dc SQUIDs with various inductances, which facilitates a unique global minimum in the voltage–flux characteristic and ensures a stable operating point, a high voltage swing and a large transfer function of the system. The noise of the amplified signal is dominated by the input stage and its voltage swing exceeds 1 mV, which reduces the demands on the readout electronics. A part of the current of the SQIF can optionally be fed back to the feedback coil of the first-stage SQUID, so that an on-chip linearization of the voltage–flux characteristic is realized. The short feedback path enables a substantially higher bandwidth compared to a flux-locked loop using a room temperature amplifier. From experimental results it can be expected that this set-up is capable of measuring magnetic fields with a high slew rate and dynamic range.

We recently demonstrated and analysed the voltage-biased SQUID bootstrap circuit (SBC) conceived to suppress the preamplifier noise contribution in the absence of flux modulation readout. Our scheme contains both the additional voltage and current feedbacks. In this study, we analysed the tolerance of the SBC noise suppression performance to spreads in SQUID and SBC circuit parameters. Analytical results were confirmed by experiments. A one-time adjustable current feedback can be used to extend the tolerance to spreads such as those caused by the integrated circuit fabrication process. This should help to improve the fabrication yield of SBC devices integrated on one chip—as required for multi-channel SQUID systems.

The ITER cable-in-conduit conductors (CICCs) are built up from sub-cable bundles, wound in different stages, which are twisted to counter coupling loss caused by time-changing external magnet fields. The selection of the twist pitch lengths has major implications for the performance of the cable in the case of strain-sensitive superconductors, i.e.  Nb₃Sn, as the electromagnetic and thermal contraction loads are large but also for the heat load from the AC coupling loss. At present, this is a great challenge for the ITER central solenoid (CS) CICCs and the solution presented here could be a breakthrough for not only the ITER CS but also for CICC applications in general. After proposing longer twist pitches in 2006 and successful confirmation by short sample tests later on, the ITER toroidal field (TF) conductor cable pattern was improved accordingly. As the restrictions for coupling loss are more demanding for the CS conductors than for the TF conductors, it was believed that longer pitches would not be applicable for the conductors in the CS coils. In this paper we explain how, with the use of the TEMLOP model and the newly developed models JackPot-ACDC and CORD, the design of a CICC can be improved appreciably, particularly for the CS conductor layout. For the first time a large improvement is predicted not only providing very low sensitivity to electromagnetic load and thermal axial cable stress variations but at the same time much lower AC coupling loss.

Reduction of the transverse load and warm-up–cool-down degradation can be reached by applying longer twist pitches in a particular sequence for the sub-stages, offering a large cable transverse stiffness, adequate axial flexibility and maximum allowed lateral strand support. Analysis of short sample (TF conductor) data reveals that increasing the twist pitch can lead to a gain of the effective axial compressive strain of more than 0.3% with practically no degradation from bending. This is probably explained by the distinct difference in mechanical response of the cable during axial contraction for short and long pitches. For short pitches periodic bending in different directions with relatively short wavelength is imposed because of a lack of sufficient lateral restraint of radial pressure. This can lead to high bending strain and eventually buckling. Whereas for cables with long twist pitches, the strands are only able to react as coherent bundles, being tightly supported by the surrounding strands, providing sufficient lateral restraint of radial pressure in combination with enough slippage to avoid single strand bending along detrimental short wavelengths. Experimental evidence of good performance was already provided with the test of the long pitch TFPRO2-OST2, which is still until today, the best ITER-type cable to strand performance ever without any cyclic load (electromagnetic and thermal contraction) degradation.

For reduction of the coupling loss, specific choices of the cabling twist sequence are needed to minimize the area of linked strands and bundles that are coupled and form loops with the applied changing magnetic field, instead of simply avoiding longer pitches. In addition we recommend increasing the wrap coverage of the CS conductor from 50% to at least 70%. A larger wrap coverage fraction enhances the overall strand bundle lateral restraint.

The long pitch design seems the best solution to optimize the ITER CS conductor within the given restrictions of the present coil design envelope, only allowing marginal changes. The models predict significant improvement against strain sensitivity and substantial decrease of the AC coupling loss in Nb₃Sn CICCs, but also for NbTi CICCs minimization of the coupling loss can obviously be achieved. Although the success of long pitches to transverse load degradation was already demonstrated, the prediction of the elegant innovative combination with low coupling loss needs to be validated by a short sample test.

Preparation and characterization of a biaxially textured Gd₂Zr₂O₇ and Ce_0.9La_0.1O_{2 − y} (CLO, cap)/Gd₂Zr₂O₇ (GZO, barrier) buffer layer stack by the metal–organic deposition route are reported. YBa₂Cu₃O_{7 − d} (YBCO) superconductor films were deposited by the pulsed-laser deposition (PLD) technique to assess the efficiency of such a novel buffer layer stack. Biaxial texture quality and morphology of the buffer layers and the YBCO superconductor films were fully characterized. The surface crystallinity of the buffer layers is studied by the electron backscatter diffraction technique. It is revealed that post-annealing GZO films in 2% H₂ in Ar is an effective way to improve the surface crystallinity. As a result, a highly textured CLO film can grow directly on the GZO film at a lower crystallization temperature. The critical current density of a YBCO^PLD film is higher than 1 MA cm ^{− 2} (@77 K, in self-field), demonstrating that the novel CLO/GZO stack is very promising for further development of low cost buffer layer architectures for coated conductors.

A three-dimensional (3D) numerical model is proposed to solve the electromagnetic problems involving transport current and background field of a high-T_c superconducting (HTS) system. The model is characterized by the E–J power law and H-formulation, and is successfully implemented using finite element software. We first discuss the model in detail, including the mesh methods, boundary conditions and computing time. To validate the 3D model, we calculate the ac loss and trapped field solution for a bulk material and compare the results with the previously verified 2D solutions and an analytical solution. We then apply our model to test some typical problems such as superconducting bulk array and twisted conductors, which cannot be tackled by the 2D models. The new 3D model could be a powerful tool for researchers and engineers to investigate problems with a greater level of complicity.

The formation of the MgB₂ superconducting compound from a mixture of Mg and amorphous B powders with various low melting point metals (Bi, Se and Te) was studied in situ by means of high-energy (synchrotron) x-ray diffraction in wires with a composite Cu/Nb sheath. In comparison with an undoped sample, it was found that the addition of Bi results in a clear lowering of the formation temperature of MgB₂, whereas Se and Te have no significant influence. T_c is slightly higher in the Bi-doped sample than in the others but the j_c in this case is lower than in the pure MgB₂ sample, probably due to the presence of remaining Mg₃Bi₂ particles that formed as an intermediate compound during reaction. Likewise, in the Se-and Te-doped samples, MgSe and MgTe respectively form below 450 °C. Whereas j_c is also depressed in the Se-doped sample, the MgTe particles do not appear to affect the performance of the Te-doped wire.

In this study, BaZrO₃ (BZO)-doped YBCO films were fabricated on SrTiO₃(100) single-crystal substrates by a fluorine-free metal–organic deposition (MOD) process. We added extra Ba and Zr organic salts, which formed well-dispersed ∼10–25 nm sized BaZrO₃ nanoparticles in the YBCO films. The in-field critical current density (J_c) and the peak pinning force (F_p) were greatly enhanced in the BZO-doped sample at 77 K relative to pure YBCO films. The optimal BZO content that gave the highest peak pinning force of ∼10 GN m⁻³ in a ∼180 nm thick film was found to be x = 0.10 for YBCO + xBZO films, where x is moles of BZO per 1 mol of YBCO. The angular dependence of in-field J_c measurements shows the BZO nanoparticles increased J_c over the entire angular range and also reduced the angular anisotropy measured at 4 T at 77 K.

The voltage-biased SQUID bootstrap circuit (SBC) is suitable for achieving simple and low-noise direct readout of dc SQUIDs. In practice, an ideal voltage bias is difficult to realize because of non-zero internal resistance R_in of the bias voltage source. In order to clearly observe the influence of R_in on the SBC parameters (namely the flux-to-current transfer coefficient (∂I/∂Φ)_SBC and the dynamic resistance R_d(SBC)) and the noise performance, we introduced an additional adjustable resistor R_ad at room temperature to simulate a variable R_in between the SQUID and the preamplifier. We found that the measured SQUID flux noise does not rise, even though R_ad increases significantly. This result demonstrates that a highly resistive connection can be inserted between the liquid-helium-cooled SQUID and the room-temperature readout electronics in the SBC scheme, thus reducing the conductive heat loss of the system. This work will be significant for developing multichannel SBC readout systems, e.g. for biomagnetism, and systems using SQUIDs as amplifiers, for example, in TES-array readout.

For a proper characterization of multi-filamentary NbTi and Nb₃Sn strands and a better understanding of their performance in short sample tests, as well as for increased understanding of inter-strand current redistribution in cabled conductors, a quantitative knowledge of the inter-filament transverse resistance is essential. In particular, in the case of strain or crack distributions among and along filaments in strain-sensitive superconductors such as Nb₃Sn cable-in-conduit conductors, a much better understanding of the voltage–current transition is required as a basis for the analysis of full-size cables.

Two particular four-probe voltage–current methods are developed to measure the transverse inter-filament resistance distribution directly, both in well-established and in state-of-the-art superconductors that are presently applied in the ITER, JT-60SA and LHC magnets. To extract values of the filament-to-matrix contact resistance from these direct experiments, some further assumptions are needed. These assumptions are based on FEM simulations and on measurement of the longitudinal strand resistance.

An overview is given of a wide range of measurements on various NbTi and Nb₃Sn strands, performed at temperatures below 10 K and at various applied magnetic fields. We present the results of the experiments and simulations and demonstrate how the extracted characteristic parameters provide a better insight into the current flow patterns within the strands.