Generation Of Accelerator Magnets Research Articles

Analysis of the quench propagation along Nb3Sn Rutherford cables with the THELMA code. Part II: Model predictions and comparison with experimental results

To improve the technology of the new generation of accelerator magnets, prototypes are being manufactured and tested in several laboratories. In parallel, many numerical analyses are being carried out to predict the magnets behaviour and interpret the experimental results. This paper focuses on the quench propagation velocity, which is a crucial parameter as regards the energy dissipation along the magnet conductor. The THELMA code, originally developed for cable-in-conduit conductors for fusion magnets, has been used to study such quench propagation. To this purpose, new code modules have been added to describe the Rutherford cable geometry, the material non-linear thermal properties and to describe the thermal conduction problem in transient regime. THELMA can describe the Rutherford cable at the strand level, modelling both the electrical and thermal contact resistances between strands and enabling the analysis of the effects of local hot spots and quench heaters. This paper describes the model application to a sample of Short Model Coil tested at CERN: a comparison is made between the experimental results and the model prediction, showing a good agreement. A comparison is also made with the prediction of the most common analytical models, which give large inaccuracies when dealing with low n-index cables like Nb3Sn cables.

Record current density of 344 A mm−2 at 4.2 K and 17 T in CORC® accelerator magnet cables

One of the biggest challenges in developing conductor on round core (CORC®) magnet cables for use in the next generation of accelerator magnets is raising their engineering current density JE to approach 600 A mm−2 at 20 T, while maintaining their flexibility. One route to increase JE could be to add more RE-Ba2Cu3O7−δ coated conductors to the cable, but this would increase the cable size and reduce its flexibility. The preferred route to higher JE is a reduction in diameter of the CORC® cable, while maintaining the number of tapes wound into the cable. The availability of very thin tapes containing substrates of 30 μm thickness enabled us to wind a 5.1 mm diameter CORC® cable from 50 coated conductors, while maintaining a tape critical current Ic of about 97% after cabling. The cable Ic was 7030 A at 4.2 K in a background field of 17 T, corresponding to a JE of 344 A mm−2, which is the highest performance of any CORC® cable so far. The magnetic field dependence allowed us to extrapolate the cable performance to 20 T to predict an Ic of 5654 A and a JE of 309 A mm−2. The results clearly show that rapid progress is being made on overcoming the JE hurdle for use of CORC® cables in the next generation of accelerator magnets. Further optimization of the cable layout will likely increase JE towards 600 A mm−2 at 20 T in the near future, while further reduction in cable size will also make them even more flexible.

Engineering current density in excess of 200 A mm−2 at 20 T in CORC® magnet cables containing RE-Ba2Cu3O7−δ tapes with 38 μm thick substrates

Conductor on round core (CORC®) cables wound from RE-Ba2Cu3O7−δ coated conductors are currently being developed for the next generation of accelerator magnets because of their high flexibility and potential for high engineering current densities JE. CORC® cables previously reached JE of 114 A mm−2 at 4.2 K and 20 T in a 7.5 mm diameter cable. Accelerator magnets require a current density of at least 300 A mm−2 and a cable-bending diameter as small as 40 mm, which has so far not been possible with superconducting tapes made on 50 μm thick substrates. CORC® cables made from thinner substrates could have significantly increased JE with greater flexibility as we here demonstrate with a CORC® cable made of tapes with 38 μm thick substrates. A custom cable machine produced higher cable quality and better retention of tape performance compared to previous cables that were wound by hand. The thinner substrate showed an almost twofold increase in engineering current density from 114 A mm−2 to 216.8 A mm−2 at 4.2 K and 20 T, at a reduction in cable diameter from 7.5 mm to 6.0 mm. The results clearly demonstrate that winding CORC® cables from tapes with thinner substrates is a straightforward method for raising their current density and one that shows great promise for use in accelerator magnets.

Microstructural factors important for the development of high critical current density Nb 3Sn strand

Nb 3Sn is the primary candidate for the next generation of accelerator magnets as well as for NMR and other applications that require magnetic fields between 11 and 20 T. Since 1999 the layer critical current density available in long length accelerator quality strand has almost doubled. The microstructural and microchemical factors that are important for high critical current density Nb 3Sn are reviewed. The highest critical current density strands have a Nb 3Sn layer that minimizes chemical and microstructural inhomogeneities and has a high fraction of the layer close to stoichiometric Sn content. Only the internal Sn process has yielded critical current densities beyond 3000 A/mm 2 at 12 T (4.2 K) and only with interfilamentary Cu thicknesses that are too low to separate the filaments after the final reaction heat treatment. The result of the reaction heat treatment is to produce a continuous ring of Nb 3Sn from hundreds of Nb or Nb-alloy filaments and thus a major ongoing challenge of Nb 3Sn conductor design is to reduce the effective filament diameter to acceptable levels for intended applications. Recent successful attempts to reduce the cost of alloying the Nb 3Sn for high field application are also examined and the potential for future improvements discussed.

Perspectives of<tex>$rm Nb_3rm Sn$</tex>in Future Accelerator Magnets

Upgrades presently being planned for high-energy hadron colliders require magnet performance beyond the capabilities of NbTi superconducting wire. Recent improvements in the performance of Nb3 Sn material and in the capability to use it in practical magnets make it the logical choice for the next generation of accelerator magnets. We discuss recent results of R&D programs, some remaining issues and possible solutions. Recent progress in understanding and in technical details of design and assembly have made Nb3Sn magnets easier to fabricate and more reliable in performance

Overview and status of the Next European Dipole Joint Research Activity

The Next European Dipole (NED) Joint Research Activity was launchedon 1 January 2004 to promote the development of high-performanceNb3Sn conductors in collaboration with European industry (aiming at a non-copper critical current densityof 1500 A mm−2 at 4.2 K and 15 T) and to assess the suitability ofNb3Sn technologyto the next generation of accelerator magnets (aiming at an aperture of 88 mm and a conductor peakfield of ∼15 T). It is part of the Coordinated Accelerator Research in Europe (CARE) project, which involveseight collaborators, and is half-funded by the European Union. After briefly recalling theActivity organization, we report the main progress achieved over the last year, which includes:the manufacturing of a double-bath He II cryostat for heat transfer measurements throughNb3Sn conductor insulation, detailed quench computations for various NED-likemagnet configurations, the award of two industrial subcontracts forNb3Sn conductor development, the first results of a cross-calibration programme of test facilities forNb3Sn wire characterization, detailed investigations of the mechanical properties of heavilycold-drawn Cu/Nb/Sn composite wires, and the preliminary assessment of a newinsulation system based on polyimide-sized glass fibre tapes. Last, we briefly reviewthe efforts of an ongoing Working Group on magnet design and optimization.

Status of the Next European Dipole (NED) Activity of the Collaborated Accelerator Research in Europe (CARE) Project

Plans for LHC upgrade and for the final focalization of linear colliders call for large aperture and/or high-performance dipole and quadrupole magnets that may be beyond the reach of conventional NbTi magnet technology. The Next European Dipole (NED) activity was launched on January 1st, 2004 to promote the development of high-performance, Nb/sub 3/Sn wires in collaboration with European industry (aiming at a noncopper critical current density of 1500 A/mm/sup 2/ at 4.2 K and 15 T) and to assess the suitability of Nb/sub 3/Sn technology to the next generation of accelerator magnets (aiming at an aperture of 88 mm and a conductor peak field of 15 T). It is integrated within the Collaborated Accelerator Research in Europe (CARE) project, involves seven collaborators, and is partly funded by the European Union. We present here an overview of the NED activity and we report on the status of the various work packages it encompasses.

Passive correction of the persistent current effect in Nb/sub 3/Sn accelerator magnets

Superconducting accelerator magnets must provide a uniform field during operation. However, the field quality significantly deteriorates due to persistent currents induced in superconducting filaments. This effect is especially large for the Nb/sub 3/Sn conductor being implemented in the next generation of accelerator magnets. A simple and inexpensive method of passive correction of the persistent current effect was developed and experimentally verified. This paper describes numerical simulations of the passive correctors and reports the test results.