In-Field Electrical Resistance at 4.2 K of REBCO Splices

Low resistance electrical splices between rare-earth barium copper oxide (REBCO) coated conductors are key technology for high temperature superconductor magnets. A splice must satisfy two main requirements: 1) reproducible low electrical resistance; and 2) high mechanical strength. Systematic measurements of the splices are required because the internal interface resistances of the REBCO-coated conductor (e.g., REBCO/Ag interface) are difficult to be predicted. In this paper, the electrical resistance of lap joints made from REBCO tapes produced by different manufacturers has been experimentally investigated at 4.2 K and in fields of up to 12.4 T. The same sample splices were also measured at 77 K and in self-field in order to define a lift factor correlating joints resistances at 77 and 4.2 K. As expected, a strong effect on the conductor orientation in the splice was found

Exploration of Two Layer Nb3Sn Designs of the Future Circular Collider Main Quadrupoles

The goal of this study is to propose an alternative FCC quadrupole design where the risk from both their fabrication and their operation in the machine is reduced compared to previous analysis. Therefore, the number of coil layers has been reduced from four to two and the load-line margin has been increased from 14% to 20% compared to previous investigations ('Design of a Nb3Sn 400 T/m quadrupole for the future circular collider,' IEEE Trans. Appl. Supercond., vol. 28, no. 3, p. 4004905, Apr. 2018). Indeed, the idea is to only challenge the ∼5000 FCC main dipoles and stay at a relatively low complexity for the ∼700 FCC main quadrupoles so they have a limiting impact on the machine operation and reliability. An exploration of the strand diameter (0.7-0.9 mm), the cable size (40-60 strands), as well as the protection delay (30-40 ms) is performed on two-dimensional (2-D) magnetic designs of the FCC main quadrupole. A discussion on cable windability allows for the selection of one design generating 367 T/m. The design is mechanically constrained with a conventional collar structure leading to collaring peak stress of 115 MPa. A single coupling-loss-induced quench unit ensures a safe magnet operation with a 300 K hotspot temperature.acceptedVersionPeer reviewe

Optimization of Nb3_{3}Sn Rutherford Cables Geometry for the High Luminosity LHC

The quadrupole and dipole magnets for the LHC High Luminosity (HL-LHC) upgrade will be based on Nb$_{3}$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$_{3}$Sn cables to achieve the nominal field with sufficient margin. The strong plastic deformation of unreacted $Nb_3Sn$ strands during the Rutherford cabling process may induce non negligible $I_c$ and RRR degradation. In this paper, the cabling degradation is investigated as a function of the cable geometry for both PIT and RRP conductors. Based on this analysis, new baseline geometries for both 11 T and QXF magnets of HL-LHC are proposed

Design, Performance and Cabling Analysis of Nb3Sn Wires for the FCC Study

The hadron collider proposed by the Future Circular Collider (FCC) study would require high-field superconducting magnets capable of producing a dipole field of around 16 T in a 50 mm aperture. To develop a suitable conductor for these magnets, CERN is coordinating a conductor development programme aiming to obtain Nb$_3$Sn wire with a non-copper critical current density of 1500 A mm$^{-2}$ at 16 T and 4.2 K, in lengths suitable for manufacturing 14 m long magnets, and able to withstand cabling without significant degradation. Here we report the superconducting characterisation and quantitative microscopy of recently-developed Nb$_3$Sn wires with novel layouts and compositions, and evaluate their suitability for Rutherford cabling based on cabling trials and rolling studies. An analysis of the influence of wire layout, materials and mechanical characteristics on cabling performance is presented, to support recommendations for future wire designs

Chapter 6A: Cold powering of the superconducting circuits

For the HL-LHC project, a novel concept for the cold powering of superconducting magnets has been developed. It is based on a new type of superconducting lines (hereafter referred to as Superconducting (SC) Links) that have been developed to transfer the current to the new HL-LHC insertion region magnets from remote distances. Power converters and current leads will in fact be located in the new underground areas (UR) excavated for the HL-LHC (technical galleries running aside the LHC tunnel), and the SC Links will provide the electrical connection between the current leads and the magnets – the latter being located in the LHC main tunnel. Each SC Link has a length of more than 100 m and transfers a total current of up to about |120| kA

Assembly and Test Results of the RMM1a,b Magnet, a CERN Technology Demonstrator Towards Nb3Sn Ultimate Performance

As part of the High Field Magnet technology development carried out at CERN, demonstrators are under construction to explore the full potential of Nb3Sn. The Racetrack Model Magnet (RMM) is one of them, building upon the successful Enhanced Racetrack Model Coil (eRMC) eRMC1a magnet which reached 16.5 T peak field or 16.3 T bore field at 1.9 K. The RMM1a,b magnet is composed of the same two previously tested eRMC racetrack-type coils and an additional middle RMM coil. This central racetrack-type coil features a closed cavity with a diameter of 50 mm and a total length of 526 mm. The coil pack is assembled in the same shell-based support structure as eRMC1a, using bladders and keys to allow for a precise control of the preload with minimal spring back and conductor overstress. The magnet was preloaded with a conservative approach limiting the equivalent peak stress in the coil blocks below 150 MPa. It was then successfully tested up to 16.7 T peak field or 16.5 T bore field at 1.9 K. This paper describes the assembly of the RMM1a,b magnet, relying on Finite Element Analysis (FEA) and mechanical instrumentation, as well as the powering test results at 4.2 K and 1.9 K.As part of the High Field Magnet technology development carried out at CERN, demonstrators are under construction to explore the full potential of Nb 3 Sn. The Racetrack Model Magnet (RMM) is one of them, building upon the successful Enhanced Racetrack Model Coil (eRMC) eRMC1a magnet which reached 16.5 T peak field or 16.3 T bore field at 1.9 K. The RMM1a,b magnet is composed of the same two previously tested eRMC racetrack-type coils and an additional middle RMM coil. This central racetrack-type coil features a closed cavity with a diameter of 50 mm and a total length of 526 mm. The coil pack is assembled in the same shell-based support structure as eRMC1a, using bladders and keys to allow for a precise control of the preload with minimal spring back and conductor overstress. The magnet was preloaded with a conservative approach limiting the equivalent peak stress in the coil blocks below 150 MPa. It was then successfully tested up to 16.7 T peak field or 16.5 T bore field at 1.9 K. This paper describes the assembly of the RMM1a,b magnet, relying on Finite Element Analysis (FEA) and mechanical instrumentation, as well as the powering test results at 4.2 K and 1.9 K

3-D Thermal-Electric Finite Element Model of a Nb₃Sn Coil During a Quench

High field superconducting magnets for particle accelerators often exhibit premature quenches. Once a normal zone is generated within the conductor, the quench may propagate causing temperature and resistive voltage rise along the coil. The resulting thermal gradients can potentially cause new peak stresses that might exceed the tolerable limits, degrading the conductor. The computation of the strain state in the coils during quench then becomes of paramount importance for magnet design, and requires a complete three-dimensional (3-D) analysis of quench phenomena. The objective of this paper is to present the first multiphysics modeling activities towards a new full 3-D methodology for the analysis of magnet mechanics during quench. As a first step, a 3-D thermal-electric finite element model of a Nb₃Sn superconducting coil is developed and explained here. The model uses direct coupled-field elements to solve the system of thermal and electrical equations. A solving algorithm has also been implemented in order to investigate the physics behind quench transients. The output from this model, built in ANSYS APDL, can be easily coupled in a later stage to a mechanical model in order to estimate the strain state in the coil windings. A very good agreement has been observed between the numerical results and experimental tests performed in individual superconducting cables and real superconducting magnets

Irreversible degradation of Nb3_3Sn Rutherford cables due to transverse compressive stress at room temperature

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb$_3$Sn technology are being developed. A pre-stress on the conductor during magnet assembly at room temperature (RT) is needed to counteract the Lorentz forces during operation. The superconducting properties of the brittle Nb$_3$Sn superconductor are strain sensitive and excessive pre-stress leads to an irreversible degradation of the superconductor. In order to determine the level of acceptable pre-stress during the magnet assembly process, reacted and impregnated Nb$_3$Sn cables were exposed to increasing transverse compressive stress up to a maximum stress level of 200 MPa at RT. After each stress cycle, the critical current of the cable specimens were characterized at 4.3 K in the FRESCA cable test station. No significant critical current degradation was observed up to 150 MPa, followed by degradation less than 4% after a nominal stress of 175 MPa. A dramatic permanent critical current degradation occurred after applying a nominal stress of 200 MPa. A comprehensive post analysis consisting of non-destructive micro-tomography followed by microscopic characterization of metallographic cable cross sections was carried out after the critical current test to reveal cracks in the Nb$_3$Sn sub-elements of the loaded specimen

Design Optimization of the Nb3SnNb_3Sn 11 T Dipole for the High Luminosity LHC

Abstract: As a part of the large hadron collider luminosity upgrade (HiLumi-LHC) program, CERN is planning to replace some of the 8.33-T 15-m-long Nb-Ti LHC main dipoles with shorter 11 T Nb$_{3}$Sn magnets providing longitudinal space for additional collimators. Whereas the present design of the 11 T dipole enables the use of RRP conductor with critical current degradation after cabling at the level of 5%, new cross sections of the cable have been studied in order to further decrease the degradation of both critical current and resistivity of the copper matrix. This change is particularly beneficial for the PIT conductor. The coil layout is reoptimized to accommodate the new cable geometry, using the ROXIE code. A set of additional design changes are implemented, such as reduction of the outer yoke diameter. In this paper, we review the main parameters of the present design, describe the changes implemented in the new design, and discuss their impact on both the electromagnetic and structural properties