Multifilamentary Conductors Research Articles

Prospects of an alternative superconductor technology for fusion reactors

New concepts for Tokamak fusion reactors are enabled by ReBCO high temperature superconductors, either to achieve toroidal magnetic fields in the range of 20 T, and/or by operating temperatures above 4.2 K, e.g., 20 K. The application of ReBCO tapes is challenging because common techniques for the manufacturing and quench protection of magnets, developed for classical multifilamentary superconductors, such as NbTi and Nb3Sn, cannot be applied directly. Less risky would be the use of a ternary molybdenum chalcogenide (TMC) superconductor, which was under development before the discovery of high temperature superconductors. Although a low temperature superconductor, the upper critical field is extremely high resulting in a comparable field dependence of the critical current to ReBCO. Because of the improved superconductor fraction of a multifilamentary TMC conductor, the expected engineering current density can be one order of magnitude higher. In addition, a TMC conductor has the potential for cost efficiency and a performance index around 1 $/kAm at 20 T seems be possible.

Enhancing Transport Performance in 7-filamentary Ba0.6K0.4Fe2As2 Wires and Tapes via Hot Isostatic Pressing

Iron-based superconductors (IBS) are considered as potential materials for manufacturing high-field magnets, for which developing multi-filamentary conductors with high performance and high strength is essential. Herein, 7-filamentary Cu/Ag composite sheathed Ba0.6K0.4Fe2As2 (Ba122) round wires and tapes were successfully prepared through the ex situ powder-in-tube (PIT) method and treated by hot isostatic pressing (HIP) process. Pure Ba122 phase with roughly homogeneous element distribution was obtained in the superconducting filaments. The wires and tapes show very small low-temperature normal-state resistivity of 0.12 μΩ cm and 0.17 μΩ cm respectively owing to the high electrical conductivity of copper and silver. The HIP process greatly enhances the mass density of the superconducting filaments and promotes the formation of well-grown plate-like Ba122 grains. Vickers hardness measurements on the cross sections of these filaments reveal a rather good uniformity of the mass density. The transport critical current density (Jc) in the 7-filamentary Ba122/Ag/Cu round wires and tapes reached 1.3 × 104 A cm−2 and 4.8 × 104 A cm−2 at 4.2 K in 10 T respectively. These results indicate that hot isostatic pressing is advantageous in developing high-performance multi-filamentary iron-based superconductors.

Experimental study on strain sensitivity of Internal-Tin Nb3Sn superconducting strand based on non-destructive technology

Fracture of brittle filaments plays a crucial role in significant degradation of critical current density of multifilamentary Nb3Sn strand. Recent researches of distribution of filament factures mostly depend on photographic analysis with SEM or TEM. However, it is hard to obtain spatial distribution of filament fractures in the mulitifilamentary Nb3Sn strand. In addition, almost unavoidable filament fracture is introduced during the sample preparation procedure and it causes deviation of filament fracture distribution. X-ray microtomography is a non-invasive, non-destructive method, which allows determination and the discrimination of internal features without destructing the sample. It is widely used as a non-destructive test technology for its high penetrability and high resolution. This present work investigated filament fracture of two types of Nb3Sn strands under different tensile strains by high energy X-ray. The influence of tensile strain on the filament fracture behavior was investigated and the results will be expected to provide reference for optimization of multifilamentary Nb3Sn strand, conductor and magnet design.

Non-twisted stacks of coated conductors for magnets: Analysis of inductance and AC losses

Almost all present High Temperature Superconducting (HTS) cable designs for magnets are based on twisted or transposed concepts that were developed for Low Temperature Superconducting (LTS) cables. However, requirements for LTS materials (like filament twisting) are in general not valid for HTS materials, which are extremely stable; for example, non-twisted multifilamentary Bi-2223 tapes have been successfully used in several magnets. Is twisting necessary for HTS cables? We investigated inductance mismatches and AC losses by numerical and analytical methods in twisted and non-twisted stacks of coated conductors; various experiments reported in the literature support the analysis. Large (hysteretic) losses are common in all magnets built with tapes and are far larger than in magnets built with LTS multifilamentary conductors, because of the aspect ratio and large width of the tape. In small magnets, losses and residual magnetisation could be reduced by replacing a wide tape with a non-twisted stack of narrow tapes. In large cables, we have found that twisting a stack of tapes reduces losses only marginally. Therefore, non-twisted stack cables could be designed to have losses comparable to those of twisted ones. Some examples of non-twisted large cables for fusion applications are discussed: non-twisted stack designs can be simpler, more robust and cost effective than twisted ones, but would require additional R&D.

Screening-Current-Induced Strain Gradient on REBCO Conductor: An Experimental and Analytical Study With Small Coils Wound With Monofilament and Striated Multifilament REBCO Tapes

Screening currents in REBCO conductors, induced by time-varying magnetic fields, not only affect the field quality of HTS coils but also cause strain gradients along REBCO tape width that may overstress REBCO conductors used in NMR and other high-field magnets. In this paper, we present results of an experimental and analytical study on screening-current-induced strain gradients, performed with small REBCO pancake coils. Because we believe that screening current effect is reduced in multifilament conductor, we have studied 2 test coils, φ150 mm, one wound with monofilament and the other with 3-striate/4-filament REBCO tapes. A 5-T/ 300-mm room-temperature bore magnet was used not only to excite a strong screening current but also apply the nonuniform Lorentz force to each coil at 4.2 K. Our experiment and analysis have quantitatively demonstrated that we can effectively suppress the screening-current effect on strain gradient, not surprisingly, by using striated multifilament REBCO conductor.

Screening current effect on the stress and strain distribution in REBCO high-field magnets: experimental verification and numerical analysis

The shielding (or screening current) effect in high-Tc coated conductors has a strong influence on its stress/strain distribution in a coil winding, especially during high-field operations. To demonstrate this phenomenon, a special experimental setup was designed. With an LTS background magnet and a small HTS insert coil, we were able to carry out direct observations on the hoop strains of a 10 mm wide REBCO sample. Measured data was compared with numerical solutions obtained from models based on T–A formulation and elastic models, showing good agreement. An analytical expression was proposed to estimate the maximum radial Lorentz force considering the screening current effect. Using the developed numerical models, we further studied two methods that are utilized for reducing the screening-current-induced magnetic field, i.e. use of multi-filamentary conductors and current sweep reversal approach.

Ternary Nb3Sn superconductors with artificial pinning centers and high upper critical fields

In this letter we demonstrate the development of ternary Nb3Sn multifilamentary conductors with artificial pinning centers (APC) which achieve high critical fields. These recently-developed conductors were tested in a 31 T magnet, and the results showed that their upper critical field (Bc2) values at 4.2 K are 27–28 T, and irreversible field (Birr) values are above 26 T, values similar to or higher than those of best rod-restack-process (RRP) conductors. The non-Cu Jc has been brought to nearly 1200 A mm−2 at 16 T and 4.2 K, comparable to RRP, in spite of the fact that the fine-grain Nb3Sn fractions in filaments are still low (20%–30%) and the grain sizes are still not fully refined (70–80 nm) due to conductor designs and heat treatments that are not yet optimized. The Nb3Sn layer Jc at 4.2 K, 16 T is 4710 A mm−2 for the APC wire with 1%Zr, about 2.5 times higher than RRP conductors, in spite of the fact that its grain size is not yet fully refined due to insufficient oxygen and unoptimized heat treatment. An analysis is presented about the non-Cu Jc that can be achieved by further optimizing the APC conductors and their heat treatments.

Optimization of the Microstructure of Nb3Sn Layers in Superconducting Composites

This paper presents a brief review of the study of the evolution of the microstructure of superconducting Nb3Sn layers prepared using different regimes of diffusion annealing in multifilamentary conductors of different designs made by the “bronze” method and by the internal-tin-source method. The work is based on original investigations of the authors and on the most significant results published by others. In this paper we consider the influence of the manufacturing method, alloying, geometry of wires, and heat treatment modes on the formation of Nb3Sn layers, their structure and morphology, and, as a consequence, on the current-carrying capacity of multifilamentary superconductors.

A 2.5-T, 1.25-m Free Bore Superconducting Magnet for the Magnum-PSI Linear Plasma Generator

DIFFER's main experiment, Magnum-PSI, is the only laboratory setup in the world capable of exposing materials to plasma conditions similar to those of future fusion reactors. The success of the Magnum-PSI experiment depends on the generation of a 2.5-T magnetic field without restricting the diagnostic access and operational aspects of the experiment. This has been achieved with a magnet consisting of five superconducting solenoids wound on a 2.5-m-long stainless steel coil former positioned in a cryostat offering a 1.25-m warm bore. A copper stabilized multifilamentary NbTi conductor with a 3.48-mm2 cross section has been used; thus the magnet exhibits a total inductance of 500 H and a stored energy of 16 MJ. This presents quite a challenge for the protection scheme that has been implemented using a mix of back-to-back cold diodes and external dump resistors. The coils generate a plateau-shaped magnetic field adjustable up to 2.5 T while the distance between the coils allows for 16 room temperature view-ports. The coils are cooled with liquid helium using a recondensing system operated with cryocoolers, while the magnet system cycles between zero and full field up to once per day. The magnetic stray field is shielded down to 1 mT outside the experimental area by iron walls that flank the magnet.

Transport Properties of Single MgB2 Filaments Extracted From Multifilamentary Conductors

Using an electrochemical bath copper and nickel metals and alloys are removed in multifilamentary MgB2 conductors to leave single MgB2 filaments surrounded by a Nb sheath. Starting from a 0.55 mm 36 filament diameter conductor, the etched filaments are 60 microns in diameter. Seven wires were characterized by transport critical current measurement, and one investigated further by resistivity when progressively etched. Their radial location in the original multifilamentary wire structure is then used to explore if the critical current is sensitive to the deformation process. The average critical current of the outer layer of filaments is 35% higher than the inner layers. The average filament critical current was 32 A which is within 10 % of the 36 filament wire critical current per filament when adjusted for self field.

Comparison of Ta3wt%W and Pure Ta as a Wire Reinforcement Element in a Multifilament Composite

In very high field magnetic applications, the windings can see very high Lorentz forces. One way to support a reacted Nb3 Sn wire under high loads is to reinforce the conductor with a high strength, high modulus element in the composite. This has been done previously with pure Ta as well as Cu-Nb multifilament elements. This work compares the results of using Ta3wt%W with pure Ta or pure Cu to reinforce a Cu-Nb multifilament conductor meant to mimic the processing of a Nb3 Sn internal tin type composite. Ta3wt% W is found to provide higher strength than pure Ta to the composite and be compatible with drawing to small diameters. A Ta3wt% W core element was successfully drawn with the Cu-Nb assembly to a diameter of 120 μm. The addition of tungsten to the tantalum also improves the microstructure of the tantalum and results in less interface distortion during processing. The mechanical properties and hardness of the individual elements were monitored during the wire processing to look at work hardening rates. A 2.7% area core made of Ta3wt%W provided more than 10% increase in the mechanical strength of the wire over a copper core and provided twice the strength increase of the same size pure tantalum core.

A feasibility study of high-strength Bi-2223 conductor for high-field solenoids

We performed a feasibility study on a high-strength BiPbxSr2Ca2Cu3O(Bi-2223) tape conductor for high-field solenoid applications. The investigated conductor, DI-BSCCO Type HT-XX, is a pre-production version of Type HT-NX, which has recently become available from Sumitomo Electric Industries. It is based on their DI-BSCCO Type H tape, but laminated with a high-strength Ni-alloy. We used stress–strain characterizations, single- and double-bend tests, easy- and hard-way bent coil-turns at various radii, straight and helical samples in up to 31.2 T background field, and small 20-turn coils in up to 17 T background field to systematically determine the electro-mechanical limits in magnet-relevant conditions. In longitudinal tensile tests at 77 K, we found critical stress- and strain-levels of 516 MPa and 0.57%, respectively. In three decidedly different experiments we detected an amplification of the allowable strain with a combination of pure bending and Lorentz loading to (calculated elastically at the outer tape edge). This significant strain level, and the fact that it is multi-filamentary conductor and available in the reacted and insulated state, makes DI-BSCCO HT-NX highly suitable for very high-field solenoids, for which high current densities and therefore high loads are required to retain manageable magnet dimensions.

Effect of annealing regimes on the structure of Nb3Sn superconducting layers in composites with internal tin sources

The structure and morphology of Nb3Sn superconducting layers that are formed in multifilamentary conductors with distributed tin sources in different diffusion-annealing regimes have been investigated by transmission and scanning electron microscopy. The composites studied differ in the design (the number of Nb filaments and their final diameter, initial tin concentration, the presence of reinforcing Cu‒Nb inserts in the stabilizing copper sheath, alloying of the copper matrix with manganese) and diffusion-annealing regime. In all of the conductors, superconducting layers arise with zones of different morphologies, namely, columnar grains are present along with fine equiaxed Nb3Sn grains. Compared to Nb3Sn-based conductors produced by the so-called bronze method, composites with internal tin layers are characterized by coarser Nb3Sn grains with a greater spread of sizes. Nevertheless, the critical current density Jc of these latter can reach values 2276 A/mm2 due to a higher Sn concentration in the superconducting phase and a larger relative amount of this phase in the conductor. Lower values of the critical current density (Jc = 850 А/mm2) were obtained in the conductor with a reduced tin concentration in the matrix and an enhanced number of Nb filaments with a smaller diameter, in which coarser Nb3Sn grains with a wide spread of sizes and wider zones of columnar grains are formed.

Finite Element Simulation of Temperature and Current Distribution in a Superconductor, and a Cell Model for Flux Flow Resistivity—Interim Results

A critical problem arises when current distribution in a high-temperature superconductor and its stability against quench shall be predicted: is it correct to assume homogeneous temperature distribution in superconductors, in general or only in LHe-cooled devices? The finite element analysis presented in this paper shows that during the very first instants following a disturbance, like single Dirac or periodic heat pulses, or large fault currents, temperature distribution in a BSCCO 2223 conductor is highly inhomogeneous. This is because disturbances, of transient or continuous, isolated or extended types in conductor volumes, create hot spots of comparatively long life cycle. As a consequence, separation between Ohmic and flux flow current limiter types, or decisions on the mechanism that initialises current sharing, cannot be made definitely. A semi-empirical cell model is presented in this paper to estimate flux flow resistivity in multi-filamentary superconductors in a successive approximation approach. Weak links are modelled, as nano- and microscopic surface irregularities and corresponding resistances, in analogy to thermal transport. Though the model requests input of a large amount of data (dimensions, porosities, field-dependent quantities) that still have to be verified experimentally, it is by its flexibility superior to ideas relying on, for example, imagination of separate, non-interacting chains of strong and weak links switched in parallel. In particular, and in contrast to the standard expression to calculate flux flow resistivity, the cell model suggests to replace solid conduction by an effective resistivity, a method that is more appropriate for multi-filamentary conductors. The paper also discusses integration time steps in numerical simulations that have to be selected in conformity with several characteristic times of current and thermal transport.

Critical current density and microstructure of iron sheathed multifilamentary Sr1−xKxFe2As2/Ag composite conductors

Iron-based superconductors have been considered to be very promising in high-field applications for which multifilamentary wire and tape conductors with high mechanical strength are essential. In this work, 7-,19-, and 114-filament Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires and tapes with silver as matrix and iron as outer reinforcing sheath were produced by the ex situ powder-in-tube method. The mass densities of Sr-122 phase in 7- and 19-filament conductors were investigated by microhardness characterization, which revealed a positive correlation between hardness and transport critical current density (Jc) in round wires and flat tapes with various thicknesses. For Sr-122/Ag/Fe 114-filament conductors, in which an average cross-sectional filament size smaller than 50 μm was achieved by drawing into round wires of 2.0 mm in diameter, the transport Jc can be significantly enhanced by flat rolling, as for the 7- and 19-filament conductors. The highest transport Jc for Sr-122 filaments in the 7-, 19-, and 114-filament Sr-122/Ag/Fe tapes reached 1.4 × 104, 8.4 × 103, and 6.3 × 103 A cm−2 (4.2 K, 10 T), respectively, showing a Jc degradation with the increase of filament number. This Jc degradation can be ascribed to the sausage effect for SR-122 filaments in longitudinal direction and the grain refinement in these very fine filaments.

Transport IV characterisation of MgB2 conductor at a bend radius of 50mm

Performance of state of the art MgB2 multifilamentary conductor at a required bend radius is essential for many applications including but not limited to magnets and motors. The characterisation is generally done with benchmark transport Ic but further detail can be seen in IV characteristics which are undertaken in this paper. Two conductors with the same architecture but different diameters, 0.89 and 0.45 mm were measured from 32 K to 20 K in self-field in conditions of as received and deformed to a 50 mm bend diameter, corresponding to strains of 1.4 % and 0.7 % respectively. The qualifying 0.45mm conductor was further measured in background fields up to 3 T. The smaller diameter wire was found to have no signs of degradation of critical behaviour in Ic or IV characteristics.

Numerical simulation of mold shape’s influence on NbTi cold-pressing superconducting joint

The cold-pressing welding methods are employed to fabricate joints between NbTi multi-filamentary conductors, and a series of joints are made with the molds of different shapes for nuclear magnetic resonance (NMR) magnet applications. The Abaqus–Explicit method was used to do a quasi-static analysis of the cold-pressing welding process. In the simulation, we analyzed four molds with different shapes: plate mold, cap mold, square mold, and radial compression. The simulation shows that the deformation of filaments is the most uniform in the case of radial compression and the square mold is the optimum one for decreasing joint resistance.

走査型ホール素子顕微鏡による超伝導線材の局所<i>J</i><sub>c </sub>分布評価

This article reviews a method for characterizing the local critical current density (Jc) distribution in superconducting tapes and wires based on scanning Hall-probe microscopy (SHPM). This method is very powerful for (1) finding the bottleneck that limits the global performance of a superconductor, (2) investigating the local inhomogeneity that may become the origin of local quench, especially in HTS applications, (3) establishing the processes for narrow and/or multifilamentary HTS conductors for reducing the magnetization in magnet applications and the AC losses in power applications. The principle and the functions of this technique are introduced by referring to characterization examples of REBCO coated conductors and an MgB2 wire.

Numerical Simulation of NbTi Superconducting Joint With Cold-Pressing Welding Technology

The cold-pressing welding methods are employed to fabricate joints between NbTi multifilamentary conductors, and a series of joints are made at different press amounts for nuclear magnetic resonance magnet applications. The Abaqus-Explicit method was used to do a quasi-static analysis of the cold-pressing welding process. In the simulation, we consider the contact area and equivalent plastic strain to determine the resistance of the superconducting joints qualitatively. The simulation shows that a press amount of 61%-65.5% should be the optimum range, in which the lowest joint resistance can be obtained. Resistances of these joints are also tested using the current decay method to verify the simulation.

Electrical properties of cold-pressing welded NbTi persistent joints

The cold-pressing welding method is employed to fabricate persistent joints between NbTi multifilamentary conductors, and a series of persistent joints are thus made at different mechanical pressures. The electrical properties of these persistent joints are tested by a joint resistance measurement device based on the current decay measurement method. Test results show that the cold-pressing welding method is reliable and applicable to NMR and MRI applications. Experiment results also shows that the joint resistances and current-carrying capacities of the joints seem to have no apparent relevance to the mechanical pressure in a wide range of 4–20MPa, but the current decay ratio due to flux creep has obvious relevance to the mechanical pressure. Further research is needed to advance this joint manufacturing technology. Besides, measured current increase under 1 T background magnetic field is observed and explained, which has a reference meaning for testing persistent joint resistances.