Sn Strands Research Articles

Thermo-Mechanics of the Hierarchical Structure of ITER Superconducting Cables

Recent experimental tests on the model coils have shown that the behavior of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn based cables is not as good as expected on the basis of the characteristics evaluated for the uncabled strands. This degradation of the cable performance seems to be due to various factors, among which the strain state of the filaments due to bending and contact phenomena inside the cable. After these results it was decided that high performance strands will be used for ITER magnets, even if they have never been tested on a full size cable. Therefore the capability of evaluating the coil performance from the strands characteristics becomes a crucial point in the Research and Development (R&D) activity. Recently some extrapolations were attempted, but the mechanical model used is rather simplified and needs some fitting parameters which are not known a priori. In this work we present a thermo-mechanical model suitably developed to evaluate the strain state of a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand inside a superconducting (SC) cable. It is based on the idea of multiscale modeling, starting from a enriched formulation of the beam kinematics to take into account the fibrous nature of a multifilamentary strand. The method consists in performing a successive substitution of discrete models involving many beams with a single equivalent beam model, which behavior is identified from the preceding cabling stage. This recursive substitution allows to perform the analysis within a reasonable computational time. Once the stress and strain fields are obtained at the higher level, a suitable unsmearing technique gives the strain till the first level, on the scale of the SC filament. The method is applied to the real case of the 3 times 3 and 3 times 3 times 5 CICC sub-size samples tested at FZK in Germany.

Study of Effects of Deformation in ${\rm Nb}_{3}{\rm Sn}$ Multifilamentary Strands

In the process that leads a flawless Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn round strand to become part of a Rutherford cable first, and of a coil next, the same cabling process affects strands of different kinds in different ways, from filament shearing to subelement merging to composite decoupling. Due to plastic deformation, after cabling the filament size distributions in a strand usually change. The average filament size typically increases, as does the width of the distribution. This is consistent with the low field transport current of strands in cables being typically lower and less reproducible than for round strands [E. Barzi et al., RRP Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Strand Studies for LARP, presented at the Applied Superconductivity Conference, 1MK07, unpublished]. To better understand the role of filament size in instabilities and to simulate cabling deformations, strands to be used in cables can be tested by rolling them down to decreasing sizes to cover an ample range of relative deformations. A procedure is herein proposed that uses both microscopic analysis and macroscopic measurements of material properties to study the effects of deformation.

Synchrotron Tomography for the Study of Void Formation in Internal Tin ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Superconductors

Synchrotron absorption tomography has been applied for the study of voids formed during the thermal treatment of internal tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 </sub> Sn strands. Possible void formation mechanisms and in particular the effect of Sn phase transformations and melting are discussed based on a quantitative void description. Sn melting changes mainly the shape and volume of the individual voids but does not increase the total void volume in the strand

Application of prebending effect to high strength Nb 3 Sn strands

It is reported that the repeated bending loads at room temperature for Nb 3 Sn wires, which show the strain dependence of superconducting properties, cause the large enhancement of a critical current, an upper critical field and a critical temperature. Those phenomena are so called “prebending effect”. It is considered that the enhancement effect is associated with the reduction of the residual strain state for Nb 3 Sn wires. In order to apply the prebending treatment to a react-and-wind processed Nb 3 Sn superconducting magnet, repeatedly bent Nb 3 Sn wires with bending strains up to 1.0% were fabricated by using pulleys, and were confirmed to demonstrate the large enhancement of the critical current in spite of the bending strain of 0.5%. From these experimental results, we intended to fabricate the high strength Nb 3 Sn strands applying the prebending effect for the cabling conductor. As a first step, we prepared triplet cables using prebent CuNb/Nb 3 Sn and Cu/Nb 3 Sn strands and evaluated critical currents and n-values of each strand. The prebent Nb 3 Sn strands show the large enhancement of critical current I c even after the cabling process. Hence, it is expected that the prebent high strength Nb 3 Sn strands may be useful for the cabling conductors.

Conceptual Design of the 12.5 T Superconducting EFDA Dipole

Within the framework of the European Fusion Programme (EFDA) a design activity has been started in late 2004 by the EFDA Close Support Unit at Garching (FRG) to design a 12.5 T superconducting dipole magnet. The final goal of this activity is to define a "minimum cost" and "minimum risk" dipole to be built within the next 3 years by European industries. The newly built magnet shall be hosted in one of the existing cryogenic laboratories in the EU to test, in particular, the full size conductor samples that shall be produced during the ITER magnets procurement. As a result of this study-carried out in cooperation with several European laboratories-different concepts have been analyzed and optimized. All designs are based on the use of high current density Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands (j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ~2000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> at 12 T/4.2 K). It is assumed that these strands can be cabled either as Cable In Conduit Conductors (CICC) or as Rutherford-type Cables (RC) and then wound as planar racetrack, non-planar (saddle) or costhetas coils. The aim of this paper is to provide an overview of this study activity with particular emphasis to the optimization steps followed in the design of each magnet concept. Four different types are described: a planar CICC/RC-based coil, a saddle CICC-based coil and a costhetas RC-based coil. Each design is described together with the salient optimization steps that include electromagnetic, structure mechanic and thermo-hydraulic analyses as well as different manufacturing constraints

Effect of Flux Jumps in Superconductor on&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Accelerator Magnet Performance

The performance of some superconducting accelerator magnets based on high-J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands suffered from flux jumps in the coils which significantly limited their maximum quench current. Detailed investigation into the possible cause of these performance-limiting quenches showed a correlation between the magnet test results and the properties of the superconducting strands used in the magnet and the magnet design parameters. This paper discusses the effect of flux jumps in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands and cables on quench performance and on field quality of accelerator magnets

Axial Tensile Stress-Strain Characterization of a 36&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Strands Cable

A newly built test apparatus for mechanical characterization of ITER (International Thermonuclear Experimental Reactor) relevant sub-size cables is described, and the first experimental results on a 36 strands cable are presented. The cable consisting of nonreacted Nb3 Sn strands was subjected to tensile axial load up to 10 kN at temperatures of 300 K and 4.2 K. The Young modulus and ultimate tensile strength of the cable specimen are compared with those measured in single strands, extracted from the cable. The cable contraction was monitored during the tensile stress-strain testing. An analytical model for wire ropes is used to predict the cable stiffness. The test results are aimed at interpretation and validation of the cable models being under development

Effect of thermo-mechanical stress during quench on nb sn cable performance

Several high field magnets using Nb{sub 3}Sn superconductor are under development for future particle accelerators. The high levels of stored energy in these magnets can cause high peak temperatures during a quench. The thermomechanical stress generated in the winding during the fast temperature rise can result in a permanent damage of the brittle Nb{sub 3}Sn. Although there are several studies of the critical current degradation of Nb{sub 3}Sn strands due to strain, little is known about how to apply the strain limitations to define a maximum acceptable temperature in the coils during a quench. Therefore, an experimental program was launched, aimed at improving the understanding of the effect of thermo-mechanical stress in coils made from brittle Nb{sub 3}Sn. A first experiment, reported here, was performed on cables. The experimental results were compared to analytical and finite element models. The next step in the experimental program will be to repeat similar measurements in small racetrack coils and later in full size magnets.

Stability and instability in high performance (metastable) composite strands during I–V measurement

Abstract Factors affecting the measurement of critical current density ( J c ) in high performance Nb 3 Sn strands are discussed. Sample holder design and sample mounting procedures are considered in the light of cryogenic heat flow considerations. Also discussed are: the steady state measurability of metastable strands at current densities ( J ) at and above their nominal J c , and mechanisms governing their quench at sufficiently high values of J greater than J c .

Correlation between voltage–current characteristics of superconducting filaments, single wires and multistrand cables

Abstract The coherence between the voltage–current characteristics (VCC) of superconducting filaments, multifilament superconducting wires and multistrand cables made from such wires is discussed. The objective of the paper is to consider how the increasing of number of superconducting elements from filaments to cables affects their VCC. The influence of different factors on VCC, like stress–strain effects or current non-uniformity is discussed also. A generalized, simplified model is proposed for the calculations of the multistrand cable VCC. The application of the obtained results for the calculations of the critical currents in LTS (steep VCC) and HTS (smooth VCC) materials is discussed. The results are important for analyzing of changes of VCC in CICC made from Nb 3 Sn strands.

The study on the I/sub c/(T, B) dependencies of Nb/sub 3/Sn strands for TF coil-insert of the ITER CS

The study on the critical current dependencies on magnetic field values in the range 3-12 T and on temperature in the range 4.2-15 K of Nb/sub 3/ Sn strands samples is presented. It is shown that experimental values of I/sub c/(T, B) are well described by relation of Summers with the deformation of -0.25%. The results of volt-temperature characteristics measurements taken in magnetic fields at different currents of a sample are given. The analysis of the dependencies has been done.

Nb/sub 3/Sn: macrostructure, microstructure, and property comparisons for bronze and internal Sn process strands

The variation in irreversibility field, B*(T), with temperature has been measured for Nb/sub 3/Sn superconducting strands manufactured for ITER using vibrating sample and SQUID magnetometers. The high performance strands were developed for both high transport critical current density, J/sub c/, and low hysteresis loss. Despite a wide variety of designs and components, the strands could be split into two distinctive groups, based on the extrapolated irreversibility fields, which lie about 10% lower than the upper critical field. "Bronze-process" strands exhibited consistently higher B*(T) (28 T to 31 T) compared with "internal Sn" process (24 T to 26 T) conductors. The intrinsic critical current density of the superconductor, J/sub c(sc)/, and the specific pinning force of the grain boundaries, Q/sub gb/, were evaluated using the measured J/sub c/, and image analysis of the macro- and micro-structures. A bronze-processed Nb(-Ta)/sub 3/Sn was found to have a higher J/sub c(sc)/ but lower Q/sub gb/ than Nb/sub 3/Sn produced from Nb filaments. This work shows that the maximization of J/sub c/ is both an intrinsic flux pinning issue and a quantitative issue of how much Nb/sub 3/Sn can be put into the composite package. The results for the ITER strands are compared to a high J/sub c/ (but also high hysteresis loss) internal Sn strand. The high J/sub c/ strand had a much higher J/sub c(sc)/ and Q/sub gb/ than any of the ITER strands.

Stainless steel sheathed forced internally cooled Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn superconductor and its coil test

Forced cooled Nb 3 Sn superconducting coil with the stainless steel sheathed conductor has been manufactured and tested in a High Field Test Facility (HFTF). Some properties of the sheath material and the conductor with stainless steel sheath have been investigated. A 316L stainless steel sheathed conductor (the outer dimension of 13 × 23 mm2) was fabricated, and one coil was completed by the wind and react method using this conductor. The coil was connected with 8 pancake coils made of copper sheathed conductor. The forced cooled magnet of 9 double pancake coils was tested under 5.9 T of the back up field in HFTF and generated 8.8 T under 7.6 kA of transport current. Experimental results show that the Nb 3 Sn strand in the copper sheathed conductor near the connector was degraded due to handling through reconnecting the conductor for several times.

Development of 12 T-10 Al-stabilized Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn conductor for TMC-II

The Al-stabilized Nb 3 Sn strand has been successfully fabricated and J c value of 400 A/mm2at 12 T is obtained on condition that Nb 3 Sn is reacted at 625°C for 200 hr. Overall residual resistivity of this strand is lowered to 77 % of that of the Cu-stabilized Nb 3 Sn strand (TMC-I)1,2at 12 T by Al-stabilizer. The 12 T-10 kA cable-in-conduit conductor, fabricated by this strands, is charged up to 20 kA at 8.7 T without appearance of normal zone. From these experimental results, this conductor satisfies almost the specification of the present target conductor (TMC-II)3.

Fabrication of 20 KA, 12 Tesla conductor using external Bronze process MF Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn

A MF Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn cabled condcutor with a design critical current in excess of 20 KA has been delivered to the Oak Ridge National Laboratory for stability test in the CWTX Facility. The 5.2 cm × 1.1 cm conductor, an adaptation of a 12 Tesla ETF design by Walker, et al is a 7 × 7 × 3 × 16 cable. The seven strand primary cables incorporate two 0.3 mm diameter external bronze process MF Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn strands and five barrier protected stabilizer strands. Critical current density in the active strands is approximately 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 Tesla, 4.2 K.

Critical properties of multifilamentary Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn between 8 and 14 tesla

The multifilamentary NB <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn strand used in the Airco/Westinghouse Large Coil Program was studied for possible direct applicability to the 12 Tesla Coil Program. These studies began with an optimization of the reaction heat treatment to improve the critical current at 12 Tesla. Subsequently the critical surface of a strand heat treated under the optimal conditions was mapped out between 8 and 12 Tesla. The results of these studies are described.

Developments in Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn forced flow conductors for large magnets

The production of a Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn forced flow conductor requires the development of the techniques for reliably fabricating long lengths of compacted cable in a stainless steel jacket. Development is needed in two major areas: 1) cabling and preliminary compaction of the cable and 2) wrapping, final compaction squaring of the full conductor. The configuration of the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn strand is presented and its performance characteristics are described. Early investigations have been made using handmade cables and the sample length was limited to 5 m by the compaction techniques used at the time. Machine made copper cable has been completed and differences are also described. Finally developments in the continuous wrapping and compacting process will be discussed.

Multifilamentary Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn conductor for fusion research magnets

In an attempt to meet the eventual needs of fusion magnets, a multifilamentary Nb 3 Sn conductor has been proposed consisting of a cabled array of multifilamentary Nb 3 Sn strands .027 cm in diameter, surrounding a stainless steel core. This cable assembly would be jacketed with soft copper to provide an overall rectangular exterior. For the purpose of testing this concept, a prototype cable was fabricated in a length of several hundred meters. The cable was designed to carry 300 amperes at 12T and have a minimum bending radius of about 6 cm. Fabrication techniques, short sample critical currents and bending test results will be presented, and the problems anticipated in scale up will be discussed.