Bronze Route Strands Research Articles

An Investigation Into the Heat Treatment Tolerance of WST Nb3Sn Strands Produced for Massive Fusion Coils

Multistep heat treatments are required to produce the superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn in the International Thermonuclear Experimental Reactor toroidal field coils; however, deviations in the temperature and dwell time during heat treatment of the big conductors are unavoidable, and these could affect the performance of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. To investigate the influence of heat treatment tolerances, both internal-Sn- and bronze-process-type Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands were heat treated with different cycles. For the internal-Sn process strands, the critical current density Jcn increases as the temperature increases from 630 °C to 650 °C and remains unchanged at 670 °C for 100 h. The Sn content in the filament increases with increasing temperature, and the grain sizes significantly increase from an average of 130-202 nm from 630 °C to 670 °C. For both the internal-Sn process strands and bronze route strands, Jcn seldom changes when the duration at 650 °C is increased from 100 to 200 h. Despite these changes, this study shows that Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands are not very sensitive to small heat treatment variations at 650 °C, and a variance of ±5 °C is acceptable for both types of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands.

Wide range pure bending strains of Nb3Sn wires

Pure bending behavior of Nb3Sn wire over a wide range of bending has been characterized. A previously developed test device designed to apply variable bending strains to Nb3Sn strands using a beam style sample holder was used. Based on finite element and experimental investigations, two sample holder beams were developed to cover pure bending strains up to 1.25% for ITER-type Nb3Sn wires. These newly designed beams were optimized to apply consistent and uniform pure bending strains to Nb3Sn strands over the entire bending range. Their performance was evaluated by testing two ITER-type Nb3Sn wires including one internal tin and one bronze route. The internal tin strands experienced around 55% critical current degradation at 1.25% bending strain while the critical current of the bronze route strands were only reduced by 40%. Upon removal of the bending load, the internal tin wires experienced significant permanent degradation whereas the bronze route wires were completely reversible. These critical current results were evaluated and explained using an existing integrated model accounting for neutral axis shift, current transfer length, filament breakage and uniaxial strain release under pure bending loads.

A simple and effective approach for thermo-mechanical modelling of composite superconducting wires

The ability to compute accurately the strain field in Nb3Sn filaments is a crucial point in cable design, due to the significant strain sensitivity of niobium–tin wires. Due to its heterogeneity, a straightforward numerical simulation of a cable, taking into account all the details of the microstructure, would result in an enormous number of unknowns. As an alternative, multiscale approaches can be used to deal with this kind of problem, to understand the behaviour across the various scales. In this framework, a simple and efficient approach to obtain the homogenized properties of a heterogeneous strand is proposed here. This approach is developed for the non-linear, thermo-mechanical field. It consists of the solutions to some boundary value problems formulated on a suitably chosen statistically representative volume element of the wire. Two bronze-route strands and one internal-tin strand are considered and the equivalent parameters are obtained. Finally, the cool down and the subsequent application of a tensile axial load are simulated taking into account the homogenized wires. Computed results are shown to be in excellent agreement with measured stress–strain curves.

Critical current scaling and the pivot-point in Nb3Sn strands

Detailed measurements are provided of the engineering critical current density (Jc) and the index of transition (n-value) of two different types of advanced ITER Nb3Sn superconducting strand for fusion applications. The samples consist of one internal-tin strand (OST) and two bronze-route strands (BEAS I and BEAS II—reacted using different heat treatments). Tests on different sections of these wires show that prior to applying strain, Jc is homogeneous to better than 2% along the length of each strand. Jc data have been characterized as a function of magnetic field (B ≤ 14.5 T), temperature (4.2 K ≤ T ≤ 12 K) and applied axial strain ( − 1% ≤ εA ≤ 0.8%). Strain-cycling tests demonstrate that the variable strain Jc data are reversible to better than 2% when the applied axial strain is in the range of − 1% ≤ εA ≤ 0.5%. The wires are damaged when the intrinsic strain (εI) is εI ≥ 0.55% and εI ≥ 0.23% for the OST and BEAS strands, respectively. The strain dependences of the normalized Jc for each type of strand are similar to those of prototype strands of similar design measured in 2005 and 2008 to about 2% which makes them candidate strands for a round-robin interlaboratory comparison. The Jc data are described by Durham, ITER and Josephson-junction parameterizations to an accuracy of about 4%. For all of these scaling laws, the percentage difference between the data and the parameterization is larger when Jc is small, caused by high B, T or |εI|. The n-values can be described by a modified power law of the form , where r and s are approximately constant and Ic is the critical current. It has long been known that pivot-points (or cross-overs) in Jc occur at high magnetic field and temperature. Changing the magnetic field or temperature from one side of the pivot-point to the other changes the highest Jc sample to the lowest Jc sample and vice versa. The pivot-point follows the B–T phase boundary associated with the upper critical field and is usually attributed to the different tin content profiles and pinning properties of internal-tin and bronze-route strands. We report that the strain dependence of the pivot-point in these strands is quite different from that of the upper critical field and suggest that its origin in optimized high tin content strands is the proximity of the tetragonal Nb3Sn phase, which has low superconducting critical parameters.

Initial damage influence of stiffness reduction for bronze route Nb 3Sn strands

Nb 3Sn strands are widely used in ITER CICCs, and the axial stiffness reduction induced by the initial damage in the form of filament breakage during processing of the composite strand is intensively related to the operation performance criteria. In this paper, an analytical model is developed to simulate this degradation of bronze route strands. The model contains three sub-models: the effective modulus of a filament with initial damage is investigated by a shear-lag model based on the global load sharing scheme and Weibull fiber strength statistics; the weakened stiffness of the superconducting layer is deduced by the Mori–Tanaka model; and the effective axial modulus of the strand is obtained by the multilayered generalized self-consistent model. The results indicate that the stiffness reduction of a strand presents obvious nonlinear behavior and depends on the initial damage parameter and the evolution of total damage.

Magnetization and Inter-Filament Contact in HEP and ITER Bronze-Route ${\hbox{Nb}}_{3}{\hbox{Sn}}$ Wires

Magnetization measurements are relevant tests for the characterization of superconductors. Practically they are the only measurements that allow estimating the critical current density at low fields of low temperature superconductors, the effective filament size and the hysteresis losses. For this purpose CERN, in collaboration with the University of Geneva, has carried out magnetization measurements on five types of Nb3Sn wires: three bronze route strands used in the ITER project; one Powder In Tube (PIT) and one Internal Tin (IT) wires used for developing next generation accelerator magnets. The field dependent magnetization has been determined using three setups: a Vibrating Sample Magnetometer (VSM), a Superconducting Quantum Interference Device (SQUID) and a special system used for the production control of LHC strands. Samples of different lengths have been tested to check the different coupling between the filaments. Unexpectedly, it was found that the magnetization of the tested bronze wires was strongly dependent on the sample length. In this paper, the results, which were obtained for different type of strands and sample lengths, are reported and compared.

Analytical model of the critical current of a bentNb3Sn strand

The critical current performance of a largeNb3Sn cable-in-conduit conductor (CICC) was degraded by periodic bending of strands due to alarge transverse electromagnetic force. The degradation of each strand due to this bendingshould be evaluated in calculations of the critical current of a CICC, but a suitable modelhas not been developed yet. Therefore, the authors have developed a new analytical modelwhich takes into account plastic deformation of copper and bronze and filamentbreakage. The calculated results were compared with test results for uniformly bentNb3Sn bronze-route strands. The calculated results assuming a high transverse resistance model(HTRM) show good agreement with the test results, a finding which confirms the validity of themodel. Because of a much shorter calculation time than for numerical simulation, the developedmodel seems much more practical for use in calculating the critical current performance of aNb3Sn CICC. In addition, simulation results show that since the neutral axis of a bent strandshifts to the compressive side due to plastic deformation of the copper and bronze, and/orfilament breakage, the strand is elongated by bending. This elongation may enhance thestrand’s critical current performance. Moreover, the calculated results indicate thatthe dependence of the critical current on the bending strain is affected by thebending history if the strand is excessively bent, especially when filaments arebroken. In a real magnet, since a strand in a CICC is normally subject to themaximum electromagnetic force prior to an evaluation of its performance at a lowerelectromagnetic force, the effect of over-bending should be taken into account incalculations of its critical current performance, especially when filament breakage occurs.

The Results of Russian ITER TF Conductor Sample Test in SULTAN

In the framework of the ITER Qualification Tests the Russian sample RFTF2 was tested in SULTAN facility. The sample is made of two identical ITER TF-type cable-in-conduit conductor sections. Bronze route strands were used for the conductor fabrication. The main characteristics of the conductor and the strand are presented as well as the sample assembly details. The test was performed in accordance with the ITER International Organization specified program. In order to evaluate the conductor performance, the current sharing temperature was measured at specified electromagnetic load cycling steps. Few voltage-current characteristics were measured with the aim of n-value estimation. Both conductor sections of the RFTF2 sample showed identical performance: the current sharing temperature is 6.4 K, that not only satisfies, but substantially exceeds ITER requirement of 5.7 K. The tests of RFTF-2 conductor sample showed that electromagnetic cyclic load as well as warm-up had negligible effect on the conductor performance. The results of the SULTAN sample test are compared with strand performance.

Analysis of Bending Effects on Performance Degradation of ITER-Relevant $\hbox{Nb}_{3}\hbox{Sn}$ Strand Using the THELMA Code

The modeling of the effects of bending on single strand DC performance (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , n index) is presented for a bronze-route strand subjected to the same loading conditions as in an experiment performed at JAEA Naka, Japan [Y. Nunoya, , IEEE TAS 14 (2004) 1468-1472]. The strand is discretized in strand elements (SE) representing groups of twisted filaments in the bronze matrix, and in portions of the outer Cu annulus, electro-magnetically coupled in the THELMA code. The 3-D strain map in the filament region is computed with a newly developed, detailed thermo-mechanical model accounting for non-linear, temperature dependent material characteristics. With respect to our previous analysis [P.L.Ribani, , IEEE TAS 16 (2006) 860-863] several new updated ingredients, besides the new thermo-mechanical model, are used here, including more accurate thermal and mechanical properties for the materials, a jacket-like model for the outer Cu layer, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and n index (interpolative) scaling from Durham University. The simulation results show an improved agreement with the experiments, in the degradation of the single-strand performance due to bending.

THELMA Code Analysis of Bronze Route&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Strand Bending Effect on&lt;tex&gt;$rm I_rm c$&lt;/tex&gt;

The THELMA (Thermal-Hydraulic-ELectro-MAgnetic) code has been developed with the aim of simulating the main aspects of superconductors to be used in the coils of the International Thermonuclear Experimental Reactor (ITER). An application of the code is presented here, where THELMA is used to simulate a single strand by considering, as cable elements, groups of superconducting (SC) filaments and the corresponding portion of the resistive matrix. This approach is used to reproduce the voltage-current characteristic of a Nb3Sn bronze route strand when subject to a bending mechanical load. Attention is focused particularly on the effect of the applied mechanical load on the critical current, which is considered a relevant item in the explanation of the degradation of the coil performance observed in several ITER Model and Insert Coil experiments. The longitudinal strain of the SC filaments is calculated by means of a composite beam model of the strand, taking into account the nonlinear, temperature-dependent material characteristics of the components. The whole load history is simulated, computing first the thermal strain due to cool-down, and then the mechanical strain due to the bending at 4.2 K

European development of high performance Nb/sub 3/Sn strand for the ITER model coils

The ITER model coils will use two types of strands, one with high Jc, HP I, one with low hysteretic losses, HP II. Main specifications for the two strands are: HP I: Jc (nonCu)>700 A/mm/sup 2/ at 12 T @ 4.2 K, hysteresis losses<600 mJ/cc (+-3 T); HP II: Jc (non Cu)>550 A/mm/sup 2/ at 12 T @ 4.2 K, hysteresis losses<200 mJ/cc (+-3 T). HP I strand performance is likely to be achieved by internal tin Nb/sub 3/Sn, HP II by a bronze route strand. About 25% of the 26 tonnes required by the model coils will be contributed by the European Community. The companies EM-LMI, Italy (for internal tin) and Vacuumschmelze, Germany (for bronze route) have been selected by ITER for the strand production. The achievement of HP II performance was already demonstrated by Vacuumschmelze on a 10 km strand manufactured for NET in the frame of a former contract. On the other hand, the HP I internal tin strand was to be developed. In view of this, a contract has been assigned to GEC Alsthom Intermagnetics, France, to have a back-up option for internal tin strand. EM-LMI (HP I strand) and Vacuumschmelze (HP II strand) have successfully manufactured Nb/sub 3/Sn multifilamentary wires complying with ITER specifications. Work is in progress at GEC Alsthom Intermagnetics to achieve HP I strand performances.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>