Nb3Sn Strands Research Articles

Local plastic deformation effects on the critical current of high-Jc Nb3Sn strands under uniaxial strain

Abstract In order to meet the target operating parameters of the toroidal field coils (TFC) for the next-generation Chinese compact burning plasma tokamak, high critical current density (Jc) Nb3Sn strand will be applied to the high-field winding-package (WP) of the TFC. To improve the transverse stiffness of the cable in withstanding the huge Lorentz force and avoiding conductor performance degradation, the Short-Twist-Pitch (STP) and Copper-Wound-Strand (CWS) cable patterns were taken into consideration. In the processes of cabling and compaction of the conductor, the tight cable configurations lead to severe local plastic deformation (LPD) within the strands. The strands in the conductor are subjected to strain caused by thermal contraction and Lorentz force during conductor cooling down and operation. So far it is unknown, whether the LPD could impact the critical current (Ic) versus uniaxial applied strain behaviour of high-Jc Nb3Sn strand. Aiming to investigate the effect of LPD on the Ic of strands under uniaxial strain, three types of high Jc Nb3Sn strand with different indentation depths were tested on a U-shaped bending spring. The axial strain ranges from -0.9% to +0.4% at 14 T and 4.2 K. The three types of strands showed more strain sensitivity and lower tensile irreversible strain limit with increasing LPD, while even irreversible degradation of the Ic could be observed in the compressive strain region. The sample preparation, test process, test results and analysis are reported.

Conceptual design study of a large bore superconducting test facility magnet, SUCCEX

A 16 Tesla, large bore superconducting magnet for testing the superconducting Cable-in-Conduit Conductor (CICC) is being designed in parallel with the design of the steady state Korea fusion demonstration reactor (K-DEMO) magnet system. The test facility magnet, named SUperConducting Conductor Experiment (SUCCEX), was designed in 2014 and some design modifications of the magnet have been conducted since 2020. The peak magnetic field of the K-DEMO toroidal field coil is about 16 T, and thus the required background field of the conductor test magnet is expected to be over 15 T in a large bore of 600 mm diameter. The background field will be in addition to the K-DEMO conductor sample's self-field. To reach the target magnetic field, high current density Nb3Sn strands (Jc > 1100 A/mm2 at 4.2 K, 16 T) were employed in the design of the high field CICC. The modified SUCCEX magnet consists of concentric solenoids, with a high field inner coil (IC) and low field outer coil (OC). They are connected in series with each other, and the operating current is about 24 kA. In this study, the overall design concept is presented in relation to the results of electro-magnetic, structural analyses.

Comparative drawability and recrystallization evaluation of Nb4Ta and Nb4Ta1Hf alloys, and the beneficial influence of Hf on developing finer Nb3Sn grain size

Nb3Sn superconducting wires with high critical current density (Jc, 4.2 K) at fields greater than 15 T are needed for next-generation superconducting magnets for high-energy physics and nuclear magnetic resonance applications. It has been demonstrated that the addition of one atomic % Hf can significantly increase the high field Jc of Ta-doped Nb3Sn and offer the potential to meet the Future Circular Collider (FCC) specification of 1500 A/mm2 at 16 T, 4.2 K if the pure Nb filaments in conventional Nb3Sn composites can be replaced by Nb4Ta1Hf alloy. In this paper we demonstrate that a commercially produced Nb4Ta1Hf alloy is ductile and drawable in a Cu matrix to a large true strain of 15 without intermediate annealing. The hardness and work-hardening rates of Nb4Ta1Hf are shown to be higher than Nb and commercial Nb4Ta, but are comparable to ductile Nb47Ti alloy at high strains, which is significant because more than 1000 tons of Nb-Ti rods are co-drawn in Cu each year. Very importantly for the Nb3Sn properties is that just 1 at%Hf shifts the recrystallization curve of Nb4Ta to temperatures above the normal 600 °C −700 °C A15 range of reaction temperatures used for Nb3Sn wires. For the Hf-based 19 filament rod in tube (RIT) conductors of this study, we observe a refined (<100 nm) Nb3Sn grain size, and an improved upper critical field of 24.6 T (4.2 K), ∼1 T higher than that in a corresponding Nb4Ta composite. The demonstration of improved performance and good drawability with commercially produced Nb4Ta1Hf alloy indicates a pathway for Nb3Sn strand development for next-generation Nb3Sn magnets.

Impact of local plastic deformation on thermo-magnetic instability of high-J c Nb3Sn strand

The future fusion reactor devices built by Institute of Plasma Physics, Chinese Academy of Science will be a compact burning plasma facility to fill the gap between ITER and the China Fusion Engineering Test Reactor, with the mission of studying the deuterium–tritium plasma in steady-state operation. The winding-package (WP) of its toroidal field coil is graded into high-field WP and low-field WP, and the high critical current density (J c) Nb3Sn strand will be applied to the high-field WP based on the current design. The thermo-magnetic instability is the main issue in the application of high-J c Nb3Sn strand, which may induce premature quenching in the low field regions. This issue can be improved by reducing the effective filament diameter (d eff) to suppress flux jumps and increasing the residual resistivity ratio (RRR) to enhance the release of heat generated by flux jumps. However, in the conductor manufacturing process, local plastic deformations (indentation) of strands can impact the sub-element layout and degrade the RRR of the strand, which would increase again the risk on instability. In this study, magnetization measurements and V–I tests were performed on samples with different indentation depths. The hysteresis loss and d eff of the indented sample was obtained from the test results. The impact of local plastic deformation on the thermo-magnetic instability of two types of Nb3Sn strand was confirmed by cross-sectional metallographic observation and quantitative microstructure analysis. It was shown that flux jumps were suppressed at indentation depths below 0.3 mm. Further indentation increase leads to severe flux jumps.

Damage experiment with superconducting sample coils - experimental setup and observations during beam impact

The damage mechanisms and limits of superconducting accelerator magnets due to the impact of high-intensity particle beams have been subject to extensive studies in the past years at CERN. Recently an experiment with dedicated sample coils made from Nb–Ti and Nb3Sn strands was performed at CERN’s HiRadMat facility. This paper describes the design and construction of the sample coils as well as the results of their qualification before the beam impact. In addition, the experimental setup will be discussed. Finally, measurements during the beam experiment like the beam-based alignment, the observations during the impact of 440GeV protons on the sample coils and the achieved hot-spots and temperature gradients will be presented.

Metallographic investigation of first full-size high-Jc Nb3Sn cable-in-conduit conductor after cyclic loading tests

In order to verify the feasibility of applying high-Jc Nb3Sn strand in fusion magnet, a full-size cable-in-conduit conductor (CICC) with short twist pitch (STP) cable pattern was manufactured and tested in SULTAN facility at SPC, Switzerland. Three levels of cyclic electromagnetic (EM) load were applied on the sample stepwise, no visible decrease of current sharing temperature (Tcs) was observed until the EM load increased to 80 kA × 10.8 T, after that the Tcs decreased dramatically with the EM cycles, which suggested that irreversible deformation, causing a change in the strain state, or even damage has occurred in the superconducting strands. For investigating the reason which caused the conductor performance degradation, the tested conductor was dissected for metallographic observation. Eight segments which subjected to different EM loads were extracted from one of the legs, the geometric feature changes of the cable cross-sections were analyzed and compared. A good correlation was found between the decrease of the Tcs and deformation of the cable cross section. A mass of cracks were found on the sub-elements of strands in the segment which subjected to highest EM load, but the amount of crack is much lower in other segments. Combining the analyses, it is speculated that the critical EM load which causes irreversible degradation is between 850 kN/m and 870 kN/m for this conductor. The results could be a reference in high-Jc Nb3Sn CICC design.

Performance tests of ITER CS conductor samples from series production

The International Thermonuclear Experimental Reactor (ITER) magnet system is one of the most sophisticated superconducting magnet systems ever designed, with a stored energy of 51 GJ. The coils are wound from cable-in-conduit conductors made of superconducting and copper strands assembled into a multistage rope-type cable, inserted into a conduit of austenitic steel tubes. The ITER central solenoid (CS) works in pulsed mode, reaching a peak field of 13 T, thus allowing the induction of a high intensity current in the plasma of the ITER tokamak. This magnet consists of a stack of six modules which include around 125 t of Nb3Sn strands. The production of all CS conductors has been completed and module manufacturing is well underway; throughout the production phase, samples were cut at the extremities of the conductor unit lengths to undergo quality control tests. About 25% of the conductor short samples were tested in current and field at cryogenic conditions at the SULTAN facility in Villigen, Switzerland. This work reports the comparative analysis of the short samples set of test results.

Manufacture and performance test result of a 95 kA-class Nb-Ti cable-in-conduit conductor for the low field winding-package of CFETR-TF coil

The engineering design of the CFETR TF prototype coil and conductors has been completed. The wind-package (WP) of the coil is graded into three regions based on the magnetic field distribution for saving cost. High-Jc Nb3Sn strand, ITER-like Nb3Sn strand and Nb-Ti strand are applied for high-field, mid-field and low-field WP respectively. In order to verify the conductor design, full-size short samples have been manufactured for the three types of conductors. The samples are tested in the SULTAN facility at CRPP in Villigen, Switzerland. At present, the test of Nb-Ti conductor for low-field WP is finished, DC and AC tests were performed. In DC test, several current sharing temperature (Tcs) measurements were performed with 500 electromagnetic cycles and one thermal cycle. Additionally, minimum quench energy (MQE) measurement was performed for investigating the stability of the conductor. The test results and analysis are reported in this paper.

Impact of indentation on the performance of high Jc Nb3Sn strand

To meet the peak magnetic field requirements of the central solenoid (CS) and toroidal field (TF) magnets in the China Fusion Engineering Test Reactor (CFETR), cable-in-conduit conductors (CICC) that are composed of Nb3Sn strands with high critical current density (Jc) have been developed. The Short-Twist-Pitch (STP) design is adopted to avoid performance degradation caused by the electromagnetic cycles. But the STP design and tight cabling will cause severe indentation at the contact point between strands. Previous studies on Nb3Sn CICC show that the indentation has a significant effect on the critical current (Ic) and residual resistivity ratio (RRR) of the superconducting strand. Therefore, the same study was carried out for the high Jc Nb3Sn superconducting strand produced by Oxford Superconducting Technology (OST) and Kis wire Advanced Technology CO., Ltd. (KAT). In this paper, the Ic and RRR measurements of sample at different indentation depths were made, and the results were compared with the previous Nb3Sn indentation strands and flat rolled samples. Also, the hysteresis loss of the samples with different indentation depth was measured, and the change of effective filament diameter with indentation depth was obtained.

FEM Modeling of Superconducting Whole Body, Actively Shielded 7 T MRI Magnets Wound Using Nb3Sn Strands

FEM Modeling of Superconducting Whole Body, Actively Shielded 7 T MRI Magnets Wound Using Nb₃Sn Strands

Conductor design and performance analysis for CFETR magnet

The design work of the China Fusion Engineering Test Reactor (CFETR) was carried out in the past years. The magnet system is composed of 16 toroidal field (TF) coils, 8 central solenoid (CS) coils, and 7 poloidal field (PF) coils. The maximum magnetic field on the TF coils is 14.4 T. Three grades of cable-in-conduit conductors (CICCs), including high-Jc Nb3Sn strand, ITER-like Nb3Sn strand, and NbTi strand, have been chosen for the TF WP winding in high, medium, and low magnetic fields. High-Jc Nb3Sn strand will also be applied in the CS conductor considering the maximum magnetic field, which is higher than the TF coil. For the PF coil, the ITER-like Nb3Sn strand will be applied in Conductor PF1 and Conductor PF7, and NbTi will be applied in others. In this paper, conductor designs for the CFETR magnet including design criteria, choice of conductor configurations, and conductor performance analysis are presented.

Niobium rod quality and its impact on the production of Nb3Sn strand for the Divertor Tokamak Test Facility toroidal coils

Niobium Tin cable is being used in the high field magnets in the Divertor Tokamak Test Facility under construction by ENEA in Italy. Kiswire Advanced Technology was selected to provide 55 tonnes of high-performance Niobium Tin wire for the 18 Toroidal Field Coils. The conductor design is based on a 0.82mm Nb3Sn strand similar to that provided for the ITER TF coils. H. C. Starck Solutions provided the pure Nb rod and Ta sheet for their conductor design. The enhanced strand requirement has both a minimum current (320 A at 12 Tesla) and maximum hysteresis loss (± 3T cycle 1000mJ/cc) and a Cu/Non-Cu of 1:1. To achieve the current and hysteresis loss reliably, the niobium rod must maintain its shape throughout the drawing process. The n value requirement (>20) is a good indication of niobium filament uniformity while the RRR requirement (>100) is a good indication of tantalum barrier performance. The HCSS rod supplied to KAT provided consistently high Ic (>320A at 12 Tesla), low hysteresis loss and the Ta sheet provided very good RRR values. We report here the consistency of more than 11 tonnes of HCSS Nb rod properties for chemistry, hardness, grain size and wire performance.

Impact of Indentation on the Performance of High Jc Nb3sn Strand

Impact of Indentation on the Performance of High Jc Nb3sn Strand

Examination of design parameters affecting Nb3Sn CICC current sharing temperature using necessary condition analysis

The performance of Nb3Sn cable-in-conduit conductor (CICC) especially its current sharing temperature (T cs) is known to depend on the various parameters of the CICC such as the Nb3Sn strand characteristics, its cabling, the final geometry of the CICC cross section, the void fraction, the current distribution and other parameters. However, the performance has not been very predictable. The change in CICC T cs performance with repeated operation or electromagnetic loading is of particular concern. Furthermore, evaluation of T cs performance with respect to parametric variation of some CICC parameters has resulted in conflicting conclusions for different CICC designs. Thus, a more comprehensive review of published CICC T cs performance with respect to the corresponding CICC cable characteristics has been performed and is reported here for CICC samples whose designs and test results have been published during the past decade and more. Necessary condition analysis as well as linear regression is applied in the interpretation of the results which has found that the first stage cable twist pitch which has been the focus of much research is not that significant in determining the T cs performance but does affect T cs degradation. The direct effect of strand critical current on T cs performance was also found to be weak. On the other hand, void fraction was found to be significant in both determining T cs performance and limiting T cs degradation. Other correlations between parameters have also been studied. Overall, it seems that there has been appropriate targeting as well as overemphasis on some CICC parameters for improving Nb3Sn CICC T cs performance. No single parameter studied has been found to be a determining factor for high stable CICC T cs performance. Research on additional parameters is warranted.

Experimental study on strain sensitivity of Internal-Tin Nb3Sn superconducting strand based on non-destructive technology

Fracture of brittle filaments plays a crucial role in significant degradation of critical current density of multifilamentary Nb3Sn strand. Recent researches of distribution of filament factures mostly depend on photographic analysis with SEM or TEM. However, it is hard to obtain spatial distribution of filament fractures in the mulitifilamentary Nb3Sn strand. In addition, almost unavoidable filament fracture is introduced during the sample preparation procedure and it causes deviation of filament fracture distribution. X-ray microtomography is a non-invasive, non-destructive method, which allows determination and the discrimination of internal features without destructing the sample. It is widely used as a non-destructive test technology for its high penetrability and high resolution. This present work investigated filament fracture of two types of Nb3Sn strands under different tensile strains by high energy X-ray. The influence of tensile strain on the filament fracture behavior was investigated and the results will be expected to provide reference for optimization of multifilamentary Nb3Sn strand, conductor and magnet design.

Suppression of the critical current degradation under the compressive stress on the internal reinforcement bronze processed Nb3Sn wire using Cu-Sn-In ternary bronze alloy matrix

Typical large current capacity conductors for fusion magnet are often manufactured by the Cable-In-Conduit (CIC) method. The CIC conductor is fabricated by twisting multiple Nb3Sn strands and pure Cu wires, and huge local stress/strain concentration alike the point contact was observed in each twisted Nb3Sn strand applied to transverse compressive load based on the electromagnetic force. These concentrations caused to the Nb3Sn filament breakage and it was also a major factor of the current sharing temperature (Tcs) and critical current density (Jc) degradation on CIC conductor. Mechanical strength improvement of the Nb3Sn strand is required to realize superconducting magnet under higher electromagnetic force for a future DEMO. Recently, we succeeded to fabricate the internal matrix reinforcement bronze processed Nb3Sn multifilamentary wire using a Cu-Sn system ternary alloy matrix containing Indium (In) element.In this study, critical current (Ic) property with the in-situ compressive stress on the internal matrix reinforcement Nb3Sn wire was evaluated. The compressive stress obtained to the Ic deterioration of 5% on the internal matrix reinforcement Nb3Sn wire was confirmed at 150 MPa, and this stress was 3 times higher than Nb3Sn wire without reinforcement. We found that the internal matrix reinforcement was one of the great advantage methods to suppress the Ic degradation by the compressive stress.

Performance test and analysis of the first large-scale cable-in-conduit conductor with high J c Nb3Sn strand for fusion reactor

The Comprehensive Research Facility for Fusion Technology (CRAFT) project has been launched in 2019, for developing the essential engineering technologies for Chinese Fusion Engineering Testing Reactor (CFETR). Within this project, a full-size toroidal field (TF) coil will be built as the prototype coil for CFETR. Based on design of CFETR magnet system, the TF coil will operate at 95.6 kA in a peak field of 14.5 T. The high-J c Nb3Sn strand is taken into consideration due to the critical current density of ITER-grade Nb3Sn is too low at 14.5 T. Considering that it will be the first time to apply the high-J c Nb3Sn strand in the large-scale cable-in-conduit conductor (CICC) for fusion magnet, a conductor sample made of high-J c Nb3Sn strand with short twist pitch (STP) cable pattern was manufactured in ASIPP and tested in SULTAN facility, to investigate the feasibility. The test campaign focuses on the impact of cyclic electromagnetic (EM) loading and warm-up cool-down (WUCD) to the performance of the conductor, the strain distribution of the conductor before and after EM cycles was measured by inductive method to make a deeper insight of the conductor performance evolution. AC losses tests have also been carried out, providing relevant information for further coil design.

Extensive Analyses of Superconducting Cables 3D Geometry With Advanced Tomographic Examinations

In the framework of activities embedding magnet design and associated R&D activities and relying on Cable in Conduit Conductors (CICC) technology, the singularity of the concept can rise some challenges versus their modelling in operation. Indeed, CICC includes thousands of superconducting strands, twisted together under a multi-staged scheme that also includes deformation by cabling and compaction during manufacture. This causes the accurate predictability of strands position in CICC extremely difficult, while it can be a key element for modelling their performances in operation. As a matter of fact the coupling losses rely on the CICC capability to establish inter-strand shielding currents, driven by the inter-strand contacts mapping which are only accessible in prediction via accurate 3D strand trajectories geometry. Same applies for prediction of CICC mechanical properties (deformation of Nb3Sn strands) and hydraulic properties (helium coolant force-flowed between the strands), making those investigations of high added-value. In this context, INFLPR installed a new set-up dedicated to micro-tomography to examine CICCs with high resolution, and get an overall overall 3D overview regarding picture of the strands location. CEA and INFLPR further developed a post-processing method to reconstruct the strands trajectories with high accuracy. The measurement and data analysis workflow was applied to two CEA middle-size CICCs with variable void fraction from which contact statistics were issued. The obtained database was exploited to reconstruct equivalent 3D resistive network. Above applications using CICC topology database are discussed, compared to experimental AC losses database issued by CEA and interpreted considering the CEA model COLISEUM. The outcomes are discussed and subsequent guidelines for future work presented.

A methodology to compute the critical current limit in Nb3Sn magnets

Numerous experiments have shown that the loads applied to Nb3Sn strands and cables can reduce their critical current. Experiments, performed on uniaxially loaded strands, allowed to define clear laws to describe the evolution of the critical surface as a function of the applied current, field, temperature and strain. It is, however, still unclear how these laws can be applied to superconducting magnets. The present paper proposes a methodology to estimate the critical current and temperature margin reduction on superconducting magnets due to stress on the superconducting material. The methodology is tested on the MQXF magnets, a quadrupole developed for the High Luminosity LHC project, and successfully validated by comparing computed strain with data from strain gauge measurements. Results suggested that, because of the stresses arising in winding during assembly, cool-down and powering, the current limit of the magnet is lower than the expected short sample limit, and that the most critical region does not coincide with the peak field location.

Prediction of effective properties for composite superconducting strand and multi-stage cables

Superconducting multifilamentary Nb3Sn strand and cables have complex microstructure and macrostructure which are used in the International Thermonuclear Experimental Reactor (ITER) superconducting magnet system. In a generic form, the superconducting strand is made of copper, multifilamentary Nb3Sn and bronze which is a composite. The twisted composite strands form the superconducting cables with typical multi-stage structures. Here we propose a numerical model focusing on the problem of characterizing the effective properties of composite superconducting strand and cables at microscopic and macroscopic scales. The effective elastic constants of the composite superconducting strand can be derived by involving the combination of the representative volume element (RVE) and the Mori-Tanaka method. Subsequently, the proposed method is applied to capture the effects of temperature on the effective elastic constants of the superconducting multi-stage cables. Results show the elastic constants predicted by this model agree well with existing theoretical predictions and available experimental data. This numerical model may be utilized to optimize the mechanical properties of the composite superconducting strand by tuning the constituent fractions and to control the tensile stiffness of superconducting cables by tuning the winding laps of the cables.