Multifilamentary Nb Research Articles

Mechanical strain triggering flux jumps of multi-filamentary Nb3Sn wires

The composite multi-filamentary Nb 3 Sn wire with a high critical current density is a preferred option for fabricating the superconducting magnet beyond the limit of NbTi wire (9-16 T). However, one crucial issue stems from the fact that electromagnetic force in superconducting coils is very strong, and the critical physical properties of Nb 3 Sn, such as J c , are more sensitive to mechanical strain than those of other possible low-temperature superconductors. We theoretically investigated the impact of mechanical strain on the thermomagnetic instabilities such as the flux jump (FJ) and quenching of Nb 3 Sn wire exposed to a static magnetic field and transport current. The good agreements with H formulation or H-φ formulation implemented on COMSOL software confirm the validity of our numerical simulations using home-made codes. It is discovered that mechanical strain can trigger flux jumps even in a static magnetic field. Furthermore, the threshold value of mechanical strain to trigger the first flux jump is a monotonic function of the static magnetic field in the case of high transport current, while it is a non-monotonic function in the case of low transport currents. It is attributed to the fact that flux can be released by the mechanical strain first, causing smooth flux penetration before triggering the flux jump. We also present the stable/unstable regions by applying mechanical strain by varying transport current, magnetic field, and working temperature, which helps in avoiding thermomagnetic instabilities while designing the superconducting magnet.

Identification of the A15 Contours in Multifilamentary Nb${3}$Sn Wires by Means of a Magnetic Method

A model for evaluating thecontours of the A15 layer in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires is proposed. These borders, crucial for the correct assessment of the layer current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and the volume layer pinning force F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> (B) are in fact non trivial to assess in some cases from scanning electron microscopy (SEM) polished surface images, eventually leading to non-negligible discrepancies between independent measurements reporting the same m-H or I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values. A self-reliant method from magnetometry based (Bean) models is proposed, allowing the outer (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ) and inner (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ) contours of the A15 phase to be assessed. The magnetic moment of short samples of a prototype powder in tube (PIT) wire with artificial pinning centres (APC) was measured between 6.5 T (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) and 6.95 T at 4.2 K in a superconducting quantum interference device (SQUID) magnetometer, allowing the calculation of r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> from the initial slope of dm/dH. Then, the derived function for the remanent magnetic moment m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rem</sub> is fit to the experimental data in order to derive r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> , under the condition that the saturation moment exactly matches the experimental value.

Effect of Intermediate Heat Treatment on the Formation and Structure of Superconducting Layers in Multifilamentary Nb&lt;sub&gt;3&lt;/sub&gt;Sn- Based Wires

In the present study, the Nb3Sn-based multifilamentary wires with coupled Nb filaments have been investigated by SEM and TEM after various regimes of intermediate annealing including short high-temperature heat treatments and after two-staged diffusion annealing. The formation of some amount of pre-reacted Nb3Sn layers has been revealed in all the wires studied, and their amount depends on the wire diameter, temperature and duration of the intermediate heat treatment. The structure of final diffusion layers is also affected by the regimes of these preliminary treatments. This research enables the revealing of the optimal heat treatment schedules for the formation of most perfect nanocrystalline structure of superconducting layers ensuring the highest critical current densities.

Fabrication and Superconducting Properties of Rod‐In‐Tube Multifilamentary Nb3Al Wire With Rapid Heating and Quenching Heat‐Treatment

In this paper, we reported the recent progress in developing practical kilometer-length rod-in-tube (RIT) multifilamentary Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al superconducting wires at WST, which included the preparation of Cu/Nb-Al precursor wires, design and manufacture of rapid heating and quenching (RHQ) equipment, and RHQ heat-treatment process of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires. Superconducting properties and microstructure of the RIT Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires have been discussed. By measuring magnetization and transport critical current properties of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires, it is found that, for all the samples, the onset superconducting transition temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> reaches 16.8-17.3 K, and at 4.2 K and 15 T, the critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 830 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires have been achieved. This paper suggests that it is possible to develop the practical Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al superconducting wires by using the RIT Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al precursor wires, in combination with RHQ heat-treatment process.

The Nb3Sn layers formation at diffusion annealing of Ti-doped multifilamentary Nb/Cu–Sn composites

The structure and morphology of superconducting Nb3Sn layers in multifilamentary Nb–Ti/Cu–Sn and Nb/Cu–Sn–Ti composite wires after various regimes of the diffusion annealing have been studied by transmission and scanning electron microscopy. The effect of the doping mode and Ti concentration on the thickness, structure and morphology of Nb3Sn layers is pronounced even after the first (low-temperature) stage of the diffusion annealing. After various regimes of the annealing the wires with doped bronze matrix (Nb/Cu–Sn–0.24Ti) demonstrate wider zones of fine equiaxed Nb3Sn grains with narrow grain size distributions compared to those with Ti-doped Nb filaments, which is of great importance for an attainment of the highest critical current densities Jc.

Microstructure and superconducting properties comparison of bronze and internal tin process Nb 3Sn strands for ITER

The long multifilamentary Nb 3Sn strands for international thermonuclear experimental reactor (ITER) have been successfully fabricated by bronze and internal Sn process, respectively. Adopted in this work, the bronze process Nb 3Sn strand with a Cu/non-Cu area ratio of 1:1.09 and the internal tin process strand with a Cu/non-Cu area ratio of 1:1.15 were designed. The microstructure details of two kinds of strands before and after heat treatment have been investigated by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The non-Cu J c (12 T, 4.2 K) value of 752 A/mm 2 for bronze process strand and 1185 A/mm 2 for internal tin process strand have been obtained. The influence of microstructure on transport property of strands has been discussed.

The key role of twist pitch and mechanical pre-treatment in the transport properties of Nb 3Sn wires subject to bending strain

In modelling the transport properties of multi-filamentary Nb 3Sn strands, the knowledge of the geometrical parameters of the superconducting filaments and the electrical and mechanical properties of the different materials composing the wire are required. In particular, the filament twist pitch and the transverse resistivity between filaments have a crucial role in the definition of the current transfer length and, consequently, in the simulation of the transport performances of superconducting wires subject to mechanical loads, as in cable-in-conduit conductors (CICC) for fusion magnets during operation. We have measured the critical current and the n-index of internal tin Nb 3Sn wires with different values of the filament twist pitch, having inserted the strand into a stainless steel jacket and under the application of pure bending strain. Results show that the degradation of the transport properties is affected by the twist pitch value and, in the limit case of non-twisted filaments, even an improvement is observed in presence of bending. Moreover, the reversibility of critical current after relaxing the mechanical load has also been checked and an improvement of the performances has been observed after pre-bending applications, presumably due to the strain relaxation. In addition, the differential analysis through the second derivative of the V– I curve evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions.

Correlation Between the Overall ${\rm T}_{\rm c}$ Distribution in Bronze Route ${\rm Nb}_{3}{\rm Sn}$ Wires and the Sn Gradient in the Filaments

From specific heat measurements of Ta + Ti alloyed bronze route multifilamentary Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires, a wide, nearly symmetric overall distribution of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> can be determined. This is apparently in contradiction with the almost linear decrease of the Sn content towards the centre of the A15 filaments, which suggests a rather sharp drop of the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distribution at the higher T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> side. In order to find an explanation to this problem, high resolution Auger spectroscopy measurements have been performed in filament bundles located at various positions of the wire cross section. It was found that the degree of reaction as well as the Sn gradient in each filament show substantial differences depending on its location, either in an innermost or outermost bundle. A model was established, assuming three different filament groups: 1) outer bundles: fully reacted filaments, almost constant Sn content, 2) intermediate bundles: partially reacted, shallow Sn gradient, and 3) inner bundles: partially reacted, steep Sn gradient. Summing together the contribution of the three groups, a total Sn distribution for the wire was obtained. Based on the relationship between T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and the Sn content in alloyed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, a T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distribution can be found which corresponds to that one measured by specific heat.

Nb 3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb 3Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb 3Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb 3Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb 3Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb 3Sn strands, with enhanced critical current density in high fields. For internal tin Nb 3Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb 3Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb 3Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb 3Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb 3Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb 3Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.

Multifilamentary Nb–Zr and V–Ti superconducting alloys prepared by diffusion reactionbetween constituent pure-metal subelements

We have succeeded in making a superconducting Nb–Zr and V–Ti alloy multifilament wireusing the diffusion reaction between constituent pure-metal subelements, which can omitthe melt-and-casting step and may reduce the fabrication cost. A third elementaddition was also performed by replacing the constituent subelements with the thirdpure-metal subelement in the monofilament; Nb–Zr–V, Nb–Zr–Ti and V–Ti–Ta.Three kinds of heat treatment were investigated; (1) an alloying heat treatment(HTalloy) which is carried outat the final size (‘fs-HTalloy ’), (2) HTalloy is followed by cold-working (W) down to the final size(‘HTalloy+W ’), and (3) an intermediate precipitation heat treatment(HTprec) is carried out during the W stage coming after theHTalloy stage(‘HTalloy+W+HTprec+W’ ).The ‘HTalloy+W+HTprec+W’ treatment was found to be most effective in enhancing the critical currentdensities for both Nb–68 at.% Zr and V–60 at.% Ti–9 at.% Ta alloys, despite theHTprec treatment having slightly degraded the critical magnetic field.

FEM Calculation of Magnetization Process in a Multi-Filamentary Superconductor

Superconductor magnetization was calculated using commercial FEM program. It is easy to create models of complex structures if we use commercial programs. Calculating superconductor magnetization using such program requires some techniques because commercially available FEM programs are made only for specific problems such as thermal, deformation and field calculation. We have successfully made a simulation of flux motion and magnetization using ANSYS. Flux motion in multi-filamentary Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor with superconducting diffusion barrier was vividly seen as a motion picture. Fluxes spreads after penetrating through barrier and penetration into each filament take place. Magnetization curves of Bean model and variable Jc models were reasonably calculated.

Critical-Current Measurements on ITER ${\hbox{Nb}}_{3}{\hbox{Sn}}$ Strands: Effect of Temperature

Transport critical-current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) measurements were made on commercial multifilamentary Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands at temperatures (T) from 4 to 17 K and magnetic fields (H) from 0 to 14 T. Samples investigated were taken from the stage 1 pre-production strand for the central solenoid of the International Thermonuclear Experimental Reactor (ITER) project. Specimens were mounted on a three-turn Ti-6Al-4V (percent by mass) mandrel, which was a shorter version of the standard ITER critical-current mandrel. The measurements covered the range of critical currents from less than 0.1 A to over 700 A. To verify the accuracy of the variable-temperature measurements, we compared critical-current values obtained on a specimen that was immersed in liquid helium at 5 K to those measured on the same specimen in flowing helium gas at the same temperature. This comparison indicated that specimen temperature was controlled to within 60 mK during the measurements. The critical-current data presented include electric-field versus temperature (E-T) characteristics, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (T) at constant H, and extrapolated effective upper critical field as a function of temperature. These data are part of what is needed for the ITER strand characterization.

Influence of the Heat-Treatment Process on the Mechanical Properties and Dimensions of Multi-Filamentary Composite Nb&lt;sub&gt;3&lt;/sub&gt; Sn Superconducting Wires

The levels of stress and strain experienced by the windings of a superconducting magnet during its manufacture and operation are dependent on the mechanical properties of the multifilamentary composite wire that the windings are comprised of. It is also influenced by the change in dimensions of the wire during the reaction heat-treatment stage of the manufacturing process. Using specimens cut from a spool of 1.5mm diameter niobium tin type superconducting wire, the influence of the heat-treatment process on the mechanical properties and dimensions have been investigated. The heat-treatment was carried out in an inert atmosphere using apparatus specially developed for the purpose. For heat-treatment durations up to that required to complete the reaction of the niobium filaments into Nb3Sn, the volume and diameter of the wires increased with increasing heat-treatment duration. The maximum increase was 2% and 0.8% respectively. The length of the wires decreased slightly for the shorter heat-treatments, but increased up to 0.33% for the longer ones. The mechanical properties were significantly different for specimens that had no heat-treatment compared to those that had only a short heat-treatment, one that was insufficient to convert much of the niobium into Nb3Sn. Before heat-treatment the bronze within the wires is in a work-hardened state, but gets annealed during the heat-treatment and this is probably the major cause of the change in mechanical properties. Apart from becoming more brittle, the mechanical properties do not change much for different durations of heat-treatment. This is quite remarkable considering that the composition changes dramatically with the length of the heat-treatment.

Mechanical Design of the Series Connected Hybrid Magnet Superconducting Outsert

An innovative hybrid magnet configuration is being developed at the NHMFL, consisting of a Florida-Bitter resistive magnet nested within a cable-in-conduit conductor (CICC) superconducting magnet to provide high fields for less power than traditional hybrid magnets. The resistive and superconducting magnets, connected in series, will be capable of producing 23.1 T and 13.8 T respectively for a total central field of 36.9 T. The CICC uses a cable of multifilamentary Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn/Cu strands inside a superalloy jacket that confines flowing supercritical helium in direct contact with the cable strands. The design of the magnet system is presented along with the design criteria used to evaluate the superconducting magnet and its integral components. The results of a structural analysis performed using finite elements for normal operational and fault loads are discussed for the most critical component, the conduit

Fracture of filaments and its influence on critical current and residual strength of fatigued Nb–Ti/Cu superconducting composite

Fatigue behavior at room temperature and its influence on critical current at 4.2 K and residual strength at room temperature of multifilamentary Nb–Ti/Cu superconducting composite wire with a filament volume fraction of 0.49 (copper ratio of 1.04) were studied. The fatigue crack nucleated in the copper in the circumferential region and propagated stably into the inner region, causing fracture of the Nb–Ti filaments in the late stage of the fatigue life. Once the fracture of the filaments started, the number of the fractured filaments increased steeply with increasing number of stress cycles, and correspondingly, the current-transportable and stress-carrying capacity of the composite decreased steeply. In this process, both the critical current and residual strength of the fatigued composite decreased nearly linearly with decreasing fraction of surviving filaments. Thus, the critical current of the fatigued composite was proportional to residual strength as a first approximation.

The study of Tc, Hc2, and Jc( T, B) in multifilamentary Nb 3Sn conductors of different design

The results of the study of the temperature-transition profile in magnetic fields and of the critical current J c( T, B) in multifilamentary Nb 3Sn conductors of different design are considered. We also analyze the contribution of the components with different critical parameters to the current-carrying capacity at high magnetic fields, and the possible causes of inhomogeneity of multifilamentary Nb 3Sn conductors of different design formed by “bronze” technology and by “internal tin technique”.

Hysteresis loss and critical current density of multifilamentary (NbW) 3Sn composite conductor for AC use

A multifilamentary Nb 3Sn composite conductor for AC use was fabricated by the internal tin-core method. In this conductor, Nb–1at.%W is used for filaments and Cu–1at.%Ni for matrix. The conductor has 33,252 filaments of a diameter of 0.36 μm. Samples of conductor were heat treated at 450–700°C for 24–740 h and the critical current and magnetization were measured at 4.2 K in transverse fields up to 5 T. Overall critical current density of 4.9 GA/m 2 at 1.5 T is obtained with a heat treated at 600°C for 200 h. The effective filament diameter was estimated to be 3–5 μm at 1.5 T.

Limitation of flux bundle volume and its influence on irreversibility fields

The irreversibility fields of multifilamentary Nb–Ti wires with different filament size were measured and compared with the theoretical prediction based on the flux creep theory. It was found that the irreversibility fields were seriously suppressed due to the flux creep at high temperatures while the critical current density of multifilamentary Nb–Ti wires was fairly large. This can be explained from the small pinning potential compared with the bulk specimen because the pinning potential is limited by the size of the filaments. In comparison with the theoretical prediction, quantitative differences occurred in the vicinity of the critical temperature. These can fit well by assuming the Gaussian distribution of the critical temperature for the multifilamentary Nb–Ti wires.

Irreversibility fields in multifilamentary Nb–Ti wires

Abstract The irreversibility field of multifilamentary Nb–Ti wires was measured and found to be seriously suppressed due to the flux creep at high temperatures while the critical current density was fairly large. This is explained with the small pinning potential as compared with a bulk specimen, since the pinning potential is limited by the size of the filaments. The experimental result is compared to the prediction based on the flux creep theory. Although the degradation of the irreversibility field is theoretically explained by the mechanism of flux creep, there exists a quantitative difference in the vicinity of the critical temperature. A theoretical analysis assuming a distribution of the critical temperature gave satisfactory results when compared to the experimental results.

Interfilament coupling loss for protection of superconducting multicoil magnets

We present experimental and analytical results of quenching for three-coil systems wound with multifilamentary Nb–Ti composites, demonstrating that AC loss, due mostly to interfilament coupling in the composite during quenching, promotes normal-zone propagation (NZP). The experiment used two three-coil systems, each comprised of inner, middle, and outer coils connected in series and shunted with resistors. Both systems shared the same outer and middle coils. Two versions of the inner coil were used, both versions wound with composites that were otherwise identical except for twist pitch lengths. One composite's twist pitch length was chosen sufficiently short to make the interfilament coupling loss to be negligible during quenching, while the other composite's twist pitch length was chosen sufficiently long to make the loss significant. Our model for quenching analysis includes the coupling loss and has been used to simulate the experiment. Agreement between experiment and simulation is excellent. The computed temperature distributions during quenching show a clear effect of heating by coupling loss in the inner coil wound with a composite having a long twist pitch length and virtually no effect of heating in the inner coil wound with a composite having a short twist pitch length. The difference in quenching behaviors between the two systems, different only in twist pitch length in the composites for the inner coils, is so compelling that we conclude that interfilament coupling loss is the key to this difference. We may further conclude that coupling loss may be used to benefit protection of multicoil systems. The twist pitch length that controls this coupling loss is thus a key design parameter; it is best determined through simulation.