Six contracts have been placed with industrial companies for the production of 1200 tons of the superconducting (SC) cables needed for the main dipoles and quadrupoles of the Large Hadron Collider (LHC). In addition, two contracts have been placed for the supply of 470 tons of NbTi and 26 tons of Nb sheets. The main characteristic of the specification is that it is functional. This means that the physical, mechanical and electrical properties of strands and cables are specified without defining the manufacturing processes. Facilities for the high precision measurements of the wire and cable properties have been implemented at CERN, such as strand and cable critical current, copper to superconductor ratio, interstrand resistance, magnetization, RRR at 4.2 K and 1.9 K. The production has started showing that the highly demanding specifications can be fulfilled. This paper reviews the organization of the contracts, the test facilities installed at CERN, the various types of measurements and the results of the main physical properties obtained on the first batches. The status of the deliveries is presented.

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Status of the LHC superconducting cable mass production

In December 2009 during its first cold test, LQS01, the first Long Nb3Sn Quadrupole made by LARP (LHC Accelerator Research Program, a collaboration of BNL, FNAL, LBNL and SLAC), reached its target field gradient of 200 T/m. This target was set in 2005 by the US Department of Energy, CERN and LARP, as a significant milestone toward the development of Nb3Sn quadrupoles for possible use in LHC luminosity upgrades. LQS01 is a 90 mm aperture, 3.7 m long quadrupole using Nb3Sn coils. The coil layout is equal to the layout used in the LARP Technological Quadrupoles (TQC and TQS models). Pre-stress and support are provided by a segmented aluminum shell pre-loaded using bladders and keys, similarly to the TQS models. After the first test the magnet was disassembled, reassembled with an optimized pre-stress, and reached 222 T/m at 4.5 K. In this paper we present the results of both tests and the next steps of the Long Quadrupole R&D.

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Test Results of the First 3.7 m Long Nb3Sn Quadrupole by LARP and Future Plans

A design study is presented for the coil layout of the EUCARD-2 Five Tesla HTS Research (FeaTHeR) magnet The angular dependence of the critical current in the used ReBCO Roebel cable is taken into account This leads to a new coil layout named aligned block This layout makes optimal use of the anisotropy of the ReBCO coated conductor, by aligning all tapes with the magnetic field lines Both two-dimensional cross sections and three-dimensional coil layouts are presented In the layouts the magnetic field angle is highest at the edges of the cable causing a large variation of the critical current over its width Different approaches to the calculation of the critical current, with and without current sharing in and between the tapes, are presented The values are compared to the values found using a non-linear network model of the cable, in which the electrical properties of the elements are calculated as a function of magnetic field and magnetic field angle The model also includes electrical contact between the strands using additional network elements

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Study of a 5 T Research Dipole Insert-Magnet Using an Anisotropic ReBCO Roebel Cable

In the quest for high-current high-temperature superconducting (HTS) cables suitable for application to high-field magnets, the Roebel cable made from (RE)-Ba2Cu3O7−δ (RE for rare earth: Y, Sm, Gd, Dy or a mixture of them) coated conductors is identified as meeting the requirements for high-current capability, compactness, transposition and good mechanical properties. In accelerator high-field magnets, Roebel cables will be operated in liquid helium at 4.2 K or lower temperatures. Previous papers have reported on the electrical characterization of Roebel cables at 77 K, but measurements at 4.2 K have not been published yet. This paper summarizes the results of the critical current measurements performed at CERN on (RE)-Ba2Cu3O7−δ Roebel cables at 4.2 K and in external fields of up to 9.6 T.

https://iopscience.iop.org/article/10.1088/0953-2048/26/6/065014/pdf

Electrical characterization of REBCO Roebel cables

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb3Sn 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 Nb3Sn 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 Nb3Sn 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 Nb3Sn sub-elements of the loaded specimen.

Irreversible degradation of Nb3Sn Rutherford cables due to transverse compressive stress at room temperature

A new test facility (FRESCA-Facility, reception of superconducting cables) is under construction at CERN to measure the electrical properties of the LHC superconducting cables. Its main features are: independently cooled background magnet, test currents up to 32 kA, temperature between 1.8 and 4.5 K, long measurement length of 60 cm, field perpendicular or parallel to the cable face, measurement of the current distribution between the strands. The facility consists of an outer cryostat containing a superconducting NbTi dipole magnet with a bore of 56 mm and a maximum operating field of 9.5 T. The magnet current is supplied by an external 16 kA power supply and fed into the cryostat using self-cooled leads. The lower bath of the cryostat, separated by means of a so called lambda-plate from the upper bath, can be cooled down to 1.9 K using a subcooled superfluid refrigeration system. Within the outer cryostat, an inner cryostat is installed containing the sample insert. This approach makes it possible to change samples while keeping the background magnet cold, and thus decreasing the helium consumption and cool-down time of the samples. The lower bath of the inner cryostat, containing the sample holder with two superconducting cable samples, can as well be cooled down to 1.9 K. The samples can be rotated while remaining at liquid helium temperature, enabling measurements with the background field perpendicular or parallel to the broad face of the cable. Several arrays of Hall probes are installed next to the samples in order to estimate possible current imbalances between the strands of the cables.

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A. Knezovic

Papers

1.9 K test facility for the reception of the superconducting cables for the LHC