Hisaki Sakamoto

Development of high-strength and high-RRR aluminum-stabilized superconductor for the ATLAS thin solenoid

The ATLAS central solenoid magnet is being constructed to provide a magnetic field of 2 Tesla in the central tracking part of the ATLAS detector at the LHC. Since the solenoid coil is placed in front of the liquid-argon electromagnetic calorimeter, the solenoid coil must be as thin (and transparent) as possible. The high-strength and high-RRR aluminum-stabilized superconductor is a key technology for the solenoid to be thinnest while keeping its stability. This has been developed with an alloy of 0.1 wt% nickel addition to 5N pure aluminum and with the subsequent mechanical cold working of 21% in area reduction. A yield strength of 110 MPa at 4.2 K has been realized keeping a residual resistivity ratio (RRR) of 590, after a heat treatment corresponding to coil curing at 130/spl deg/C for 15 hrs. This paper describes the optimization of the fabrication process and characteristics of the developed conductor.

High-strength and high-RRR Al-Ni alloy for aluminum-stabilized superconductor

The precipitation type aluminum alloys have excellent performance as the increasing rate in electric resistivity with additives in the precipitation state is considerably low, compared to that of the aluminum alloy with additives in the solid-solution state. It is possible to enhance the mechanical strength without remarkable degradation in residual resistivity ratio (RRR) by increasing content of selected additive elements. Nickel is the suitable additive element because it has very low solubility in aluminum and low increasing rate in electric resistivity, and furthermore, nickel and aluminum form intermetallic compounds which effectively resist the motion of dislocations. First, Al-0.1wt%Ni alloy was developed for the ATLAS thin superconducting solenoid. This alloy achieved high yield strength of 79 MPa (R.T.) and 117 MPa (4.2 K) with high RRR of 490 after cold working of 21% in area reduction. These highly balanced properties could not be achieved with previously developed solid-solution aluminum alloys. In order to achieve higher strength than the above, Al-Ni alloys of up to 2.0 wt% Ni content were investigated. Al-2.0wt%Ni alloy achieved yield strength of 120 MPa (R.T.) and 167 MPa (4.2 K) with RRR of 170 after cold working of 20% in area reduction.

Progress in Production and Performance of Second Generation (2G) HTS Wire for Practical Applications

Rare earth (RE) based second generation (2G) high temperature superconducting (HTS) wire is an emerging material that has been commercially available to a wide range of energy, scientific, industrial and medical applications including high-field magnets, fault current limiters, motors and generators, transformers, and power cables. High engineering critical current density, high mechanical strength, and high irreversibility field are the major advantages of 2G HTS wires over low temperature superconducting (LTS) wires and bismuth-based first generation (1G) HTS wires. 2G HTS wires with critical current Ic (77K, self-field) of 300 ~ 500 A/cm-w and piece lengths of a few hundred meters are being routinely produced. Past and ongoing demonstration projects using the 2G HTS wires suggest that the practical application of this new material is appealing, promising and challenging. Further improvement in wire performance is desired and wire price is to be reduced. In this paper, important properties of 2G wires such as uniformity, in-field Ic and electromechanical behaviors are described. Recent technology advancements in using 2G HTS wire for practical applications are discussed.

Very high critical current density of bronze-processed (Nb,Ti)/sub 3/Sn superconducting wire

From the viewpoint of fabricability, the Sn content in the bronze matrix of bronze-processed Nb/sub 3/Sn wire is limited to below 15 wt%. This limitation results in a higher matrix-to-niobium ratio of bronze-processed Nb/sub 3/Sn wire than other high-Sn-content wires, such as the tube-processed Nb/sub 3/Sn wire. This is a reason why the non-Cu critical current density (J/sub C/) of the bronze-processed Nb/sub 3/Sn wire is lower than that of tube-processed Nb/sub 3/Sn wire. Therefore, increasing the Sn content in the bronze matrix and reducing the bronze ratio is the key to enhancing the non-Cu J/sub C/ of the bronze-processed Nb/sub 3/Sn wire. A bronze-processed Nb/sub 3/Sn superconducting wire with a Cu-16wt%Sn-0.2wt%Ti matrix was successfully fabricated on a production scale. The wire achieved a high non-Cu J/sub C/ of approximately 1000 A/mm/sup 2/ at 12 T and 4.2 K.

(Nb,Ti)/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer

An in-situ CuNb composite shows a high strength and a high electrical conductivity. Therefore, it is a suitable material for the reinforcement of Nb/sub 3/Sn superconducting wire. Many studies about mechanical and electrical properties of Nb/sub 3/Sn wire with CuNb reinforcing stabilizer have been carried out, but a fabricability of the long-length wire was not confirmed. In this study, 1 mm-diameter (Nb,Ti)/sub 3/Sn superconducting wire with Cu-20 wt%Nb reinforcing stabilizer was fabricated 22 km in length. The wire showed a good fabricability throughout the production. Mechanical properties and critical currents are evaluated in comparison with un-reinforced one.

(Nb,Ti)/sub 3/Sn superconducting wire reinforced with Nb-Ti-Cu compound

Reinforcement of Nb/sub 3/Sn wires is required for high field superconducting magnets because the Nb/sub 3/Sn superconductor is very sensitive to mechanical strain and its critical current degrades drastically at a higher absolute strain than 0.3%. Recently, Nb/sub 3/Sn superconducting wires reinforced with tantalum, Cu-Nb alloy and Al/sub 2/O/sub 3/-dispersed copper, have been developed. But, these high-strength wires do not show very high strength at an absolute strain of 0.3% because these reinforcements have a similar Youngs modulus to copper. In addition, Nb/sub 3/Sn layers of these wires experience an additional compressive strain caused by the differences in thermal contraction between reinforcements and Nb/sub 3/Sn layers. Such a compressive strain also decreases the critical current of the wire. An intermetallic compound was selected as a reinforcement for Nb/sub 3/Sn superconducting wire because such a compound is expected to have a high Youngs modulus and a similar thermal contraction to the Nb/sub 3/Sn compound. Fortunately, the Nb-Ti-Cu compound can be formed from the Nb-Ti and copper composite during reaction heat treatment for Nb/sub 3/Sn layers. Therefore, (Nb,Ti)/sub 3/Sn superconducting wire reinforced with the Nb-Ti-Cu compound has been fabricated. The wire shows a 0.2% proof stress of 400 MPa and maintains a critical current density equal to the unreinforced wire.

Properties of bronze-processed multifilamentary Nb/sub 3/Sn wires for fusion experimental reactor

The fabrication of bronze-processed multifilamentary Nb/sub 3/Sn wires having a critical current density of 840 A/mm/sup 2/ (4.2 K, 12 T) and an effective filament diameter of 6 mu m is reported. These values satisfy the specification required for the ITER Central Solenoid Scalable Model Coil. The measured time constant of the coupling loss in the wire is 4 ms at 12 T, in good agreement with the calculated result using B. Turcks formula (1979).<<ETX>>

Stress–Strain Relationship, Critical Strain (Stress) and Irreversible Strain (Stress) of IBAD-MOCVD-Based 2G HTS Wires Under Uniaxial Tension

The mechanical behavior of a REBCO-coated conductor wire under uniaxial tension is largely determined by the two thickest component layers in the architecture, namely, the substrate and the stabilizer. A rather complicated stress-strain relationship is often observed when the composite conductor is under uniaxial tension, which is the result of the differences in elastic modulus and yield stress between the Hastelloy substrate and the electroplated Cu stabilizer. In this paper, the stress-strain relationships of a free standing Cu stabilizer and a bare REBCO wire, i.e., a tape without any stabilizer, were measured. The results are utilized for the calculation of the stress-strain relationships of electroplated Cu stabilized wires using a two-component composite model. Given the thicknesses of the substrate and stabilizer, the calculated results agree well with the measured stress-strain curves, which can be well fitted with the Ramberg-Osgood equation. The tensile strain (stress) dependence of the critical current (Ic) was measured at 77 K in liquid nitrogen. Depending on whether the critical current was measured with the wire under a stress or after unloading, a critical strain (stress) and an irreversible strain (stress) were determined, respectively.

Hot Spot Behavior of Y123 Coated Conductors

We measured the time evolution of voltage and temperature of a CVD-Y123 coated conductor monolayer coil with Cu stabilizer under the cryocooled condition. The thermal runaway occurred locally at operate currents of 1.05-1.3 at 38 K and 1-10 T. In this case, the hot spot originated from the inhomogeneity of was observed. The thermal runaway generated from different parts of the tape at high and low fields at 38 K, respectively. This is due to the different heating behavior related to the distribution. The critical heating power of the thermal runaway obtained in this study were about 30 mW at 38 K and 10 T. It weakly depends on magnetic filed but is independent to the temperature. The analysis suggests that the cooling condition for the transient phenomena such as the thermal runaway is poor.

Superconducting and Mechanical Properties of Impregnated REBCO Pancake Coils Under Large Hoop Stress

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the BJR relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at Iop = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.