Nb3Sn Conductors Research Articles

Two-Layer 16 T Cos Dipole Design for the FCC

The future circular collider or FCC is a study aimed at exploring the possibility to reach 100 TeV total collision energy that would require 16 T dipoles. Upon the conclusion of the high luminosity upgrade, the US LHC accelerator upgrade project in collaboration with CERN will have an extensive Nb3Sn magnet fabrication experience. This experience includes robust Nb3Sn conductor and insulation scheming, 2-layer cos 2θ coil fabrication, and bladder-and-key structure and assembly. By making improvements and modification to existing technology, the feasibility of a two-layer 16 T dipole is investigated. Preliminary designs indicate that fields up to 16.6 T are feasible with conductor grading while satisfying the HE-LHC and FCC specifications. Key challenges include accommodating high-aspect ratio conductor, narrow wedge design, Nb3Sn conductor grading, and especially quench protection of a 16 T device.

Superconducting Magnets for High Performance ECR Ion Sources

Modern heavy ion accelerators require intense heavy ion beams with high charge state. Electron cyclotron resonance (ECR) sources are the primary tool for generating such beams, making them indispensable devices for heavy ion physics research. Remarkable performance advancements were obtained during the last decades. At present, state of the art “third generation” ECR ion sources are based on magnets built with NbTi conductor. However, future nuclear physics facilities will require significant improvement in beam intensity and quality, which can only be met by ECR sources operating at higher frequency and field. These fourth generation machines will need to incorporate a sophisticated sextupole magnet built with high performance Nb3Sn or NbTi conductor. In this paper, we review the technical challenges and worldwide progress in developing high performance superconducting ECR ion sources, and discuss the development of both third and fourth generation ECR ion source magnets at IMP.

Overview of the Design Status of the Superconducting Magnet System of the CFETR

The concept design of the China Fusion Engineering Test Reactor (CFETR) was started in 2012 in two stages. For CFETR-Phase I, the major radius is 5.7 m, the minor radius is 1.6 m, and the magnetic field at the plasma region, BT , is 4–5 T. There were 16 toroidal field coils and six central solenoid coils designed by a Nb3Sn cable-in-conduit conductor (CICC) with the maximum operation current of 64 and 50 kA, respectively. Three types of plasma equilibrium configuration (ITER-like single null, super-X, and snowflake) were designed. The maximum flux provided by the central solenoid is set at 180 V⋅s. However, to get much higher operation parameters such as steady-state operation, particle and heat exhaust, disruption mitigation and avoidance, ELM control, and material damage by high heat flux and neutron, the superconducting magnet system of CFETR-phase II has been updated based on the larger machine with R = 6.7 m−7.0 m, a = 2.0 m, and BT = 6.5–7 T. Such a new design makes over 1-GW fusion power and better plasma performance possible. Hybrid TF coils are designed by a high-performance Nb3Sn conductor, since the maximum magnetic field can reach to 14–15 T. Besides, in order to save the space for the blanket system and get higher flux, a high-temperature superconducting Bi-2212 magnet with better current-carrying performance under high field is supposed to be employed for the CS coils of CFETR-phase II. The Bi-2212 CICC sample has been tested at 4.2 K with a critical current of 26.6 kA under its self-field.

Design and Construction of the Full-Length Prototype of the 11-T Dipole Magnet for the High Luminosity LHC Project at CERN

The luminosity upgrade of the Large Hadron Collider (LHC) at CERN requires the installation of additional collimators in the dispersion suppressor regions of the accelerator. The upgrade foresees the installation of one additional collimator on either side of interaction point 7 (IP7) at the location of the existing main dipoles (MBs) that will be replaced by shorter and more powerful dipoles, and of one additional collimator on either side of IP2 at the location of existing empty cryostats. This paper describes the design and the construction status of the full-length prototype of the 11-T dipole magnet, which is needed for IP7. This magnet features a two-in-one structure, like the LHC MB, impregnated coils made of Nb3Sn conductor, an inner bore of 60 mm, and a magnetic length of about 5.3 m. Two 11-T magnets are needed to replace a 15-m long MB. A by-pass cryostat placed in between the two magnets allows creating a room temperature space for the additional collimators. The magnet is designed to provide the same integrated field as the MB at nominal field. However, due to the difference in transfer function at lower field, a correction by means of a trim current has been considered. A full-length prototype is currently under construction at CERN with the goal of developing the manufacturing and inspection procedures prior to launch the series production. For this, new tooling has been developed and optimized during the fabrication of fully representative practice coils. This paper describes the design of the magnet, the main manufacturing steps, and corresponding quality indicators, which will be used to monitor the series production. Finally, the production and installation schedule will be presented.

Mechanical Design of a Nb3Sn Superconducting Magnet System for a 45 GHz ECR Ion Source

Lawrence Berkeley National Laboratory in collaboration with the Institute of Modern Physics, Lanzhou, China, has developed a Nb3Sn superconducting magnet system for a fourth-generation electron cyclotron resonance (ECR) ion source operating at the microwave frequency of 45 GHz. This paper presents a mechanical design capable of supporting the magnet up to the required operational level, resulting in peak coil fields in the order of 12 T, without exceeding the stress limits of any of the components—particularly, the brittle Nb3Sn conductor. The coil system, based on the sextupole-in-solenoid configuration, is supported by a shell-based structure that uses an aluminum cylinder pretensioned using Bladder & Key Technology (with water-pressurized bladders) during the magnet assembly. High thermal contraction of the shell attains the target preload level at cryogenic temperatures during operation without overstressing the coils during roomtemperature assembly. We present the optimization progression of the support structure, describe its main components and assembly procedure, and examine the predicted coil stress at each step of the magnet assembly and operation using the three-dimensional finite-element mechanical model.

Improvement of stability of Nb3Sn superconductors by introducing high specific heat substances

High-Jc Nb3Sn conductors have low stability against perturbations, which accounts for the slow training rates of high-field Nb3Sn magnets. While it is known that adding substances with high specific heat (C) into Nb3Sn wires can increase their overall specific heat and thus improve their stability, there has not been a practical method that is compatible with the fabrication of long-length conductors. In this work, we put forward a scheme to introduce such substances to distributed-barrier Nb3Sn wires, which adds minimum difficulty to the wire manufacturing process. Multifilamentary wires using a mixture of Cu and high-C Gd2O3 powders have been successfully fabricated along this line. Measurements showed that addition of Gd2O3 had no negative effects on residual resitivity ratio or non-Cu Jc, and that flux jumps were remarkably reduced, and minimum quench energy values at 4.2 K, 14 T were increased by a factor of three, indicating that stability was significantly improved. We also discussed the influences of the positioning of high-C substances and their thermal diffusivity on their effectiveness in reducing the superconductor temperature rise against perturbations. Based on these results, we proposed an optimized conductor architecture to maximize the effectiveness of this approach.

Rotation analysis on large complex superconducting cables based on numerical modeling and experiments

The conductors used in large fusion reactors, e.g. ITER, CFETR and DEMO, are made of cable-in-conduit conductor (CICC) with large diameters up to about 50 mm. The superconducting and copper strands are cabled around a central spiral and then wrapped with stainless-steel tape of 0.1 mm thickness. The cable is then inserted into a jacket under tensile force that increases with the length of insertion. Because the cables are long and with a large diameter, the insertion force could reach values of about 40 kN. The large tensile force could lead to significant rotation forces. This may lead to an increase of the twist pitch, especially for the final one. Understanding the twist pitch variation is very important; in particular, the twist pitch of a cable inside a CICC strongly affects its properties, especially for Nb3Sn conductors. In this paper, a simplified numerical model was used to analyze the cable rotation, including material properties, cabling tension as well as wrap tension. Several rotation experiments with tensile force have been performed to verify the numerical results for CFETR CSMC cables. The results show that the numerical analysis is consistent with the experiments and provides the optimal cabling conditions for large superconducting cables.

Design Strategy and Recent Design Activity on Japan’s DEMO

The Joint Special Design Team for Fusion DEMO was organized in 2015 to enhance Japan’s DEMO design activity and coordinate relevant research and development (R&D) toward DEMO. This paper presents the fundamental concept of DEMO and its key components with main arguments on DEMO design strategy. Superconducting magnet technology on toroidal field coils is based on the ITER scheme where a cable-in-conduit Nb3Sn conductor is inserted in the groove of a radial plate. Development of cryogenic steel with higher strength is a major challenge on the magnet. Divertor study has led to a baseline concept based on water-cooled single-null divertor assuming plasma detachment. Regarding breeding blanket, fundamental design study has been continued with focuses on tritium self-sufficiency, pressure tightness in case of in-box LOCA (loss of coolant accident) and material compatibility. An important finding on tritium permeation to the cooling water is also reported, indicating that the permeation to the cooling water is manageable with existing technology.

Strain dependence of critical superconducting properties of Nb3Sn with different intrinsic strains based on a semi-phenomenological approach

A semi-phenomenological approach, which combined the microscopic properties calculated by first-principles and macroscopic critical characteristics determined from empirical relations, is suggested to investigate the superconducting critical properties of the low temperature superconductor Nb3Sn with different intrinsic strain modes like uniaxial tension, shear and torsion deformations. Firstly, the microscopic properties of the electronic structure and density of state for Nb3Sn are numerically obtained by first-principles calculations using density-functional theory in the generalized gradient approximation. These are further incorporate with the macroscopic empirical relation of the unified scaling law for predicting critical parameters of the strained Nb3Sn superconductor. The superconducting critical profiles of critical temperature, magnetic field and current, in such a way, are achieved for Nb3Sn under different strains. The predictions on the critical parameters of the superconductor bulk in uniaxial tension/compression state exhibit obvious degradations and bell-shaped curves with maximum critical values at zero strain and a slight asymmetry between the tensile and compressive strains, which show quite good agreements with the experimental data. As for Nb3Sn under shear and torsion deformations, the similar degradations on critical parameters also are presented which are monotonously decreased with the applied strains. The first-principles calculations and results in this work are based on an assumption which the superconducting critical properties from the strain-induced variations in the electronic density of states. Furthermore, the modified critical surfaces of Nb3Sn, determined by the critical temperature, current and magnetic field dependence upon the applied different strains are depicted. The present study will be helpful to identify the scaling relation for the critical parameters and understanding the origin of strain sensitivity in Nb3Sn conductors.

Quench Protection Study of the Eurocircol 16 T cosθ Dipole for the Future Circular Collider (FCC)

After Large Hadron Collider will be turned off , a new, more energetic machine will be needed in order to explore unknown regions of the high-energy physics. For this reason, the project Future Circular Collider (FCC) has started, with the goal of developing a 100-km-circumference collider of 50 TeV proton beams. The Eurocircol collaboration is part of the FCC study under the European Community leadership, and it aims to develop a conceptual design of FCC till 2019. One of the main targets is to design a bending dipole able to reach 16 T operation magnetic field, in order to accomplish the size and energy constraints. Such a magnetic field can be reached using Nb3Sn conductors. One option under exploration is the Cost dipole, by INFN of Milano and Genova. Because of the high stored energy and the large current densities due to the conductor performances, quench protection is one of the most challenging aspects of the design. In this paper, the quench protection of the cost design is presented. A standard quench protection study is accompanied by a less conservative study which includes ac effects on the power dissipation inside the coils and on the magnet inductance, in order to not exclude preventively more convenient designs, and to develop a more performing magnet as possible.

Quench Level of the HL-LHC Nb3Sn IR Quadrupoles

The scope of the Large Hadron Collider Hi-Lumi Project at CERN includes the installation of several superconducting magnets wound with Nb 3 Sn Rutherford cables. The quench level of these magnets (i.e. the maximum energy that a cable can tolerate without quenching) is a key value required to set magnet protection from beam losses, and is expected to be significantly different from the computed and measured levels of the LHC NbTi magnets. In this work, we applied a one-dimensional numerical model of multi-strand Rutherford cables to simulate the electro-thermal instabilities caused by the heat released by the particle beam losses. Two models have been applied, one based on the analysis of the single strand, and the other accounting for all the strands in the multi-strand cable. The results of these two models are compared to analyze the effects of heat and current redistribution during quench. A comparison between the quench energy values obtained for the Nb 3 Sn conductor in the working conditions of the LHC Hi-Lumi inner triplet low-β quadrupole (MQXF) and those of the NbTi Rutherford cable of the LHC main quadrupole magnet (MQ) is presented.

Extrapolative Scaling Expression: A Fitting Equation for Extrapolating Full Data Matrixes From Limited Data.

Scaling analysis of several thousand Nb3Sn critical-current measurements is used to derive the extrapolative scaling expression (ESE), a fitting equation that can quickly and accurately extrapolate (or interpolate) limited datasets to obtain full three-dimensional dependences of on magnetic field , temperature , and mechanical strain . Unlike nonextrapolative fitting equations, the ESE relation is determined completely by fundamental raw scaling data from master pinning-force curves. The results show that extrapolation errors with ESE approach typical measurement errors. The scaling expression is simple and robust, providing straightforward extrapolation capability for conductor characterization and magnet design. The ESE relation offers the prospect for extrapolations in several new areas, including: a reduction in the measurement space required for full characterization to about one fifth the size; ability to combine data from separate temperature and strain apparatuses; extrapolation of transport data from above 4 K to lower temperatures, where heating effects and instabilities are problematic for transport measurements; and extrapolation of full datasets from as little as a single curve when several core parameters have been measured in similar conductors (particularly applicable to qualifying production wires). Accuracies are evaluated for concatenations of these different extrapolation capabilities. Examples are given for practical Nb3Sn conductors, including those for high luminosity-LHC magnets, ITER, and cryo-cooled NMR magnets.

EuroCirCol 16 T Block-Coils Dipole Option for the Future Circular Collider

EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. In the frame of the high field accelerator magnet design work package of this study, three different layouts for double-aperture dipole magnets made of Nb 3 Sn conductors and providing a field of 16 T in a 50-mm aperture are being considered: block-coils, common-coils, and cosine-theta. All options are being explored and will be compared based on the same assumptions, in particular with regards to conductor performance, operating temperature and margin. This paper details the block-option presently under development at CEA. After an exploratory phase of various block-coils possibilities (cable dimensions, number of layers, cable grading) the design converged to a four layer magnet. An internal grading-two different geometries of cables are used in the coil-driven by a conductor saving is required. An electromagnetic analysis of the magnet cross-section in a 2-in-1 configuration is performed, coupled to a protection investigation. Preliminary results of a mechanical study on a bladder-and-key single aperture magnet are also reported.

New design of inter-layer splice joint for the TF coils of the DEMO- EUROfusion tokamak

Since the year 2013, the Swiss Plasma Center (SPC) has proposed a Toroidal Field (TF) layout for the DEMO- EUROfusion tokamak, based on a graded winding pack made of layers of Nb3Sn (react-and-wind) conductors. In summer 2015, a new reference baseline is issued for the DEMO- EUROfusion tokamak, leading to an update of the TF coil requirements, e.g. the operating current has been reduced from 80 kA to 63kA. Consequently, the conductor layouts for every graded layer of the TF coil winding pack is re-designed in order to match the new requirements.Each layer of TF coil winding pack has to be connected electrically in series to form the coil. The inter-layer Nb3Sn splice joint design which does not exceed the conductor dimensions is proposed in this paper for the updated 63kA Nb3Sn TF conductor. This proposed joint design allows a continuous winding of TF coil winding pack from layer to layer, housing the joint at the zone of inter-layer transition. Ultimately, the all inter-layer joints should be arranged within the winding pack at the low-field and low mechanically stressed region of D-shaped TF coil.

Central solenoid winding pack design for DEMO

The present study aims to reduce the outer radius of the central solenoid (CS) with respect to its nominal size specified by EUROfusion for a maintained CS magnetic flux. A reduced outer CS radius would allow the reduction of the overall size and cost of the DEMO magnet system. The proposed outline design of the winding pack for the CS1 module is based on layer winding. To achieve the same magnetic flux in a CS coil of significantly reduced outer radius the peak magnetic field at the CS conductors needs to be substantially increased. The use of high-temperature superconductors is therefore envisaged in the highest field sections of the CS coil. It is planned to use react & wind Nb3Sn conductors for intermediate field sections and NbTi at the lowest fields. In order to make a most economic use of the superconductors the proposed winding pack design considers a superconductor grading.

Analysis of the DC performance of the ITER CSI coil using the 4C code

The DC performance of the ITER Central Solenoid Insert (CSI) coil, a single layer solenoid wound using the same Nb3Sn conductor that will be adopted for the 3L module of ITER CS, was measured during the 2015 test campaign in different magnetic field and current operating conditions, before and after electromagnetic and thermal cycles, as well as before and after quench tests. The 4C thermal-hydraulic code is applied here to the analysis of the CSI performance: first, the free parameters of the model are calibrated; then, the model is validated against measurements not used for its calibration. The model is then used to compute the current sharing temperature, to be compared with the measured jacket temperature, and to assess the performance after quench tests.

Conductor Specification and Validation for High-Luminosity LHC Quadrupole Magnets

Author(s): Cooley, LD; Ghosh, AK; Dietderich, DR; Pong, I | Abstract: The high-luminosity upgrade of the large hadron collider (HL-LHC) at CERN will replace the main ring inner triplet quadrupoles, identified by the acronym MQXF, adjacent to the main ring intersection regions. For the past decade, the U.S. LHC Accelerator RaD Program, LARP, has been evaluating conductors for the MQXFA prototypes, which are the outer magnets of the triplet. Recently, the requirements for MQXF magnets and cables have been published in [P. Ferracin et al., IEEE Trans. Appl. Supercond., vol. 26, no. 4, Jun. 2016, Art. no. 4000207], along with the final specification for Ti-alloyed Nb3Sn conductor determined jointly by CERN and LARP. This paper describes the rationale beneath the 0.85-mm-diameter strand's chief parameters, which are 108 or more subelements, a copper fraction not less than 52.4%, strand critical current at 4.22 K not less than 631 A at 12 T and 331 A at 15 T, and residual resistance ratio of not less than 150. This paper also compares the performance for ∼100 km production lots of the five most recent LARP conductors to the first 163 km of strand made according to theHL-LHCspecification.Two factors emerge as significant for optimizing performance and minimizing risk: a modest increase of the subelement diameter from 50 to 55 μm, and a Nb:Sn molar ratio of 3.6 instead of 3.4. The statistics acquired so far give confidence that the present conductor can balance competing demands in production for the HL-LHC project.

Semi-analytical modeling of the coupled strain and low-temperature dependence of the normal-state resistivity in Nb3Sn

A semi-analytical modeling framework on the microscopic basis is proposed in this paper to predict the low-temperature transport properties of strained Nb3Sn superconductors. The theoretical predictions agree well with experimental observations, which indicate that the competitions between the strain state-dependent variations in the phonon spectrum and the electron density of states (DOS) are an important consideration in interpreting the coupled low temperature-strain sensitivity of resistivity in superconducting Nb3Sn. The model is helpful for identifying the scaling law describing the anomalies in the strain dependence of superconducting critical properties of Nb3Sn conductors.

A multiple-field coupled resistive transition model for superconducting Nb3Sn

A study on the superconducting transition width as functions of the applied magnetic field and strain is performed in superconducting Nb3Sn. A quantitative, yet universal phenomenological resistivity model is proposed. The numerical simulation by the proposed model shows predicted resistive transition characteristics under variable magnetic fields and strain, which in good agreement with the experimental observations. Furthermore, a temperature-modulated magnetoresistance transition behavior in filamentary Nb3Sn conductors can also be well described by the given model. The multiple-field coupled resistive transition model is helpful for making objective determinations of the high-dimensional critical surface of Nb3Sn in the multi-parameter space, offering some preliminary information about the basic vortex-pinning mechanisms, and guiding the design of the quench protection system of Nb3Sn superconducting magnets.

Analysis of the cooldown of the ITER central solenoid model coil and insert coil

A series of superconducting insert coils (ICs) made of different materials has been tested since 2000 at JAEA Naka in the bore of the central solenoid model coil at fields up to 13 T and currents up to several tens of kA, fully representative of the ITER operating conditions. Here we focus on the 2015 test of the presently last IC of the series, the central solenoid (CS) insert coil, which was aimed at confirming the performance and properties of the Nb3Sn conductor, manufactured in Japan and used to wind the ITER CS modules in the US. As typical for these large scale applications, the cooldown (CD) from ambient to supercritical He temperature may take a long time, of the order of several weeks, so that it should be useful, also in the perspective of future IC tests, to optimize it. To that purpose, a comprehensive CD model implemented in the 4C code is developed and presented in this paper. The model is validated against the experimental data of an actual CD scenario, showing a very good agreement between simulation and measurements, from 300 to 4.5 K. The maximum temperature difference across the coil, which can only be roughly estimated from the measurements, is then extracted from the results of the simulation and shown to be much larger than the maximum value of 50 K, prescribed on the basis of the allowable thermal stress on the materials. An optimized CD scenario is finally designed using the model for the initial phase of the CD between 300 and 80 K, which allows reducing the needed time by ∼20%, while still satisfying the major constraints. Recommendations are also given for a better location/choice of the thermometers to be used for the monitoring of the maximum temperature difference across the coil.