Sn 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.

Advantages of Cu-Nb Reinforced Nb$_{\text{3}}$Sn Strands in Large Multi-Stage Stranded Cables With Low-Void Fraction

Advantages of Cu-Nb Reinforced Nb$_{\text{3}}$Sn Strands in Large Multi-Stage Stranded Cables With Low-Void Fraction

Development of Internal Tin Nb3Sn Strand for CRAFT in WST

Development of Internal Tin Nb₃Sn Strand for CRAFT in WST

Conceptual Design Study of a Test Facility Magnet for Fusion Conductor Samples

Design studies of the steady-state Korean fusion demonstration reactor (K-DEMO) magnet system have been conducted since 2013. The maximum magnetic field of the K-DEMO toroidal field (TF) coil is expected to be over 16 T. A high field superconducting magnet and a facility for testing the K-DEMO TF conductor samples are also being designed to prepare for the development of the K-DEMO TF conductor. To test straight fusion conductor samples compatible with the K-DEMO TF conductor, it is essential to have a large, 150 mm wide by 120 mm high sample space in the magnet. The required maximum magnetic field of the fusion conductor test magnet is expected to be over 15 T. The field consists of about a 15 T background field and some self-field. Both 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 and high temperature superconductor (HTS) tapes are candidates for the superconducting materials for the magnet. In this work, two conceptual design study models of the high field magnet are introduced - a planar racetrack dipole based on HTS tape (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 200 A/4-mm at 4.2 K, 18 T) cable-in-conduit conductor (CICC), and a non-planar flared end saddle dipole based on Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 2600 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K, 12 T) CICC. 2-dimensional magnetic and mechanical analyses of both magnet models were performed and the main features of the models are presented.

RRP Nb3Sn Subelement Shear Dependence on Hexagonal Subelement Stack Orientation and the Strand's Position Within a Rutherford Cable

Superconducting strands built into Rutherford cables undergo high deformation at the cable edges. This deformation negatively impacts subelement integrity and subsequently the Residual Resistance Ratio (RRR) of the stabilization Cu in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands after heat treatment due to compromised diffusion barriers [1]. Rutherford cables for the high luminosity upgrade of the LHC at CERN (HL-LHC) use Restack Rod Process (RRP) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires with a 108/127 subelement stack. Numerous transverse cross-sectional samples from the cables made under the HL-LHC Accelerator Upgrade Project (“HL-LHC AUP”) [2] have been mounted, polished, and analyzed with light microscopy. The number of sheared subelements within a strand was seen to depend not only on a strand's position in the overall cable (i.e. center vs. edge), but also on its specific position within the cable edge. Moreover, the rotational angle of the subelement stack within a strand, relative to the cable's centerline plane, appears to play a role. There is a need to understand these dependencies, as they might impact the applicability of any criterion for a maximum number of sheared subelements (typically < 15% of subelements) applied to series production quality assurance, often imposed to ensure acceptable RRR degradation. By gathering data across many HL-LHC AUP cables and utilizing a more statistical approach, we aim to establish more accurate success criteria for future R&D cables using RRP wire when the available cable runs (and thus samples) are limited.

Metallographic and Bending Strain Property Analysis of Reinforced Nb3Sn Strand Candidates for Japanese DEMO

The influence of a large electromagnetic force on the conductor of a demonstration power plant (DEMO) reactor magnet is a crucial issue. The present study summarizes the metallographic and bending strain property analysis of DEMO-candidate reinforced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires in Japan. Here, EBSD strain analysis was used as a new microscopic method to evaluate the material properties of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire components. Two reinforcement options have been considered: (1) the use of a brass matrix for distributed-tin (DT) wire (made by Kobe Steel) and (2) the use of CuNb reinforcement in the outer sheath of a bronze-route Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire (made by Furukawa Electric). Both the reinforced wires exhibit better <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stress</i> tolerance against bending load because of their higher mechanical strength than Cu. The high elasticity of the brass matrix of the DT wire and the CuNb area in the bronze-route Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire have been confirmed through electron backscatter diffraction (EBSD) strain analysis. The results strongly support the use of reinforced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires to maintain their high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> performance in Japan's DEMO designed with a larger operating current and higher magnetic field than International Thermonuclear Experimental Reactor (ITER).

Performance of the Cable-in-Conduit Conductors for Super-X Test Facility

Following the conceptual design, the engineering design of the dc magnet and cable-in-conduit conductor (CICC) for the Super-X test facility has been done in 2021. Totally three types of conductors with different structures were designed for the three pairs of coils in the dc magnet, respectively. High- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ∼2200 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) was applied to high field and middle field coils for reducing the radius of the dc magnet. In order to qualify if the designed parameters of the conductors could fulfill the performance criteria, three pairs of short samples have been manufactured and tested successfully in the SULTAN facility at SPC, Switzerland. Test results and analysis show that dc performance of the three types of conductors can meet the design criteria. The conductor qualification process including the sample preparation, test results, and analysis are presented in this article. The HFC and LFC conductors exhibit different ac loss behavior to the MFC conductor, which is discussed in the article.

DC Characterization of a Low-Field Nb3Sn Prototype Conductor for a DEMO TF Coil

A Low Field W&R (Wind and React) conductor, developed in the frame of the conceptual design studies for the Toroidal Field (TF) coils of DEMO, has been designed to be constituted of a small number of superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands and a high number of stabilization copper wires. It has been extensively characterized at the SULTAN facility at Swiss Plasma Center (SPC) in February 2021. The conductor has been operated up to its target current, 70.8 kA, and characterized up to a background magnetic field of 10.78 T. The test program included AC loss measurements, current sharing temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_cs</i> ) and critical current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> ) measurements before and after cyclic loading. Critical temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> ) measurements were performed on the virgin conductor at the beginning and at the end of the test campaign. In this work we are focusing on the DC characterization, while the AC measurements analysis will be the object of a following work. Concerning DC tests, premature quench phenomena prevented from operating the conductor at the target current – field conditions. Nevertheless, at lower currents <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_cs</i> stability was observed, with no performance degradation due to cycling. The present paper investigates these results and tries to give an explanation through measurements and tomographic analysis. Furthermore, the achieved strain distribution in the conductor, obtained by means of inductive measurements analysis, will be shown. The implications of the design approach on the performance, based on the outcome of such tests, are discussed.

Heat Treatment Optimization on Nb$_{3}$Sn Strands Based on Electrical and Physical Properties

The electrical and physical properties of Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn strands are strongly dependent on the heat treatment during which tin diffuses into niobium by solid-state diffusion. During diffusion, Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn grains grow at the Nb/bronze interface. The shape and size of the grain depend on the temperature of the last step of the heat treatment, its duration and the size of the Nb filaments. The volume of reacted Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn together with the grains’ structure influence the non-copper critical current density J <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{c}$</tex-math></inline-formula> and the magnetization. Therefore, an optimization of the heat treatment with respect to J <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{c}$</tex-math></inline-formula> and hysteresis loss is important when working on the design of superconducting cables. This contribution presents the results of a heat treatment optimization performed on a 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\,$</tex-math></inline-formula> mm diameter, internal Sn Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn strands produced by Kiswire Advanced Technology (KAT) for two React&Wind conductor prototypes for the Toroidal Field Coil of the EUROfusion DEMO: a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {66}\,$</tex-math></inline-formula> kA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {12}\,$</tex-math></inline-formula> T and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {105}\,$</tex-math></inline-formula> kA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {12}\,$</tex-math></inline-formula> T prototype. For the optimization, four heat treatment schemes were considered and their evaluation based on I <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{c}$</tex-math></inline-formula> measurements at 4.2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\,$</tex-math></inline-formula> K, in the range of 9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\,$</tex-math></inline-formula> T to 15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\,$</tex-math></inline-formula> T, on SEM micrographic studies on grain size and shape and on hysteresis loss measurements on a vibrating sample magnetometer (VSM). Based on these results, a heat treatment schedule is proposed for the prototype DEMO conductor and the scaling law for J <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{c}$</tex-math></inline-formula> is updated.

Mechanical Analysis of Full-Scale Nb3Sn CICC Designs for Tokamaks

Several designs of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Cable-In-Conduit Conductors (CICCs) have been proposed so far for high-performance tokamak magnets. The Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands composing the conductors are subjected to mechanical stresses of electromagnetic (EM) and thermal origin, inducing local deformations and affecting the strands critical current carrying capability. In the last ten years a numerical tool based on a finite element (FE) code has been developed to simulate the mechanical behavior of the CICCs subjected to operating loads. The main goal of this tool is to predict the electro-mechanical performance of the conductor in operation as a function of the design parameters such as the void fraction, the twist pitches and the conductor shape. In this work, a detailed model of a full-cable ITER TF CICC is presented. This model proves useful for a deeper understanding of the mechanical phenomena occurring among the sub-cables during the conductor operation. Moreover, the numerical modelling of different conductors from the ITER, DTT and JT60 projects is also presented to highlight the versatility of the code.

Construction and Test of the Enhanced Racetrack Model Coil, First CERN R&amp;D Magnet for the FCC

Racetrack model coils (RMC) have been built at CERN during the past decade, as an R&D tool to qualify conductors and technologies developed for high field superconducting accelerator magnets (Perez <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , 2016). RMC, assembled in a dipole magnet configuration, proved to be an efficient instrument reducing cost and feed-back time while developing new magnets. In a similar way, as for the High-Luminosity Large Hadron Collider (HL-LHC) project, CERN has designed the enhanced RMC (eRMC) made of two flat coils using 40 (1 mm diameter) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand cable produced with Rod Restack Process (RRP) technology. This conductor geometry, originally designed and produced to build the block coil dipole magnet FRESCA2 (Rochepault <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , 2019), was chosen to reduce the production time and shorten the road towards the feasibility demonstration to reach 16–18 T magnetic fields in a dipolar configuration. Like previous model coils built at CERN (Short model coils (SMC) & RMC), eRMC1a has been built using the “bladders and keys” type mechanical structure. This paper describes the main construction steps and the powering test results. At 1.9 K the magnet produced 16.5 T peak field in the conductor, the highest ever for a dipole magnet of this configuration.

Properties of Nb3Sn Superconductors of Various Designs

To manufacture a high field magnet it is required to have Nb₃Sn superconductors with high current-carrying capability in magnetic fields up to 18 T. For example, CERN has chosen Nb₃Sn together with HTS as potential material for high field magnet of Future Circular Collider (FCC) project. Also, these superconductors must have an RRR of more than 150 and a low level of energy loss. One of the ways to achieve high current density is to use Internal Tin method to manufacture Nb₃Sn superconductors. In this paper, Internal Tin Nb₃Sn strands with 37, 54, 84 and 120 sub-elements are studied. Using TEM and SEM analysis, the structure and elemental composition of the superconducting layer were investigated. The analysis of the measurement results of electrophysical parameters showed that with an increase in the number of subelements from 37 to 120, at the same heat treatment mode of 370&#x00B0;C, 100 h &#x002B; 665&#x00B0;C, 40 h, the critical current density measured in the range of magnetic fields from 12 to 23 T for strands of different designs practically does not changes significantly, and the RRR parameter decreases from 280 to 110 units and the effective filament diameter decreases from 139 to 70 micrometers.

Experimental Research of the New Developed High-Jc Nb3Sn Superconducting Strand for 14 T MRI Magnet

The Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) has announced a project to design and manufacture a high field MRI magnet with a 900 mm warm bore and comprised of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables in 2017. The maximum magnetic induction of magnet system is 14.3 T and magnetic field homogeneity in a spherical volume with 22 cm diameter is better than 0.5 ppm. The high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand is the potential options to use for such high field magnet system. ASIPP cooperates with Western Superconducting Technologies Company (WST) to develop high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand whose critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is higher than 2000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K, 12 T. To obtain the actual performance of WST high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand, three short samples are prepared with different non-copper ratio or different heat treatment, the critical current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> or quench current of all sample is measured at different field by the voltage-current (V-I) method, the residual resistivity ratio (RRR) of all sample is also measured. To study the strand whether it will be affected by low field instability, a small coil is built with WST strand, and the experimental result is presented and discussed in this paper.

Study of the ITER TF CICC Mechanical Behavior Under Cool-Down and Repetitive EM Loadings

High-current and high-field superconducting magnets sustain huge electromagnetic loads. On top of that, the high field constraint pushes towards the use of high-performing superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands, which are strain sensitive. In recent fusion magnets, the loads reach few hundreds of kN/m in static and sometimes cyclic regimes. These loads modify the strands local strain state and the overall mechanical properties of the cable. In this article, the finite element mechanical code MULTIFIL is applied to model an ITER Cable-In-Conduit Conductor (CICC) petal for the Toroidal Field (TF) coil under operating loadings. The local strain maps and the macroscopic mechanical behavior of the cable are analyzed for both thermal and electromagnetic loadings. A possible explanation for the initial drop of the current sharing temperature observed in some of the TF CICCs tested in the SULTAN facility (PSI, Villigen) is proposed. The impact of important model parameters, such as the void fraction and the friction coefficient between strands, on the mechanical behavior of the conductor is investigated.

AC loss and contact resistance of different CICC cable patterns: Experiments and numerical modeling

• The AC loss and interstrand contact resistance measured on three prototype Nb 3 Sn Cable-In-Conduit Conductors with significantly different cabling twist pitch patterns. • Simulations, done by using the JackPot model are in fair agreement, confirming the consistency of data and allowing reliable predictions with the JackPot model. • For CICC cables, a design petal wrap coverage of 70 % should be maintained for CS cables to reduce the AC loss. A critical threshold value of void fraction exists, which makes the presence of petal wraps ineffective in reducing coupling loss. • The JackPot simulations with derived corrections for inter-petal contact resistance representing 70 % petal wrap coverage show that the Twente cable design has the lowest coupling loss. • So far, the Twente cable design seems a suitable candidate for CFETR or DEMO conductors, made of strain-sensitive strands such as Nb 3 Sn or other materials, for minimization of coupling loss and strand indentation. For upcoming nuclear fusion energy reactors, like the China Fusion Engineering Test Reactor (CFETR) and EU-DEMO, the superconducting Cable-In-Conduit Conductors (CICC) are in the design phase, and the operating conditions like electromagnetic forces can be higher than in previous devices like ITER. The prototype conductors for the Central Solenoid (CS) coils in the CFETR, for example, are designed to produce a peak field of 19.9 T and are expected to be made of high current density Nb 3 Sn strands. Investigations are also ongoing on the application of bismuth strontium calcium copper oxide (BSCCO) and MgB 2 strands for CICCs in fusion reactors. The latter material, MgB 2, could be applied for superconductors subjected to lower magnetic fields, such as Poloidal Field coils, Correction Coils, and Feeders. The performance of all these strands is sensitive to strain, and the mechanical strength of the brittle filaments is relatively weak. This requires a thorough analysis of the cable pattern in terms of the mechanical support of the strands along their length in combination with the minimization of the interstrand coupling currents and strand indentation. As an initial step to finding the most appropriate cable pattern for CICCs, three prototype CICCs made of ITER type Nb 3 Sn strands with significantly different cable twist patterns are tested experimentally for AC coupling loss, interstrand contact resistance, and strand indentation. The three cabling patterns referred to as the Twente, CWS (copper wound superconducting strand), and CFETR-CSMC (CFETR Central Solenoid Model Coil) design. The numerical code JackPot ACDC developed at the University of Twente is used to analyze the interstrand coupling loss and contact resistance. The new ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) triplet modified CWS design is aimed at reducing strand pinching during cabling, which causes degradation of transport properties during compaction and cyclic loading. The Twente design has the same objective but also aims at reducing the coupling loss while maximizing the mechanical lateral support for the strands by making the twist pitch ratio of the sequential cabling stages close to one. The CFETR-CSMC, taken as a reference for comparison, has cable a pattern mostly similar to the ITER CS cable design.

Measurements and Modeling of Mechanical Properties of Nb3Sn Strands, Cables, and Coils

High-field Nb3Sn magnets for the next generation of hadron colliders are known to have long trainings and to be limited at 90% of their short sample limit at best. This means that the conductor current is at 60% or less of its critical value. Homogeneous finite-element models are typically used for the complex system that includes coils, wedges, poles, yoke, rods, keys, shoes, fillers, shell, etc. Through both modeling and measurements of mechanical properties, we show how a more accurate sub-modeling of the composite structure allows for a better understanding of the stress and strain distributions. This is extremely important for Nb3Sn performance since state-of-the-art high critical current wires of this superconductor are particularly sensitive to strain.

First Experimental Results on Damage Limits of Superconducting Accelerator Magnet Components Due to Instantaneous Beam Impact

The energy stored in the particle beams of an accelerator such as CERNs LHC is substantial and requires a complex machine protection system to protect the equipment from damage. Despite efficient beam absorbers, several failure modes can lead to beam impact on superconducting magnets. The energy deposition from these beam losses can cause significant temperature rise and mechanical stress in the magnet coils, which can lead to a degradation of the insulation strength and critical current of the superconducting cables. An improved understanding of the damage mechanisms is important for the LHC when considering its planned increase in beam brightness, as well as for other accelerators using superconducting magnets. The degradation mechanisms of Nb-Ti and Nb $_3$ Sn strands and the cable stacks insulation have been assessed based on magnetization and breakdown voltage measurements in three experiments at room temperature. The degradation of the insulation when exposed to high temperatures for several hours was measured. The second experiment assessed the effect of a millisecond temperature rise on superconducting strands using a fast capacitor discharge. In the third experiment, cable stacks and single strands have been exposed to a 440 GeV proton beam. In this paper, the experimental results of these tests are presented and discussed.

Ongoing Efforts at Internal-Tin Nb3Sn Strand With Higher Jcn and Lower Hysteresis Loss

Western Superconducting Technologies Co. Ltd. (WST) has been keeping in the research and development 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 strand for higher performance level, for the future fusion reactor after ITER. Deformation optimization, Cu splits, and Sn spacers were applied to prepare Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand with higher J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> and lower hysteresis loss, and the effect of these three factors on performance was investigated and discussed. Compared with the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand for ITER, strand distortion was obviously reduced by deformation optimization and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> was improved, but at the same time hysteresis level was increased. To reduce the level of hysteresis loss, Cu splits were used to replace a few Nb filaments in each subelement. Cu splits could break the bridging Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filament rings and as a sequence hysteresis loss was dramatically reduced along with the reduction of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> , which is attributed to low reaction degree of Nb filaments in the outermost subelements. Therefore, Sn spacers between the outermost subelements and the Ta barrier were added, and a 37-subelement structure was used to shorten the diffusion path of Sn. The reaction degree of the filaments was enhanced as well as J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> . Finally, it could be concluded that a 37-subelement structure with Cu splits is effective to reduce the level of hysteresis loss and raise the level of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> .

Evaluation of Thermal Strain Induced in Components of Nb3Sn Strand During Cooling

Nb3Sn strands, whose properties are very sensitive to stress/strain, are utilized for ITER cable-in-conduit conductor (CICC) of the central solenoids. The Nb3Sn strands experience temperature range of ∼1000 K from the temperature of the heat treatment with the initiation of the Nb3Sn reaction to the operation temperature of ∼4 K. Due to this large temperature range, large thermal strain is induced in the Nb3Sn filaments due to the differences between the coefficients of thermal expansion and Young's moduli of the components of the strand. Therefore, it is considered that initial performance of the CICC is influenced by the thermal strain on the Nb3Sn, and it is important to evaluate the strain state of the Nb3 Sn strand at low temperature. In this study, the thermal strain of the components of free Nb3Sn strand was measured by neutron diffraction and stress/strain state was assessed from room temperature to low temperature. As the results of diffraction measurements, it was found that 0.111% and 0.209% compressive strain were generated in Nb3Sn filaments at 300 and 10 K, respectively.

Optimization of Nb3Sn Rutherford Cables Geometry for the High-Luminosity LHC

The quadrupole and dipole magnets for the high-luminosity large hadron collider (HL-LHC) upgrade will be based on Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables that operate at 1.9 K and experience magnetic fields of up to about 12 T. An important step in the design of these magnets is the development of the high-aspect-ratio Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cables to achieve the nominal field with sufficient margin. The strong plastic deformation of unreacted Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands during the Rutherford cabling process may induce non-negligible I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and Residual Resistivity Ratio degradation. In this paper, the cabling degradation is investigated as a function of the cable geometry for both Powder-in-Tube and Restack Rod Process conductors. Based on this analysis, new baseline geometries for both 11 T and quadrupole magnets of HL-LHC are proposed.