Tape Face Research Articles

Strategies for conformal REBCO windings

A high-field winding can be fabricated from a cable of non-insulated REBCO tapes stacked face-to-face without twisting. If the cable is oriented within each turn of a winding so that the tape face is closely parallel to the magnetic field at its location, the supercurrent capacity of that cable is enhanced ∼3x greater than in a transverse or twisting orientation. This concept for a conformal winding was presented in a previous paper pertinent to the body winding of a REBCO based high-field dipole. Strategies are presented and simulated for how the same orientation can be sustained in the flared ends of a high-field hybrid dipole.

Dependences of AC Loss in Stacked REBa2Cu3Oy Superconducting Tapes on the Interval among Tapes under Perpendicular Magnetic Field

In this study, we focused on the dependence of the AC loss in REBa2Cu3Oy (REBCO, RE = rare earth, Y, Eu, Gd) superconducting tapes stacked perpendicular to the tape face on the interval between the superconducting layers. The AC loss of the 2-layer stacked tapes with various interval in perpendicular field was investigated by pickup-coil method. The AC loss for external applied magnetic field, Bm, below the penetration field, Bp, increases with increasing the interval. In the case that the ratio of the distance between the neighboring superconducting layer, D, to the width of the tape, w, (D/w) is smaller than 0.1, the AC loss of 2-layer tapes decreased to 1/2 of that of 1-layer tape. On the other hand, in the case of D/w being larger than 0.1, the AC loss of 2-layer tapes was larger than 1/2 and it approached the AC loss of 1-layer tape with increasing D/w. From the result obtained in this study, it is predicted that, for Bm below Bp, the AC loss of stacked REBCO tapes with large D/w, such as multifilamentary tapes, is larger than the AC loss of stacked non-scribed REBCO tapes. However, for Bm above Bp: a large AC loss region, it is known that the AC loss of the multifilamentary tapes decreases compared with that of non-scribed tapes. Next step, AC loss of the multifilamentary tapes for Bm below Bp will be investigated experimentally and theoretically.

Electromechanical Properties of 1-mm-Wide Superconducting Cables Comprised of Exfoliated YBCO Filaments

Development of multifilament YBCO cables is an important step toward the manufacture of practical superconducting magnets based on second generation high-temperature superconductor technology. Currently, magnets manufactured from wide 2G tapes suffer from high magnetization loss and poor reliability. The presence of an insulation substrate is to insulate YBCO layers from each other, which allows only lateral (along the tape face) current sharing. Here we present an exfoliated multilayer cable with electrically coupled (1 mm wide) filaments that permit normal (across the tape) current sharing within the cable. This development can potentially enable fast ramping and stable high-field magnets for applications in fusion, energy storage, and transportation.

Numerical Analyses on the Influences of Armature Winding Shape and Yoke Arrangements on Total Losses in Fully Superconducting Synchronous Motors Using REBCO Tapes

In the fully superconducting synchronous motors using REBCO tapes, the magnetic fields in the direction of perpendicular to the face of the tapes in armature windings generate large ac losses in the windings. Therefore, it is effective to reduce magnetic fields in the direction of perpendicular to tape face for the reduction of ac losses. A one of the methods is to get closer a yoke to the armature windings as possible. However, installing the yoke in the cooling area in order to get closer the yoke to the armature windings substantially increase the iron loss in the yoke by the cooling penalty. Therefore, in general, the yoke is placed at room temperature. The purpose of this paper is to evaluate the effect of installing the yoke in low temperature. So, we analyzed the properties of 500-kW fully superconducting synchronous motor by finite element method. We estimated the influences of installing yoke into low temperature in the two cases of armature winding shapes, solenoid windings and pancake windings. In this calculation, we assumed that superconducting material is EuBCO, and the operating temperature is 65 K.

Design, Construction, and Test of HTS/LTS Hybrid Dipole

This paper presents the design, construction, and test results of a hybrid dipole magnet. The inner coils were of second generation (2G) high-temperature superconductor (HTS) ReBCO tape and the outer coils were of low-temperature superconductor (LTS) Nb3Sn Rutherford cable. The HTS and LTS coils were independently powered and protected using different power supplies. The HTS coils were quenched many times with no degradation in performance observed. The hybrid field reached ∼8.6 T, which is believed to be a record for a hybrid dipole. The maximum field was limited by the stable operation of the leads in the LTS coil at 8000 A. The HTS coils were independently ramped to 800 A, and the LTS coils to 10 000 A. With improved leads and instrumentation, this hybrid dipole is expected to produce over 13 T when the ReBCO tape in the HTS coil is aligned nearly parallel to the field. One major purpose of this program was to perform magnetization studies in the coils made with the HTS tape. Magnetization-induced field errors are expected to be small when the field is nearly parallel to the wide face of the tape. The magnetization measurements were performed at 77 K with the two racetrack coils in two orientations, with field predominantly either parallel or perpendicular to the wide face of the HTS tape. In addition, measurements were also performed at 4 K in different background fields provided by the outer Nb3Sn coils. This paper will summarize the magnetization measurements and present the quenching experience of the HTS coils in this hybrid magnet system.

AC Loss Distribution in Two-Layer HTS Cable

We measured the influence of the twist pitch, twist direction, and transport current balance of the inner and outer layers on the ac loss characteristics in a two-layer twisted high temperature superconducting (HTS) cable. To measure the ac loss in each layer, we placed the thermo-couples on each tape face with thermal insulation. In the HTS cable, the inner layer is exposed to a longitudinal magnetic field generated by the twisted outer layer, and the outer layer is exposed to a circumferential direction magnetic field generated by the inner layer. When the inner and outer layers connect in series, the ac loss characteristics in the inner layer exhibit the influence of the outer layer arrangement. When the transport current balance of the inner and outer layer changes, the lowest loss condition exists when the inner transport current exceeds the outer transport current. These phenomena are possible explanations for the amplitude and direction of the magnetic field in each layer.

Modeling of Magnetization Loss in HTS Tape Exposed to All Magnetic Field Directions

In this paper, we suggest a new magnetization loss model for all magnetic field directions. We measured three types of HTS tapes, two different critical current Bi2223/Ag tapes, and an YBCO tape to compare the measurement with the magnetization loss model. Our new model is based on the measured magnetization losses, because the loss is affected by the intrinsic critical current distribution and the cross-section. The studied conditions of the base magnetization losses are: the magnetic field applied perpendicular to the tape axis and perpendicular to the tape face, perpendicular to the tape axis and parallel to the tape face, and parallel to the tape axis. This model enables us to describe magnetization losses in response to a change in the amplitude of the magnetic field, the angle of the tape axis, and the angle of the tape face.

Experimental Investigation of AC Loss Characteristics in HTS Tape With Rotating Magnetic Field

In this work, we exposed a high-temperature superconducting (HTS) tape to a rotating magnetic field, and measured ac losses using a calorimetric method based on the temperature rise caused by the ac loss. To compare the characteristics of the other ac loss conditions, we measured the magnetization losses in the uniform ac magnetic field perpendicular and parallel to the HTS tape face. The losses in the rotating magnetic field are the same as in the perpendicular magnetic field. In addition, we measured the influence of the rotating magnetic field waviness on the ac loss characteristics.

Decay of Shielding Current in Stacked REBCO Superconducting Tapes

We are developing superconducting magnets for MRI with $\mbox{Re}_{1}\mbox{Ba}_{2}\mbox{Cu}_{3}\mbox{O}_{7-\delta}$ (Re: rare earth, Y, Gd, Eu, REBCO) tapes. Superconducting magnets for MRI need the uniformity of the produced magnetic field in time and space. The disturbance is mainly caused by the induced shielding current in the wound superconducting tapes, which corresponds to the magnetization of the REBCO superconducting tapes. We have made a test coil with REBCO tapes and evaluated the influence of the shielding current on the produced magnetic field. In this paper, applying external magnetic field perpendicular to the EuBCO superconducting tape face, we investigated the magnetization and its relaxation properties of the tapes at 20–40 K using a saddle-shaped pickup coil. Increasing an applied magnetic field and interrupting the field sweep temporarily at 0, 0.5, 1, 1.5, 2, and 2.5 T, we observed the variation of the magnetization for each step of magnetic field. The magnetization decayed at a speed of a few to tens of percent per minute. It was faster beyond expectation. The relaxation rate of the magnetization depended on magnetic field, temperature, and the number of stacked tapes. It became faster with increasing temperature and magnetic field and with decreasing number of stacked tapes.

Magnetic field direction dependence of AC losses in Bi2223/Ag tape

In this paper, we applied a magnetic field in multiple directions to a Bi2223/Ag tape and measured the transport current and magnetization losses in each position using the four terminal and pickup coil methods. We rotated the tape face and the tape axis to the ac magnetic field by 15° segments. We report the magnetic field direction dependence of ac losses. When the magnetic field direction is parallel to the tape face and the tape axis is rotated in the magnetic field, each magnetization loss shows a crossing point. When the magnetic field is not parallel, the magnetization losses are increased as the angle of the tape axis to the magnetic field increases. We compared the total losses with additions of the transport current loss without the magnetic field and the magnetization loss without transport current in multiple magnetic field directions, and these results did not correspond.

Estimation of the critical current of race-track HTS magnet considering angular dependency

Abstract The high temperature superconducting (HTS) magnet has been developed for the high magnetic field applications such as NMR , MRI and other industrial machinery. In designing process of these HTS magnets, the accurate estimation on the critical current ( Ic ) is essential to predict and secure electromagnetic performance.the The critical current of 2G HTS tape has anisotropic Icdegradation characteristics with the application of magnetic field angular dependency of critical current. It is known that the – perpendicular magnetic field to the face of HTS tape makes dominant degradation on the critical current for conventional 2G HTS tape. However, recently developed 2G HTS tapehas more complex characteristics due to the artificial pinning center. Therefore, the method for estimation reflecting such characteristics of 2G HTS tape needs to b Ic e devised. The method considering the angular dependency is introduced in this paper. And the result of newly devised method is compared with that of previous method.

The Utilization of Genetic Algorithm on High Temperature Superconducting Magnet Design

In high temperature superconducting (HTS) magnet design, it well known that the critical current (Ic) is sensitive to the direction of the local magnetic field. The perpendicular magnetic field component to the face of tape have a larger effect on the Icof a HTS tape than the parallel component. Thus in HTS magnet design, the magnetic field distribution is the first considering factor. This work presents a HTS magnet design using genetic algorithm to obtain the object. When the length of the tape is certain, the results show that the process gives the optimal number of the double pancake and the inner radius of the magnet.

A Comparative Study on Hysteresis Losses in High $T_{ \rm c}$ Tapes for Superconducting Fault Current Limiter Applications

Hysteresis loss is an important parameter of high-temperature superconductors while designing inductive type superconducting fault current limiters (SFCLs). Furthermore, in high temperature superconducting (HTS) tapes, the major part of the magnetic field in a coil configuration is parallel to the tape face. To optimize the design of an SFCL based on HTS, the hysteresis losses should be minimum at the operating temperature and current. Therefore, we investigated M-H curves for Bi2223-based 1st generation (1G) and YBCO-based 2nd generation (2G) HTS tapes in magnetic fields applied parallel and perpendicular to the tape surface to obtain hysteresis losses. The temperature (35-77 K) and magnetic field (0-3 T) dependence of hysteresis loss and critical current together are analyzed for both 1G and 2G HTS tapes to assess their applicability to SFCL.

Development of REBCO Superconducting Transformers With a Current Limiting Function

We designed and fabricated a 10 kVA single-phase transformer with RE1Ba2Cu3O7-δ (REBCO, RE:Rare Earth, Y, Gd, etc.) superconducting tapes. It had a 4-winding structure at the beginning. In this study, we removed the auxiliary windings and made a test as a general 2-winding transformer. Making repeated short circuit tests, we further investigated the response of the REBCO superconducting windings against a fault excess current. Although the short circuit current decayed satisfactorily, the whole of the REBCO superconducting windings did not shift to normal state even if the short circuit current exceeded the critical current all over the length of REBCO tapes. The induced normal resistance depended on the primary voltage. To study the phenomena, we derived the basic equations and made numerical simulations taking into account of flux-flow state, in which the equivalent resistance and generated heat are much smaller than those in normal state, and also the dependences of the critical current on temperature, applied magnetic field and the applied field angle to the tape face. It was clarified that short circuit current was limited not only by normal resistance but also by flux-flow one. The introduction of the concept of flux-flow state made it possible to quantitatively explain the behavior of the REBCO superconducting transformer at a sudden short circuit.

AC Loss Properties of Laser-Scribed Multi-Filamentary GdBCO Coated Conductors with Artificial Pinning Centres

RE 1 Ba 2 Cu 3 O 7-∂ (RE: Rare Earth, Gd, Y and so on) coated conductors have a large aspect ratio and also anisotropy in critical current density. For practical application of REBCO tapes, we evaluated J c and ac loss properties in a wide range of temperature and magnetic field. The induced ac losses due to the perpendicular magnetic field to a tape face are considerably huge. Therefore, in case that REBCO coated conductors are applied electric power machines and devices, ac loss induced in the windings should amount to a great part of the total heat load. We proposed an ac loss reduction method, which is combined with a laser scribing technique and a special winding process into a multi–filamentary structure. Additionally, we are trying to enhance critical current density J C, by doping artificial pinning centre. In this study we investigated the ac loss properties of multi-filament structural GdBCO tapes with BZO nano–lods as artificial pinning centers. We measured ac losses of 5, 10, 20-filament GdBCO tapes by using a saddle-shaped pickup coil. Consequently we confirmed the ac loss decreased in inverse proportion to the number of filaments even in the GdBCO tapes with enhanced J C by introducing artificial pinning centers.

Self‐assembly of amphiphilic β‐sheet peptide tapes based on aliphatic side chains

Amphiphilic β-sheet nanotapes based on the self-assembly of 9mer and 7mer de novo designed β-strand peptides were studied in the dilute regime. The hydrophobic face of the tapes consisted predominantly of aliphatic (leucine) side chains, while the hydrophilic tape face contained polar side chains (glutamine, arginine and glutamic acid). Both peptides underwent a transition from a monomeric random coil to a self-assembled β-sheet tape upon increase of peptide concentration in aqueous solutions. P(9) -6 exhibited lower critical concentration (c*) for self-assembly and thus higher propensity for self-assembly in water, compared to the shorter P(7) -6. At neutral pH where there was little net charge per peptide, self-assembly was favoured compared to low pH in which there was a net + 1 charge per peptide; the net charge decreased overall intermolecular attraction, manifested as an increase in c* for self-assembly in low compared to neutral pH aqueous solutions. Propensity for self-assembly and β-sheet formation was found to be greatly enhanced in a polar organic solvent (methanol) compared to water. These studies combined with future more extensive comparative studies between amphiphilic tapes based on aliphatic amino acid residues and amphiphilic tapes based on aromatic residues will throw more light on the relative importance of hydrophobic versus aromatic interactions for the stabilisation of peptide assemblies. Systematic studies of this kind may also allow us to throw light on the fundamental principles that drive peptide self-assembly and β-sheet formation; they may also lead to a set of refined criteria for the effective design of peptides with prescribed combination of properties appropriate for specific applications.

Unique behaviour of RE1Ba2Cu3O7−δ superconducting tapes producing drastic reduction of pinning loss

The reduction of flux-pinning loss is a paramount concern for power applications ofRE1Ba2Cu3O7−δ (RE: rare earth, Y, Gd, etc; REBCO) superconducting tapes. We have discovered a uniqueelectromagnetic behaviour of such tapes where the magnetization loops in an obliquemagnetic field showed a drastic deviation from the conventional theoretical prediction. Itbrought about a drastic reduction of the pinning loss contrary to common knowledge thatpinning loss is proportional to a critical current density. The deviation was enhanced atlower temperatures, at higher magnetic fields, when the applied magnetic field angleto the tape face was smaller, in the tapes with higher critical current densities,and with a higher in-plane alignment of REBCO grains. We also verified thisunusual behaviour is intrinsic to REBCO superconducting thin films and inferredthe origin lies in the fact that magnetic flux has a tendency to penetrate into,and exude out of, REBCO superconducting films in a direction parallel to theCuO2 planes in its crystal structure. The results suggest the possibility of reducing the pinningloss by improving the in-plane alignment of REBCO grains instead of reducing thegeometrical width of superconductors as suggested by the conventional theory.

Improvement of coil performance by optimal use of multifilamentary superconducting tapes

It was theoretically confirmed that improvement of superconducting-coil performance can be obtained by making optimum use of multifilamentary tapes with high aspect-ratio cross-sections. Superconducting tapes, such as NbTi and MgB2 multifilamentary tapes, are known to have anisotropic electromagnetic properties, i.e., critical current densities and ac losses; under external transverse magnetic fields which are parallel to the tape face, the critical current density is higher than that of the multifilamentary round wire, but the ac loss is lower. For tapes with a high aspect-ratio cross-section, the anisotropic properties are still more remarkable, and therefore it is predicted that such a tape with superior electromagnetic properties will prove valuable as the windings of high performance coils for practical applications. In this paper, some examples of coil arrangements such as single or plural solenoid coil systems are shown in order to highlight the usefulness of this superior tape performance. The effect of the high aspect-ratio of tape cross-section on the electromagnetic coil performance was also discussed, taking the cooling condition of coil windings into account.

Improvement of Electromagnetic Properties of ${\hbox{MgB}}_{2}$ Filaments Due to Deformation to Tape Shape

We have proposed MgB2 multifilamentary tapes with high critical current densities and low ac losses under external magnetic fields which is parallel to the tape face. The purpose of this proposal is to develop the MgB2 new conductors composed of plural tapes as windings of superconducting coils with both large current capacities and low losses for fusion reactors, superconducting magnetic energy storage systems, and so on. Our MgB2 multifilamentary tapes are fabricated by rolling process from round wires. The tapes are fabricated by using in-situ method. The purpose of this paper is to clarify the effects of deformation to tape shape on the electromagnetic properties of MgB2 filaments in multifilamentary wires. For the purpose, measurements of both critical currents and magnetizations have been carried out on MgB2 fine single filament wires which have simulated a filament in multifilamentary wires. Measurements on wires with large diameter have been also carried out, in order to clarify the effect of diameters of original round wires. We prepared pure and SiC-added MgB2/Nb/Cu composite wires as the original round wires before rolling process. The diameters of round wires have been varied and then the aspect ratios of tapes have been varied. The observed critical current densities of the tapes are about 7 times larger than that of the original round wire at a maximum. The ratios of Jc of wires to tapes don't have dependence of the diameter of the original round wire. In addition, the results of measurements of magnetizations have shown that magnetization losses in MgB2 tapes decrease to 1/3 compared with original round wires.

Transverse Thermal Conductivity in an Epoxy Impregnated ${\rm MgB}_{2}$ Coil

Superconducting magnets operate at low temperatures, and therefore, even small heat pulses can ruin their stable operation. For example, resistive joints or changes in the operation current generate heat which must be extracted to prevent a quench. In impregnated magnets, the transverse thermal conductivity inside the coil has a vital influence on the heat extraction, and it dominates the 3D quench propagation. In this study, the transverse heat conductivity is measured from the cross-section of a small epoxy impregnated MgB2 coil at temperatures between 10 and 35 K. Finally, the results are analysed and compared with the results of a computational model based on heat conduction equation solved with the finite element method. The results show that effective thermal conductivity is over two orders of magnitude higher in the parallel direction with conductor axis compared to the perpendicular direction. The measured effective thermal conductivities at 20 K parallel to the broad tape face and perpendicular to the broad tape face were 1.55 W/mK and 0.31 W/mK, respectively. The fill factor of the measured coil was 60.5% for the whole conductor.