AC Loss Reduction Research Articles

Superconducting–magnetic heterostructures: a method of decreasing AC losses andimproving critical current density in multifilamentary conductors

Magnetic materials can help to improve the performance of practical superconductors onthe macroscale/microscale as magnetic diverters and also on the nanoscale as effectivepinning centres. It has been established by numerical modelling that magneticshielding of the filaments reduces AC losses in self-field conditions due to decouplingof the filaments and, at the same time, it increases the critical current of thecomposite. This effect is especially beneficial for coated conductors, in whichthe anisotropic properties of the superconductor are amplified by the conductorarchitecture. However, ferromagnetic coatings are often chemically incompatible withYBa2Cu3O7 and(Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, inMgB2 conductors an iron matrix may remain in direct contact with the superconducting core.The application of superconducting–magnetic heterostructures requires consideration of thethermal and electromagnetic stability of the superconducting materials used. On one hand,magnetic materials reduce the critical current gradient across the individual filamentsbut, on the other hand, they often reduce the thermal conductivity between thesuperconducting core and the cryogen, which may cause destruction of the conductor in theevent of thermal instability. A possible nanoscale method of improving the critical currentdensity of superconducting conductors is the introduction of sub-micron magnetic pinningcentres. However, the volumetric density and chemical compatibility of magneticinclusions has to be controlled to avoid suppression of the superconducting properties.

Magnetic flux penetration and AC loss in a composite superconducting wire withferromagnetic parts

The current distribution and the AC loss in a composite superconducting tape containing alayer of magnetic material is calculated and compared with experiments, showing a verygood agreement. The situations of an alternating uniform applied field or a transportcurrent are studied. The newly developed numerical model is an approximation to thecritical state model, adapted for applicability to commercial finite element codes that solvethe vector potential. A substantial feature of this procedure is that it can be carried out inthe case when the critical current density in the superconductor depends on the magneticfield and the magnetic layer material is nonlinear. Additionally, the hysteresis loss inthe magnetic material is estimated, based on its measured magnetization loops.Measurements on Bi-2223 multifilamentary tapes covered on edges by nickel confirmed ourpredictions, showing a substantial ac loss reduction in both the investigated regimes.

Influence of magnetic materials on the transport properties of superconducting compositeconductors

Magnetic materials can help to improve the performance of practical superconductors onthe macro/microscale as magnetic diverters and also on the nanoscale as effectivepinning centres. It has been established by numerical modelling that magneticshielding of the filaments reduces ac losses in self-field conditions due to decouplingof the filaments and, at the same time, it increases the critical current of thecomposite. This effect is especially beneficial for coated conductors, in whichthe anisotropic properties of the superconductor are amplified by the conductorarchitecture. However, ferromagnetic coatings are often chemically incompatible withYBa2Cu3O7 and (Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, inMgB2 conductors an iron matrix may remain in direct contact with the superconducting core.The application of superconducting–magnetic heterostructures requires consideration of thethermal and electromagnetic stability of the superconducting materials used. On the onehand, magnetic components reduce the critical current gradient across the individualfilaments but, on the other hand, they often reduce the thermal conductivity between thesuperconducting core and the cryogen, which may cause the destruction of the conductor inthe event of thermal instability. A possible nanoscale method of improving the criticalcurrent density of superconducting conductors is the introduction of sub-micron magneticpinning centres. However, the volumetric density and chemical compatibility of magneticinclusions has to be controlled to avoid suppression of the superconducting properties.

AC Loss Reduction in Bi2223 Tapes with Interfilamentary Oxide Barriers

AC Loss Reduction in Bi2223 Tapes with Interfilamentary Oxide Barriers

Development of YBCO HTS cable with low AC loss

High temperature superconducting (HTS) cables using YBCO tapes are expected to be more economical because AC losses will be much smaller than conventional cables. To reduce AC loss, 10 mm wide YBCO tapes were divided into five strips using a YAG laser. Using narrower strips and optimizing the space between the strips were effective in reducing AC loss. A 1 m conductor was fabricated, and AC loss was 0.048 W/m at 1 kA and 50 Hz. Based on the successful AC loss reduction in the 1 m conductor, we will fabricate a 10 m HTS cable with a three-layer HTS conductor, electrical insulation, and a one-layer HTS shield and cupper protection layer for overcurrent. In addition, we have developed a prototype of the HTS cable joint that can withstand an overcurrent condition of 31.5 kA for 2 s.

Design Considerations for Fast-Cycling Superconducting Accelerator Magnets of 2 T B-Field Generated by a Transmission Line Conductor of up to 100 kA Current

Recently proposed synchrotrons, SF-SPS (Super- Ferric SPS) at CERN and DSF-MR (Dual Super-Ferric Main Ring) at Fermilab, would operate with a 0.5 Hz cycle (or 2 second time period) while accelerating protons to 480 GeV. We examine possibilities of superconducting magnet technology that would allow for an accelerator quality magnetic field sweep of 2 T/s. For superconducting magnets the AC losses in the coil compromise magnetic field quality and require high level of cryogenic cooling power. We outline a novel magnet technology based on HTS superconductors that may allow reduction of AC losses in the coil possibly up to an order of magnitude as compared to similar applications with LTS type conductors.

AC Loss Characteristics in Bi2223/Ag Tapes With Interfilamentary Oxide Barriers for Filament Decoupling

In this paper, we prepared Bi2223/Ag tapes with interfilamentary oxide barriers for filament decoupling and also for reducing AC losses under AC external magnetic field. To suppress the side effect on Bi2223 phase formation inside the filaments and superconducting properties in final tapes by introducing oxide barriers, we selected Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> as barrier materials. In addition, the 30 wt% Bi2212 powders were mixed into Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , to improve their deformation properties for cold working such as drawing and flat rolling. The mixtures of Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Bi2212 were introduced around the twisted Bi2223 filaments. AC losses in barrier tapes under parallel transverse magnetic field were measured at 77 K, and compared with the several analytical models. Based on the results, the effect of barrier introduction on the AC loss characteristics in twisted multifilamentary tape is discussed.

Effects of Cr Diffusion on $RRR$ Values of Cr-Plated Nb$_{3}$Sn Strands Fabricated by Internal-Tin Process

Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands which will be used for the ITER TF coils should be chrome plated thickness of 1.8 mum to reduce AC loss and improve the thermal stability of cable. During heat treatment, the RRR value can be affected by Cr diffusion. In this work, the effect of Cr diffusion was systematically studied, using the KSTAR and ITER candidate strands. For the KSTAR strand, the Cu/non-Cu ratio was systematically varied from 1.53 to 0.69 by chemical etching whereas the samples were heat treated by the same schedule. For the ITER candidate strand, on the other hand, the Cu/non-Cu ratio was fixed to the specification value of 1.0, and the variation of the RRR value was studied with respect to the heat treatment schedule, especially the retention time of the 650degC plateau was varied from 100 to 200 hours. It was also compared with the Cr diffusion distance obtained from EPMA (electron probe microanalysis). We found that the RRR value of the ITER candidate strand becomes lower than the ITER requirement of 100, if the heat treatment at 650degC is longer than 200 hours. The RRR can be reached up to 161 when the duration of 650degC plateau is reduced to 100 hours, while satisfying all other ITER requirements including the critical current density.

Results of the Excitation Test of the LHD Helical Coils Cooled by Subcooled Helium

Large helical device, the largest superconducting stellarator, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is produced by a pair of pool-cooled helical coils, in the innermost layers of which a normal-zone had been induced several times at the bottom of the coil at higher currents than 11.0 kA. Since the field is not the highest there, the local cooling conditions are probably deteriorated by bubbles gathered by buoyancy. In order to improve the cryogenic stability by subcooling, an additional cooler with two-stage cold compressors was installed at the inlet of the coil in 2006. The inlet and outlet temperatures of the coils were successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. In spite of a half charging rate to reduce AC losses, a normal-zone was induced near the top of the coil at 11.45 kA. It propagated to one side and stopped near the inner equator, where the field is the highest. In comparison with the stability tests with a model coil, the local temperatures of the innermost layers near the top is considered to have been raised up to almost the saturated temperature of 4.4 K by charging. The excitation method was revised to waiting cool-down at 11.0 kA, and the excitations up to 11.5 kA have been attained.

AC loss properties in YBCO model coils for loss reduction

We have strategically promoted R&D activity for the development of elemental technologies in order to put into practical HTS power transformers applying YBa 2Cu 3O 7 (YBCO) coated conductors, focused on AC loss reduction in HTS coils. This paper shows AC loss reduction in model coils of YBCO tapes divided into five filaments by laser ablation. At first experiment, magnetization loss of YBCO tape with five filaments and YBCO tape with non-divided were firstly measured using a pick-up coil technology. Then, AC loss measurement is performed using two types of model coils of YBCO tapes with five filaments and non-divided, by two different technologies: an electric technique (i.e. 4-probe) and a calorimetric technique (i.e. boil-off). The results of the experiment show that the AC loss in the model coil of YBCO tapes with five filaments was reduced to one-fifth when compared with model coil of YBCO tapes with non-divided.

Fabrication and characterization of Bi2223 tapes with Al 2O 3 barriers for reduction of AC losses

We fabricated and characterized the Bi2223 tape with Al 2O 3 as interfilamentary resistive barriers for the filament decoupling and also for the reduction of AC losses under AC external magnetic field. The tapes with thin Al 2O 3 barrier layers around the Bi2223 filaments were prepared by using a standard powder-in-tube (PIT) method. Al 2O 3 powders with mean grain size below 1 μm were used and they are coated with the slurry on the outside surface of hexagonal monocore rod before stacking. The phase formation inside the filament was evaluated by XRD measurement. The uniformity of transport properties ( J c) for Al 2O 3 barrier tapes was measured on the order of several metre lengths and compared with data for the tapes without barriers and with Ca 2CuO 3 + Bi2212 barriers. To evaluate the effect of introducing Al 2O 3 barriers on the filament decoupling, frequency dependence of AC losses under AC transverse magnetic field for barrier tapes were investigated.

Influence of the conductor’s arrangement and current on the AC loss characteristics of a fault current limiting coil

Researches and developments on a superconducting fault current limiting (SFCL) system have been done for several decades. The SFCL system, which is using a high temperature superconducting (HTS) conductor such as Bi-2212 bulk and YBCO film, has recently become technically available. In this work, we are developing a 22.9 kV/25 MVA SFCL system for a power distribution network. A Bi-2212 bulk SFCL element, which has the merits of large current capacity and high allowable electric field during fault of the power network, becomes a candidate for our resistive SFCL system. AC loss of the HTS conductor has a strong influence on the economic viability of the SFCL system, which offers an attractive means to limit short circuit current in the power network. As an earlier stage of our 22.9 kV/25 MVA SFCL system development, we prepared several samples of the fault current limiting elements in a coil shape to reduce AC loss of the SFCL system. The effect of conductor’s arrangement and current direction on AC loss characteristics of the fault current limiting coils has been investigated experimentally. The test result shows that the measured AC loss depends significantly on the conductor’s arrangement and also on the current direction even in the same arrangement. The AC loss of a face-to-face coil in bifilar mode, which is even smaller than the self-field loss of the conductor, is smallest among our fabricated coil samples. Moreover, the measured loss in the face-to-face coil in bifilar mode is invariable regardless of thickness in an insulator which is placed between conductors.

Development of scribing process of coated conductors for reduction of AC losses

Coated conductors (CCs) are prospective for electric power applications intended for not only better cost performance but also high critical current density ( J c) under magnetic fields comparing with Bi 2Sr 2Ca 2Cu 3O y (BSCCO) tapes. Furthermore, they also have a possibility to reduce AC losses by various methods of post-treatments since the CCs are of a laminated structure with the insulating buffer layers. Multifilamentation by narrowing the tape width is quite effective to reduce the AC losses and therefore this approach is valid for the standard process to fabricate practical CCs with an appropriate architecture. In this study, we developed a tape scribing technique applying a technique of YAG laser with/without chemical etching to control the filament width in YBa 2Cu 3O y (YBCO) CCs for the multifilament. The AC loss was reducible to one-twentieth by 20-multifilament structure in a short sample test, and to one-third by 3-multifilament structure in the coil using 28 m long tapes in total. The authors measured the resistance between the filaments at 60–300 K after post-treatment to investigate the effective bridged materials, which is essential for decoupling of the filaments. We improved the resistance value between the filaments at liquid nitrogen temperature up to the order of 10 Ω/cm using the YAG laser with high-temperature oxygen anneals. Furthermore, the resistance over 10 6 Ω/cm was evident by the combination of laser scribing and chemical etching, which is much higher than the requirements of all the applications.

Overview of Nb 3Sn and V 3Ga conductor development in Japan

In the early stage of high-field A15 conductor development in Japan, different type of V 3Ga conductors were fabricated. Then, Ti-doped Nb 3Sn conductors have been developed, and widely used for high-field generation. Increase of Sn concentration in the bronze produces an appreciable progress in the performance of bronze processed (Nb,Ti) 3Sn conductors. New internal Sn processed (Nb,Ti) 3Sn conductors with modified cross-sectional configurations have been produced, which exhibit large J c in high fields as well as reduced AC loss. Both bronze processed and internal Sn processed (Nb,Ti) 3Sn conductors satisfy recent ITER magnet specifications. As for new type Nb 3Sn conductors, powder core and Jelly Roll processed (Nb,Ta) 3Sn wires with improved high-field performance have been fabricated.

Fabrication and characterization of Bi2223/Ag tapes with interfilamentary oxide barriers for reducing AC losses

We prepared and characterized Bi2223/Ag tapes with interfilamentary oxide barriers for AC loss reduction in external AC magnetic field. Ca2CuO3, Al2O3, and SrZrO3 were selected as barrier materials. 20-30wt% Bi2212 were added to Ca2CuO3 and SrZrO3 powders to improve their deformation properties for cold working. The multifilamentary tapes with interfilamentary oxide barriers were fabricated with a conventional powder-in-tube method. The influence of introducing different oxide barriers on the final tape properties such as the phase formation in filaments, critical current density Jc and its uniformity along a tape length, was evaluated. Based on the results of the preliminary studies, we determined the oxide materials for fabrication of barrier tapes with twisted filaments. AC losses in barrier tapes with twisted filaments were measured at 77 K in transverse AC magnetic field. The AC loss characteristic of the tape with barrier was compared to the one of the tape without barrier.

Scan measurements on ROEBEL assembled coated conductors (RACC)

The application of RE-BCO coated conductors (cc's) in high field magnets as well as in electrical machinery decisively depends on the ability to produce conductors with high current densities and small losses. The production process of the conductors restricts the thickness of the superconducting layer and therefore the current. It is possible to enhance the current carrying capability by combining several conductors to a cable. The ROEBEL bar concept is the most suitable solution because of reduced ac losses. Cc's are suitable for shaping ROEBEL strands and to perform the cabling. We employed different scan techniques to get an overview of the conductor properties. The magnetoscan, which was successfully applied to cc's, was optimised for scanning a prototype of such a RACC-cable. The resulting map of the magnetic field reflects the superconducting properties of the conductor and allows analysing the defect structure and the critical currents. Due to the complex configuration of the RACC-cable some considerations concerning the effects at conductor edges and the transitions between the strands were necessary. The field map of the ROEBEL-cables can be used to optimise them.

Numerical calculation of AC losses in multi-layer superconducting cables composed ofcoated conductors

Two types of four-layer cables composed of coated conductors, with practicalcross-sectional geometries and various critical currents, were designed. The AC lossesin the cables were numerically calculated using a numerical model employing aone-dimensional FEM (1D model). The AC loss of each layer in the four-layer cable withw = 4.0 mm was compared with Norris’s analytical values,QNS, for a widerange of It/Ic. The AC loss in the first (innermost) layer was smaller than itsQNS, whereas the losses in the outer layers were close to, or much larger than, theirQNS. These results indicate that, in multi-layer cables, the AC loss in an outerlayer increases because of the magnetic field generated by the current inthe inner layers, and the entire AC loss of the cable is dominated by theAC losses in the outer layers. The AC losses in the four-layer cables withw = 3.3 mm, which had smaller gaps between the conductors, were smaller than those for the cable wherew = 4.0 mm. These results showed the effectiveness of using narrower conductorsand decreasing the gaps to reduce AC loss in multi-layer cables composedof coated conductors. The AC losses of the four-layer cable withw = 3.3 mmand Icc/w = 500 A cm−1 at 1 and 3 kArms were much smaller than the measured AC losses in cables fabricated withYBCO-coated conductors or BSCCO tapes. The AC loss in a four-layer cable with practicalcross-sectional geometries should approximately be within an order of magnitude of theirQNS.

バリア入りBi2223多芯線材の作製と交流損失特性

Synopsis: Bi2223 multifilamentary tapes with interfilamentary oxide barriers were fabricated and their AC loss properties were examined under AC external magnetic field. In order to avoid the side effect on Bi2223 phase formation during heat treatment, Ca2CuO3 and SrZrO3 with average grain sizes of 1-2 μm were selected as barrier materials. Furthermore, an additional Bi2212 powder corresponding to 20wt% were mixed with them to improve their deformation properties for cold working. The mixed oxide powders were introduced among the twisted Bi2223 filaments. The critical current densities (Jc) in the barrier tapes were 10-20% lower than those in the tapes without barriers. In a parallel field, the loss reduction in the SrZrO3/Bi2212 barrier tape is most remarkable because the continuity of the SrZrO3/Bi2212 barriers is far superior to the Ca2CuO3/Bi2212 barriers. At 50 mT around power-grid frequencies, parallel field losses in a twisted tape with SrZrO3/Bi2212 barriers are 80% lower than the non- twisted tape. From the frequency dependence of AC losses in a perpendicular field, it was confirmed that the transverse resistivity in the SrZrO3/Bi2212 barrier tape is 8-9 times higher than the tape without barriers. However, around the power-grid frequencies, the introduction of barrier reduces AC losses only slightly in a perpendicular field. The twist pitch of the present barrier tape (= 16 mm) is probably too long to decouple the filaments in a perpendicular field.

A new approach for AC loss reduction in HTS transformer using auxiliary windings, casestudy: 25 kA HTS current injection transformer

AC loss is one of the important parameters in HTS (high temperature superconducting)AC devices. Among the HTS AC power devices, the transformer is an essential partin the electrical power system. The AC losses in an HTS tape depend on themagnetic field. One of the techniques usually adopted to mitigate the unwantedmagnetic field is using a system of coils that produce a magnetic field oppositeto the incident one, reducing the total magnetic field. In this paper adding twoauxiliary windings to the HTS transformer to produce this opposite magneticfield is proposed. The proper use of these auxiliary windings could reduce theleakage flux and, therefore, the AC loss. A mathematical model is used to describethe behaviour of a transformer operating with auxiliary windings, based on thetheory of electromagnetic coupled circuits. The influence of the auxiliary windingson the leakage field is studied by the finite element method (FEM) and the ACloss of an HTS transformer was calculated. Also, the simulation results showthat employing auxiliary windings will improve the HTS transformer efficiency.

Development of a laser scribing process of coated conductors for the reduction of AClosses

Expectations are high for using coated conductors for electric powerapplications not only because of better cost performance but also higherJc–B propertiescompared with Bi2Sr2Ca2Cu3O10 (BSCCO) tapes. Furthermore, the coated conductors could also reduce AC losses bythe use of various methods in post-treatment. When a YAG laser was used forscribing the superconductive layer, an AC loss reduction due to a decrease inthe width of the superconductivity layer could be confirmed in short samples.In the case of AC applications using long tapes, high resistance between scribed filaments isnecessary. However, the resistance between the filaments scribed by a laser was as low as10−3 Ω cm−1. The reason for the low resistance was the existence of dross in the filament spacings,which was confirmed by a cross-sectional compositional mapping observation.Although the superconductivity layer was divided by the laser, the dross of the metalsubstrate made a bridge over the superconductivity filaments. Post-annealing in anO2 atmosphere to oxidize the dross was carried out in order to increase the resistance.Consequently, the resistance between the filaments was improved to be as high as50 Ω cm−1. We succeeded in controlling the resistance value between filaments in a range five ordersof magnitude higher than that of the as-scribed YBCO film, through oxygen atmospherepost-annealing.