Département des accélérateurs, de cryogénie et de magnétisme

Development of high-density critical current wires from Nb3Sn

The development is articulated in two phases. The first phase concerned the development of a Nb3Sn wire meeting the specifications of the ITER model coils so as to allow Alstom/MSA to catch up on ground lost to its competitors, in particular American ones, both in terms of performance and in terms of yield and production costs. This objective was met in 1999 with a wire achieving a critical current density of 750 A/mm2 at 4.2 K and 12 T. To remain competitive at world level, Alstom/MSA is now developing, in a second phase, a 2000 A/mm2 wire at the same temperature and magnetic field. This second phase is also meant as an intermediate step towards the even more ambitious goal of 1500 A/mm2 at 4.2 K and 15 T (corresponding to 3000 A/mm2 at 4.2 K and 12 T) set up by the EU-funded CARE/NED Joint Research Activity.  

Fabrication of a quadrupole model

The Nb3Sn compound has a critical temperature and field that are almost double those of NbTi. However, it does have the disadvantages of being brittle and having critical parameters – such as current density - which are sensitive to deformations or stresses. By comparison therefore with NbTi, design and manufacture of the coils have to be completely rethought so as to limit risk of degradation. Since 1995, SACM has undertaken the design, construction and cold testing programme of a quadrupole magnet model based on the design of the NbTi quadrupole magnets of the Large Hadron Collider developed by SACM for CERN. The bore is 56 mm and nominal field gradient is 223 T/m for a current of 11,870 A. The model magnet coils will be wound from the phase 1 cable developed by Alstom/MSA. The last three years have been devoted to the finalisation of the design and the procurement of the main components and tools needed for the quadrupole model. All cable lengths have been delivered in February 2004. Winding of the first pole is set to start in August 2004 with cold testing of the magnet foreseen at the end of 2005.  

Developing ceramic insulation

The main objective of the development of ceramic-type insulation is the study and production of an electrical insulation able to withstand the high temperature (600 to 700°C) heat treatment needed to react niobium-tin. The conductors are wrapped in glass fibre tape with a protective coating of precursors that react during the high temperature heat cycle to provide insulation and ensure that the coil has the correct mechanical characteristics. At the same time the niobium-tin compound is formed. In this way, two tricky stages of manufacture are eliminated: handling of the coil after heat treatment to take it out of its mould at a stage when it is very fragile; and then the vacuum impregnation stage using an epoxy resin. This development will unlock one of the doors to the technology of Nb3Sn compact coil windings by simplifying their scale of manufacture and making it more accessible to industrial processes.  

Since January 2001, development has continued as part of research work co-financed by CEA-Région Languedoc Roussillon and the Laboratoire des céramiques et des composants avancés (CEA, le Ripault) and the Institut européen des membranes (CNRS, Montpellier). The main achievements are: optimization of components and of rheological parameters of the solution of precursors, and design and set-up of the impregnation bench for the glass fibre tapes. The first stages of insulation manufacture, that is tape impregnation and wrapping of superconducting cable, have been completed. The first electrical tests have shown that the presence of the insulator during the thermal reaction did not modify the properties of the copper included in the superconducting wires making up the cable. Electrical and mechanical characterizations are ongoing. 

Two patents concerning “fabrication procedure for an electrically insulating and mechanically structuring jacket for an electric conductor” have been applied for by the CEA in July 2001 and in May 2003.