Yoshinori Tsuchiya

First test results for the ITER central solenoid model coil

Abstract The largest pulsed superconducting coils ever built, the Central Solenoid (CS) Model Coil and Central Solenoid Insert Coil were successfully developed and tested by international collaboration under the R&D activity of the International Thermonuclear Experimental Reactor (ITER), demonstrating and validating the engineering design criteria of the ITER Central Solenoid coil. The typical achievement is to charge the coil up to the operation current of 46 kA, and the maximum magnetic field to 13 T with a swift rump rate of 0.6 T/s without quench. The typical stored energy of the coil reached during the tests was 640 MJ that is 21 times larger than any other superconducting pulsed coils ever built. The test have shown that the high current cable in conduit conductor technology is indeed applicable to the ITER coils and could accomplish all the requirements of current sharing temperature, AC losses, ramp rate limitation, quench behavior and 10u2008000-cycle operation.

Force free strain exerted on a YBCO layer at 77 K in surround Cu stabilized YBCO coated conductors

The stress/strain behavior of the surround Cu stabilized YBCO coated conductors and its influence on critical current were precisely investigated. The internal strain exerted on the superconducting YBCO layer was determined at 77 K by using a neutron diffraction technique at JAEA. The initial compressive strain decreased during tensile loading and changed to a tensile component at the force free strain (Aff), where the internal uniaxial stress becomes zero in the YBCO layer. The Aff was evaluated to be 0.19–0.21% at 77 K. The critical current measurements were carried out under a uniaxial tensile load at 77 K. The strain dependence revealed a characteristic behavior, where a maximum was observed at 0.035%. Thus it was made clear that the strain at the critical current maximum does not correlate with Aff for YBCO coated conductors.

Completion of CS insert fabrication

The central solenoid (CS) model coil program is in progress with an international collaboration under the frame of the ITER-EDA. The purpose of the CS insert coil is to test the performance of the ITER-CS conductor. The CS insert coil is installed in the bore of the CS model coil and tested at a magnetic flux density of 13 T. The installation work is underway with the inner and outer module of the CS model coil. The superconducting characteristics of the CS conductor, the critical current and the current sharing temperature are evaluated under the operating load. The AC loss characteristics of the conductor are also evaluated under pulsed magnetic field. The fabrication of the CS insert coil was completed on May 1999. The winding tools and the results of the winding of CS insert coil are reported. The heat treatment for Nb/sub 3/Sn processing was performed successfully with no SAGBO (stress accelerated grain boundary oxidation). The procedure of the heat treatment is also reported.

Development of a cryogenic load frame for a neutron diffractometer

Strain measurements under loading at cryogenic temperatures are much requested for investigation of the stress/strain effect on the critical current for composite superconductors. In order to provide in-situ measurements of the lattice strain, a cryogenic load frame with a GM refrigerator has been developed, which is suitable for the neutron diffraction facility RESA equipped at JRR-3 in JAEA. The lowest temperature of 4.8 K was achieved, while the capacity of the load frame was 10 kN. Using the present cryogenic load frame, plane spacing measurement was performed under loading for two specified samples of 316 stainless steel and engineering Y–Ba–Cu–O (YBCO) coated conductor. The relation between applied stress/strain and lattice strain has been made clear in a wide range of temperatures.

Completion of the ITER CS model coil-outer module fabrication

The fabrication of the outer module has been completed, which is one of the central solenoid (CS) model coils being developed as the most important R&D for ITER construction. The inner and outer diameters are 2.8 m and 3.6 m, respectively. The weight is 63 tons. The conductor is a Nb/sub 3/Sn cable-in-conduit conductor with a square Incoloy 908 conduit. The coil that consists of 8 layers was fabricated with a solenoid winding with two conductors-in-hand. The outer module will be connected with the inner module coil in series and contribute to the generation of 13 T at 46 kA. The test will be started in November 1999.

Local strain and its influence on mechanical–electromagnetic properties of twisted and untwisted ITER Nb3Sn strands

It is important to evaluate the local strain exerted on superconducting filaments in Nb3Sn strands, because it influences both superconducting and mechanical properties, in particular for the ITER (International Thermonuclear Experimental Reactor) project. The local strain in the twisted and untwisted Nb3Sn strands was directly measured at room temperature as well as at low temperatures by means of quantum beam techniques. The local strain consists of thermal strain and lattice strain. The latter changes as a function of external strain. The interrelation between the force-free strain and the intrinsic strain showing a maximum critical current was considered on the basis of the present experimental data as well as the recent theory. The thermal strains along both directions parallel and transverse to the strand axis were numerically evaluated. Their evaluated results could explain well the observed values, when To is the recovery temperature of pure Cu. The force-free strain along the axial direction is deduced to be distributed among grains with different crystal orientation with respect to the axial direction. It is suggested that this fact affects the definition of intrinsic strain.

Thermal strain exerted on superconductive filaments in practical Nb3Sn and Nb3Al strands

Practical superconductive (SC) wires such as the Nb3Sn and Nb3Al strands used in fusion reactors are typical composite materials consisting of brittle superconducting intermetallic compounds. Thermally induced strain is inevitably generated in the composite due to the different coefficients of thermal expansion and different moduli of elasticity among the constituent components. In order to evaluate quantitatively the thermal strain, local strain measurements during heating by means of quantum beams, and room temperature tensile tests were carried out. The stress versus strain curves of both Nb3Sn and Nb3Al strands showed a typical elasto–plastic behavior, which could be numerically evaluated on the basis of the rule of mixtures. The local strain exerted on SC filaments along the axial direction was compressive at room temperature and tensile at high temperatures, which is common for Nb3Sn and Nb3Al strands. Their temperature dependence was numerically evaluated by means of the iteration method. As a whole, it has been established that the temperature dependence of thermal strain can be reproduced well by the numerical calculation proposed here. It is pointed out that the thermal strain in SC filaments is affected by the creep phenomenon at high temperatures above a threshold temperature.

Residual strain measurement using neutron diffraction for practical Nb3Sn wires under a tensile load

Residual strains for practical Nb3Sn superconducting wires were measured directly using neutron diffraction. Seven wires, stacked with epoxy resin, were measured under a tensile load at room temperature. As a result, the three-dimensional strain was obtained up to 0.5% axial tensile strain. We found that the neutron diffraction result is consistent with the strain measured using strain gauges and extensometers at the same time, although the strains increased due to the long measurement time. The ratio between the axial and lateral strains is about 0.33, although the Cu?Sn, Cu and CuNb were plastic in this region. The ratio is an important value for evaluating three-dimensional strain effects on the superconducting properties of Nb3Sn wires.

Homogeneous performance and strain tolerance of long Bi-2223 HTS conductors under hoop stress

Two types of high-strength industrial Bi-2223 conductor, one laminated by copper alloy and the other laminated by stainless steel, have been tested to examine the effect of hoop stress on the transport property. The specimens (~2 m long) were prepared by winding one layer around a GFRP mandrel and the measurements were made in a liquid helium bath with the hoop stress calculated from the BJR product applied by external magnetic field. A careful measurement wire configuration was necessary to cancel the noise pick-up from the environment for more accurate determination of Ic and n-value. We show for the first time that both conductors showed homogeneous voltage–current characteristics over a long length and degradations with hoop stress occurred uniformly, which is crucial information for the development of HTS magnet technology. The onset of degradation occurred at 200 MPa and 220 MPa, with additional bending stress present from the winding diameter of 108 mm, for copper alloy laminated and stainless steel laminated conductors, respectively. After considering the effect of bending strain, our result agrees well with the previously measured data.

Microtwin Structure and Its Influence on the Mechanical Properties of REBCO Coated Conductors

The elastic properties of REBCO tape (i.e., REBa₂Cu₃ O_7-d, RE = <i>Y</i>, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain <i>A</i>_hkl^l, which consists of thermal strain <i>A</i>_hkl^T and lattice strain <i>A</i>_hkl, was analyzed using a microscopic structure model, with two types of microtwin configuration along the tensile axis, i.e., with the [100] or [110] crystal axis oriented parallel to the longitudinal direction of the tape. Experimental results were: (a) slope <i>dA</i>_h00^l/<i>dA</i> is larger than <i>dA</i>_0k0^l/<i>dA</i> (where <i>A</i> is the external tensile strain), which is attributed to the elastic modulus <i>E</i>_a along the <i>a</i>-axis being smaller than <i>E</i>_b along the <i>b</i>-axis; (b) both <i>dA</i>_h00^l/<i>dA</i> and <i>dA</i>_0k0^l/<i>dA</i> are smaller than unity, but <i>dA</i>₁₁₀^l/<i>dA</i> is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as <i>A</i>_h00^T ≠ <i>A</i>_0k0^T due to the elastic behavior of the superconducting layer being modified because it is constrained by the substrate and the outer metallic layer; and (d) a large Poisson ratio is attributed to this constraint state. It is suggested that the microtwin structure is critical to understand and model this unusual stress/strain behavior.