Sn Conductors Research Articles

Computation of the Strain Induced Critical Current Reduction in the 16 T Nb3Sn Test Facility Dipole

A test facility dipole is being developed at LBNL, targeting a 16 T field in a 144 mm wide aperture. The magnet uses a block design, with two double-pancake coils. In order to minimize motion under the large Lorentz forces, the coils are preloaded against a thick aluminum shell and iron yoke using bladder and key technology. It is then crucial to verify that the performance of the magnet is not degraded due to strain induced on the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor during assembly, cool-down and powering. The critical current of extracted strands was measured in a varying background magnetic field and as a function of the applied longitudinal strain. Finite element analysis was used to extract the strain state inside the superconducting strands during magnet assembly and operation. This strain was then compared to the measurements to evaluate potential reversible and irreversible effects on the magnet performances. The results suggest that the magnet can reach 16 T with sufficient margin, with no irreversible degradation in the high field region.

A Method for Measuring Interface Roughness from Cross-Sectional Micrographs

Uniform co-deformation of Nb, Ta, and their alloys in a Cu matrix is vital for performance yield for most internal-Sn Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors, however, multiple factors can result in instabilities during extrusion and drawing. A better understanding of the variability can be attained by quantifying the interface roughness, however, there has been no consistent method developed to do this. Here we present a method to derive traditional stylus-based roughness measurements, including root-mean-square and average roughness values from micrographs of curved interfaces by image and signal processing analysis. Deformation creates interface roughness with different wavelengths. The longer wavelength or waviness and shorter wavelength or roughness are separated in this method. We apply the technique to 12 different commercial Nb filaments drawn in a Cu matrix. We find good correspondence between the roughness and initial Nb grain size of the rods. The procedure developed has been made available with the open-source code made available to the community. The procedure developed could be used to optimize the wire architecture for next-generation high-field magnets that use Nb alloys.

Recent Progress in Developing Nb3Sn Conductors With Increased Specific Heat

A possible approach to reducing Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnet training is to increase the energy margin of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors by enhancing their specific heat ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> ). We have been developing Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors with increased <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> by incorporating substances with high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> at 2-10 K based on a conductor design that is compatible with standard Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand production. In the past couple of years our efforts have been mainly focused on improving strand design (e.g., position of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> filaments, thickness of the Cu tube for the high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> filaments, ratio of the Cu powder to the high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> substance, filament spacing, etc.) in order to obtain good strand drawability and to reduce degradation after rolling, which is needed for production of Rutherford cables. We also tried a new high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> substance, Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S, and verified that it has much higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> over the whole magnetic field range than the Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> we used before. With that development work we can now produce high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> strands with good drawability and low levels of degradation after rolling. This paper reports our findings and the current status of the development of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors.

Strategy for Developing the EU-DEMO Magnet System in the Concept Design Phase

Fusion power plants offer the prospect of a new sustainable source of energy for future generations. The design and R&D of future reactors is expected to largely benefit from the experience gained in the design, construction and operation of ITER. However, deploying reliable fusion power plants, requires to overcome the design challenges and to address the remaining readiness gaps. Europe is starting the Conceptual Design Phase for building a superconducting DEMOnstration Fusion Power Plant (DEMO) and starting operations around the middle of the century. The aim is demonstrating the production of 500 MW of net electricity, with a closed tritium fuel cycle and adequate plant availability. For the design of the Superconducting Magnet System several variants of coils have been investigated in the pre-conceptual design phase, which has concluded in 2020. Some of them are very close to ITER design, therefore have a relatively high technology readiness level. The “alternative” proposed solutions, that are very promising in terms of costs and performances, need a validation to industrial scale in order to be eligible for the down-selection expected in 2024. In particular, the validation regards the design of the layer-wound graded TF winding pack based on React-and-Wind Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors, and the hybrid, layer-wound graded CS coil. The challenging aspect is that the CS magnet is made of REBCO conductors in high-field, React-and-Wind Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors in medium field and NbTi conductors in low field. Also innovative techniques need to be studied for insulating critical parts of the coils, as penetrations, discontinuities and joints. The main issues of the technological development are illustrated. A R&D work plan is presented for the Conceptual Design phase to validate the technology within 2024 and demonstrate the operational capabilities by testing superconducting insert coils, that is about 50-m long wound conductors, by 2027.

Influence of Zn Addition in Cu Matrix on the Mechanical and Superconducting Properties of Nb3Sn Conductor

New generation accelerators, such as future circular collider (FCC), require superconducting wires with non-copper <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 1500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 16 T and 4.2 K. Kobe Steel Ltd. developed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors with elemental addition to achieve the target <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for FCC. The effect of zinc addition to the copper matrix on the superconducting properties and mechanical strength of the distributed tin (DT) method processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire is studied. In the present study, the <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> -strain characteristics were evaluated using Walters springs (WASP) at 16 T and 4.2 K to investigate the impact of copper (Cu) and brass (Cu-Zn) matrix on further <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> improvement and mechanical robustness.

Results of ITER TF Coil Double Pancake Heat Treatment in Japan

In Japan, 9 ITER TF coils were manufactured for the ITER project. Each TF coil consists of seven Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor double pancakes (DPs). In DP manufacturing, heat treatment is one of the most important processes, because the insertion of a heat treated conductor into the radial plate groove (i.e., transfer) is complicated owing to the elongation of the conductor under heat treatment. A critical current distribution over the DP may also occur owing to the temperature distribution and disturbance in the heat treatment process. To address these issues, this study developed a procedure wherein the conductor length elongation caused by heat treatment is precisely considered, and transfer can be successfully achieved for all the DPs. In addition, strand witness samples were simultaneously heat treated with the DPs, and their critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) was measured. Subsequently, it was confirmed that the average I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is equivalent to those in the strand acceptance test while the standard deviation of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for the witness samples is approximately 4% larger than that for the strand acceptance test. It was also confirmed that the disturbance of the heat treatment was negligible. Related results are described herein.

Start of the ITER Central Solenoid Assembly

The Central Solenoid (CS) is a key element of the ITER Magnet system, including six identical coils, called modules, assembled together to form a 4 m outer diameter, 13 m high solenoid. It is a superconducting magnet, using a 45 kA Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor internally cooled by circulation of supercritical helium at 4.5 K with a peak field up to 13 T. It is enclosed inside a structure providing vertical pre-compression and mechanical support. Procurement of the components and the special assembly tooling of the ITER CS is the responsibility of US ITER, the ITER Domestic Agency of the USA, while the ITER Organization (IO) will carry out the assembly of these components. US ITER has awarded several contracts since 2011 to supply seven modules, including a spare, structure components, and the special tooling required for the CS pre-assembly. All deliveries are organized with the objective to start the CS assembly at IO site by the end 2021. IO has organized special process training and is now starting first phases of the assembly mostly focused on on-site assembly contractor. The paper describes the CS module manufacturing and delivery status, reports the special process final development and training, and assembly status. In particular, the outcomes from first modules factory acceptance tests results will be reported as well as the development and training for special activities to start the first module stacking. The bus bar joint and pre-compression processes with their related tooling qualifications will be detailed.

Cable Design and Development for the High-Temperature Superconductor Cable Test Facility Magnet

A large bore “High-Temperature Superconductor Cable Test Facility Magnet” for testing advanced cables and inserts in high transverse field is in its design phase. This magnet will be the core component of a facility for developing conductors and accelerator magnets operating above 15 T, an enabling technology for next-generation fusion devices using magnetic confinement of plasma and for future energy frontier colliders. The procurement of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor, fabrication of cables, winding of coils, and assembly of the dipole magnet will be done at Lawrence Berkeley National Laboratory (LBNL) and the test pit and cryostat will be constructed at Fermi National Accelerator Laboratory. This article will present the conductor element of the LBNL project, specifically cable design parameters (based on the Bruker OST RRP <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\bigcirc \!\!\!\! {\hbox{R}}}$</tex-math></inline-formula> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting wire) and the development phase cable fabrication experience. Challenges of the cable fabrication will be discussed. The wire and cable planned for this magnet are similar to those under study for the Future Circular Collider and other large facility magnets. The successful fabrication of the development cable has positive implications for these other projects.

Design and Characterization of the Interlayer Joint Between Low-Field Nb3Sn Conductors of a Layer Wound DEMO TF Coil

In the frame of the conceptual design studies for the Toroidal Field (TF) coils of DEMO, a solution based on a layer-wound magnet, rectangular-shaped Cable-in-Conduit conductors and W&R manufacturing approach, is being developed. The feasibility and performance of a large-size Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor operating at about 82 kA in a 13 T field has been proven in the past. Another key technology to be demonstrated for a layer-wound TF coil, is that of a joint between two different conductor grades, to be possibly manufactured in-line during winding. The proposed joint solution would provide the minimum room occupancy, with the joint embedded within the winding pack volume. A joint between two low-field conductor grades, i.e., constituted of a small number of superconducting strands and a high number of stabilization copper wires, has been designed and manufactured. The two conductor lengths are characterized by a different number of superconducting wires and different outer dimensions. A joint sample has been assembled and instrumented, in the configuration allowing testing at the SULTAN facility of the Swiss Plasma Center. Both DC and AC performance of the joint has been characterized at the DEMO TF operating conditions. The present paper reports the main manufacturing steps for the joint and its main test results. The implications on the performance and design approach of the TF coil are also discussed, based on the outcome of such tests.

Development of Coax Compacted Joint Assembly Process for the ITER Central Solenoid

The Central Solenoid (CS) is the core element of the ITER magnet system, contributed as in-kind procurement by the US Domestic Agency. Made up of six modular inter-exchangeable module coils, vertically stacked, forming a 15 m high and 4 m outer diameter solenoid, to be inserted inside the central tokamak core, after assembly of the 18 D-shaped Toroidal Field (TF) coils. Each module uses a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor internally cooled by circulation of supercritical helium at 4.5 K and supplied, for each module, with a 45 kA current by the way of two vertical leads exiting from the module outer radius. The available space between the CS and TF magnets being very limited, the US DA has developed a compact - so called Coax Joint (CJ) - devoted to connect the respective twelve module leads to the feeders located at top and bottom of the CS via dedicated busbar extensions. The CS coax joint assembly procedure as developed by US DA would make use of CS assembly onsite soldering process, to be executed under supervision of the ITER Organization (IO) in the tokamak assembly hall. In order to mitigate risks related to these soldering activities needed at assembly stage, IO has proposed an alternative assembly process based on indium wires compaction - so called Coax Compacted Joint (CCJ)- and initiated its prototype development and qualification in time. The hereby presented CCJ design solution is based on four copper quadrants, each including an embedded straight Rutherford-type superconductor cable-strip to transport the current. The current is transferred from the lead to the quadrants by the way of compacted indium wires. A steel jacket welded around the joint ensures the mechanical support as well as the needed leak tightness. The paper describes developments made by CEA under IO supervision from the conceptual design to the testing of joints in relevant cryogenic and operative conditions.

Powering Performance and Endurance Beyond Design Limits of HL-LHC Low-Beta Quadrupole Model Magnets

For the High Luminosity Upgrade project (HL-LHC) of the CERN Large Hadron Collider (LHC), lower β* quadrupole magnets based on advanced Nb 3 Sn conductors will be installed on each side of the ATLAS and CMS interaction points. To quantify the endurance and technological limits of these magnets, beyond their maximum operational conditions, two short length model magnets have been extensively tested at the CERN SM18 test facility. Both magnets were subjected to eight thermal cycles. One of them was trained beyond its ultimate current (17.89 kA, corresponding to 143 T/m field gradient and 12.2 T peak field), reaching a maximum of 19.57 kA at 1.9 K (corresponding to 155 T/m, 13.4 T peak field and 95.4% of the short sample limit) in a 150 mm diameter bore. This magnet currently has the record highest field gradient of this quadrupole magnet class. The second short model had zero re-training quenches up to nominal (16.47 kA) and ultimate current at 1.9 K during the thermal cycles; more than 1000 current cycles to nominal current; and provoked quenches to simulate the most severe failure scenarios of the protection system. After all these tests, both magnets continue to perform beyond requirements for operating current and temperature. In this paper, the tests performed on the two magnets are discussed.

Development of the Turn Releasing Technology for the Nb3Sn Pancake Coil of CFETR CSMC

A central solenoid model coil (CSMC) is being developed to verify the large-scalar superconducting coil manufacture technology for China Fusion Engineering Test Reactor (CFETR) in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The turn insulation wrapping of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn pancake coil is conducted after the coil heat treatment. The turn releasing technology is necessary to be developed and verified to insure there is no damage to Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor during the turn insulation wrapping of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn pancake coil. This paper introduces the design, analysis and verification for the safe stretch distance, the design and debugging of the special equipment, and the process experiment with the 4 × 4 mockup coil.

Inter-Layer Joint of Nb3Sn React&amp;Wind Cables for Fusion Magnets

The Swiss Plasma Center (SPC) has developed a layout of Toroidal Field (TF) coil for EUROfusion DEMO tokamak, based on the reference tokamak baseline of 2015. Each TF coil winding pack is wound with graded Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors and consists of 12 single layers, connected in series by means of inter-layer graded joints. The design of inter-layer joints takes into account the react-and-wind (R&W) manufacturing technique for fabrication of TF coil winding pack, i.e., the inter-layer joint is prepared with use of two already heat treated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors. The development, preparation and test of inter-layer joint at SPC is performed in frame of R&D program for TF coil of EUROfusion DEMO tokamak. The high-grade Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn TF conductor, operating at 63.3 kA and 12.2 T (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> >6.5 K) was tested at SPC, and afterwards was used for fabrication of inter-layer joint. The developed TF inter-layer joint is an “overlap-type” joint, which can be fit within the dimensions of TF winding pack. Each end of two conductors is copper cladded by a thermal-spraying technique and then bonded together along the surfaces of cladded copper with use of a high-frequency inductor. This paper describes the design of inter-layer joint, process of joint fabrication and test results obtained at SPC at the SULTAN test facility.

Further Heat Treatment Optimizations for Nb3Sn Conductors: From Wires to Cables

Accelerator magnets made with Nb3Sn superconducting coils are currently the most viable candidate technology for increasing the luminosity and energy of particle accelerators like the LHC. Today, the most developed Nb3Sn wire technology is the restacked rod process (RRP) wire manufactured by Bruker. RRP wires normally undergo a reaction heat treatment after coil winding which consists of three stages, one at 210 °C, one at 400 °C, and one at ∼660 °C spending roughly 48 h at each stage. The last stage of this heat treatment activates the solid-state diffusion which forms the Nb3Sn, whereas the first two stages have been historically used to “mix” the Cu and Sn in order to avoid liquefaction of the low melting temperature phases. Recently, the second stage was studied and optimized to improve critical current density in RRP wires by controlling the kinetics triggered when the Cu–Nb–Sn ternary phase (“Nausite”) forms; the first stage however, has a negligible impact on the critical current density of RRP wires, and instead focuses on reducing the probability of Sn leaking from the wires. These instances of Sn leaks are often referred to as “Sn bursts.” In this paper, we explore Sn bursting on Rutherford cables made with RRP wires in an attempt to better understand the threat of Sn bursts for future accelerator magnets using modern RRP wires. It was found that the Sn burst prevention mechanism, facilitated by the 210 °C stage, can be achieved in a fraction of the time, and that the modern RRP wires are less likely to observe Sn bursting. This new knowledge has enabled us to further understand and optimize the heat treatment of RRP wires for future accelerator magnets.

Magnetization and Flux Penetration of YBCO CORC Cable Segments at the Injection Fields of Accelerator Magnets

High-temperature-superconducting (HTS) cables are being considered for use in particle accelerator beam steering magnets. Knowledge of the magnetic properties of these HTS cables is needed for accelerator magnet design. Here, we present the results from a Hall probe magnetometer made for measuring the M-μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H of short sections of superconducting cables in applied magnetic fields up to 12 T. The M-μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H of a CORC cable was measured with field sweeps of ± 4.5 T and also a -2- to 8-T loop. Relevant to a particle accelerator with an injection field of 1 T, the magnetization M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">inj</sub> of the CORC cable was ≅ 900 kA/m and the penetration field B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ≅ 1 T. The magnetization and penetration field of the cables are compared to tape from which the cables were wound, and the results are compared to NbTi and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors.

Preliminary Tests of Soldered and Diffusion-Bonded Splices Between Nb3Sn Rutherford Cables for Graded High-Field Accelerator Magnets

High-field accelerator magnets of the next generation will require coil grading in order to decrease the amount of superconductor and hence the magnet cost. The joints between Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor grades must have low electrical resistance (<;1 nΩ) and reasonably good mechanical strength. The joints are classified as “Wind & React” because they will be assembled during the coil winding, prior to the heat treatment. Various techniques have been initially considered for the preparation of joints between Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables, including soldering, diffusion bonding with and without interleaved copper foil, electroplated cables (to “smooth” the surface of the cable overlap), and crimped cables. Initial investigations included examinations of cross-sections (cut by electro-erosion). In a second stage, joint samples are prepared and assembled for test in the SULTAN test facility, with background field up to 11 T and operating current up to 14 kA. Besides the resistance test results, the selection of the most promising joint layout will also consider the restriction imposed by the coil manufacturing, e.g., available space for assembly, access after heat treatment, applicable quality control, etc.

Russia's Contribution to the ITER TF Magnets

The ITER toroidal field (TF) magnet system consists of 18 D-shaped coils, each of which is formed by five regular double pancakes (rDP) and two side double pancakes (sDP). Manufacturing of conductors for the TF coils, which are Nb 3 Sn-based cable-in-conduit conductors, has been shared among six ITER Domestic Agencies, including the Russian Federation (RF). The RF share includes manufacturing of eighteen 760-m conductors for rDP and eight 415-m conductors for sDP, i.e., almost 4 out of 18 TF coils will be produced from the conductors supplied by RF. In order to fulfil project obligations, facilities for Nb 3 Sn strand, cable, and conductor production had been established, and after successful qualifications, the manufacturing phase began. The manufacturing activities went on for six years, starting with the first strand production in 2009 and finishing with a successful wrap-up of conductor manufacturing and delivery in 2015. In this paper, highlights of the TF conductor production from the superconducting strand manufacturing up to the results of conductor full-size sample tests in the SULTAN facility are presented.

Review of Scientific Results Obtained During Production of ITER TF and PF Conductors in Russia

The Russian Scientific R&D Cable Institute was obliged to produce and to deliver 20% toroidal field (TF) conductors and 20% poloidal field (PF) cables for the ITER magnet system. The task has been completed in October 2015. This work was a great technological challenge in the development of tricky technologies. Development of manufacturing processes was accompanied by extensive scientific studies directed to an improvement of technologies and a search of causes resulting in change of cables and conductors properties during manufacturing and testing. In this paper, we present the extended review and summary of results obtained for Russian superconductors that permitted improving technologies and finding out the reasons of changes of properties of cables and conductors during manufacturing and testing. The study of rotation and untwisting of TF cables during insertions into jackets allowed developing the model describing the rotation process and suggesting the device which keeps untwisting within demanded levels. Analyses of mandatory SULTAN high-field electromagnetic tests of PF and TF conductors from Russia confirmed good quality and adequacy of manufacturing processes. Micrographic studies of Nb 3Sn strands before and after SULTAN tests permitted qualitatively understanding the reason for the stability of RF TF conductor behavior during multiple electromagnetic and thermal cycling. Statistical studies of residual resistance ratio (RRR) of Nb 3Sn and NbTi strands permitted tracking of its degradation during manufacturing processes and SULTAN tests. The optimal heat treatment process for Nb 3Sn conductors has been confirmed after RRR statistical study. The hydraulic performance has been measured in the short samples of TF conductor. By using the parameters obtained, it is possible to predict hydraulic performance of helium flow in a conductor at any temperature. These results led to deeper understanding and to improving manufacturing processes and quality of TF and PF conductors for the ITER magnet system and could be useful for suppliers of superconducting cables and conductors using ITER-like technologies.

Performance Analysis of Mass-Produced Nb3Sn Conductor for Central Solenoid in ITER

The performance of two mass-produced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors for the ITER central solenoids was tested for the first timmass-produced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductorse. One was cut from the forward end of an 80-m-long conductor, and the other was cut from the forward end of a 918-m-long conductor. The fifth-stage twist pitches of these conductors were lengthened by approximately 16% during cable insertion and compaction with conduit. The current sharing temperatures Tcs were measured over 20 000 electromagnetic cycles, including four thermal cycles between 4.2 K and room temperature. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> of the former conductor increased and became almost constant through the cycling. In contrast, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> of the latter conductor not only increased but also decreased slightly against cycling. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> decline rate after 10002 cycles was -4.50 × 10-6 K/cycle. If this rate is assumed to continue after 20000 cycles, Tcs would decrease by -0.27 K over 60000 cycles. Even so, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> is higher than an acceptance criterion of 6.5 K at 60 000 cycles. The ac losses Q of both conductors at a current of 0 kA were almost the same as or slightly lower than Q of a short sample conductor whose fifth-stage twist pitch was not lengthen. On each conductor, Q at 40 kA was approximately 10% higher than that at 0 kA; thus, the effect of the transport current on Q was not large.

Nb3Sn High Field Magnets for the High Luminosity LHC Upgrade Project

The high luminosity upgrade of the Large Hadron Collider at CERN requires a new generation of high field superconducting magnets. High field large aperture quadrupoles (MQXF) are needed for the low-beta triplets close to the ATLAS and CMS detectors, and high field two-in-one dipoles (11-T dipoles) are needed to make room for additional collimation. The MQXF quadrupoles, with a field gradient of 140 T/m in 150 mm aperture, have a peak coil field of 12.1 T at nominal current. The 11-T dipoles, with an aperture of 60 mm, have a peak coil field of 11.6 T at nominal current. Both magnets require Nb 3 Sn conductor and are the first applications of this superconductor to actual accelerator magnets. Collaboration between the US LARP (LHC Accelerator Research Program) and CERN is developing the MQXF magnets, whereas the 11-T dipole magnets are being developed by CERN and Fermilab. This paper reviews the status of Nb 3 Sn technology for accelerator magnets, discusses its main challenges, and discusses how the MQXF and 11-T designs are addressing them.