A New Superconducting Wire for Future Accelerators

The CARE/NED project has developed a new superconducting wire that can achieve very high currents (1400 amps) at high magnetic fields (12 teslas).

Cross-section of the CARE/NED wire produced by SMI.

As we prepare to enter a new phase of particle physics with the LHC, technological development is a continuous process to ensure the demands of future research are met. The next generation of colliders and upgrades of the present ones will require significantly larger magnetic fields for bending and focusing the particle beams.

NED (Next European Dipole) is one of the projects taking on this challenge to push technology beyond the present limit (see: More about NED). The magnets in the LHC rely on niobium titanium (NbTi) as the superconducting material, with a maximum magnetic field of 8 to 10T (tesla). In order to exceed this limitation, a different material together with the corresponding technology needs to be developed. NED is assessing the suitability of niobium tin (Nb3Sn), which has the potential to at least double the magnetic field achievable with NbTi. Working in collaboration with European industry, the NED partners are primarily focusing on the development of high-performance Nb3Sn cables for use in high-field accelerator magnets. NED's conductor development programme led by CERN aims to deliver superconducting cable of consistently high performance and quality, supplied in unit lengths of several hundred metres.

Meeting NED's ambitious conductor specification brings its own challenges. It calls for a high critical current density within a 1.25 mm diameter wire. Each wire is made up of Nb3Sn filaments, surrounded by a low resistivity copper matrix. The effective filament diameter of Nb3Sn wires is usually significantly larger than the 6 to 7μm diameter achieved in NbTi wires. Reducing the filament diameter while preserving a high current density presents one of the many difficulties in the technological development process. One of NED's goals is to lower the Nb3Sn filament diameter to 50μm. Further challenges arise when it comes to shaping the wires. Once formed, the material becomes brittle, which prohibits subsequent bending.

Under contract with CERN, ShapeMetal Innovation (SMI) - a company in the Netherlands - has successfully developed a NED wire comprising 288 Nb3Sn filaments. This wire achieved a record critical current of ~1400A, at 4.2K and 12T (corresponding to a critical current density of ~2500A/mm2 in the non-copper area of the wire). The high amperage and the fine filament size developed by SMI are unprecedented at this current density level. We may compare this performance with other Nb3Sn wires currently used in two other projects. The International Thermal Experimental Reactor (ITER) uses wires of 0.81mm diameter, but the critical current is seven times lower (200A at 4.2K and 12T). The US-LHC Accelerator Research Program (LARP) uses wires of 0.7 mm diameter, with effective filament diameters of 70 to 80μm, and less than half the critical current (600A at 4.2K and 12T).

Wires developed for the accelerator will also need to sustain the physical process of cabling. To evaluate this ability, the SMI wires were put under deformation tests, which involved rolling under different conditions. They successfully passed the tests and are now undergoing electrical specification tests.

The next step for SMI is to perform cabling tests and to demonstrate its ability to produce the wire in large quantities. NED plans to use these cables and those under development by another company, Alstom/MSA in France, to manufacture short demonstration coils and to eventually produce 1-metre-long dipole magnet models for 2009. If all goes well, the Nb3Sn technology could be implemented in the LHC in 2015, when some of its interaction region magnets will need to be upgraded. The ground-breaking result reported here is a promising step towards the manufacture of NED magnets.

More about NED:

NED is coordinated by CEA (France). The project collaborators include CCLRC/RAL (UK), CERN, CIEMAT (Spain), INFN/Genoa and INFN/Milan (Italy), Twente University (The Netherlands) and Wroclaw University of Technology (Poland).

It has a total budget of 2 million euros, half of which comes from the Coordinated Accelerator Research in Europe (CARE) programme - a large EU funded project that started in 2004, which involves most European universities and laboratories working on accelerator physics and technology.