Leroy, D

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CABLE DESIGN AND R&D FOR NED  D. Leroy CERN, Accelerator Technology Department, Geneva - Switzerland Abstract This paper presents the work package on conductor development as a part of the Next European Dipole (NED) program supported by EU. The results of a dipole preliminary magnetic design to reach 15 T in the bore are summarised From this preliminary design, a strand and cable specification has been derived to place orders in European industries. The various steps foreseen for the strand development are indicated. 1. INTRODUCTION The NED project is one of the Joint Research Activities (JRA) accepted by EU in view of developing in Europe high field accelerator magnets [1]. Six institutes have agreed to collaborate to the NED JRA:   - CCLRC-RAL (United Kingdom)  - CEA/DSM/DAPNIA (France) - CERN/AT (International)  - INFN – Milano/LASA and – Genova (Italy) - University of Twente (The Netherlands)  - University of Wroclaw (Poland).  The Phase I of NED, which is supported by EU is articulated around three main work packages: (1) Thermal Studies and Quench Protection (TSQP), (2) Conductor Development (CD) and (3) Insulation Development and Implementation (IDI).  The CD work package is coordinated by CERN. The core of the CD work package will be devoted to Nb3Sn strand and cable development. It includes the preliminary designs of a large-aperture and high-field (up to 15 T) Nb3Sn dipole magnet in order to derive the Nb3Sn meaningful strand and cable specifications suitable for large field magnet applications. A technical specification will be written and an order will be placed according to the CERN procedures. The high critical current density of 3000A/mm2 at 12 T, 4.2K, represents a new technical challenge for the European superconductor industry, which will also invest in this ambitious program. Within the limited funding, it was then decided to concentrate the Nb3Sn strand fabrication on two routes: the Powder In Tube (PIT) and the Internal Tin Diffusion (ITD). CERN will make the follow-up of the contract. The characterization measurements will be performed in the associated laboratories: - Strand critical current measurements (following a standardized protocol) at CEA/Saclay, INFN-Milano/LASA, University of Twente. - Strand magnetization measurements at INFN-Genova. - Cable critical current measurements at University of Twente.  2162. RESULTS OF THE PRELIMINARY MAGNETIC STUDIES A preliminary design of a large-aperture (up to 88 mm), high-field (up to 15 T), Nb3Sn dipole magnet has been made to derive conductor specifications. The design is based on a two layer cable distribution.  2.1  Magnet Overall Structure  Fig.1.Magnet overall lay-out Fig.1 shows a schematic magnet overall layout. The coil aperture is 44 mm; there is 3 mm space between the two layers. The iron yoke is separated from the coils by an intermediate spacer (collars) of 25 mm thickness and is surrounded by a 28 mm thick stainless steel shrinking cylinder. The inner layer consists of 4 blocks of conductors while the outer is made of 3. The adjacent blocks are separated by a distance of at least 2 mm. The iron yoke is 350 mm thick. 2.2 Results of an impact study An impact study of a few variables on the conductor has confirmed that the main parameter influencing the bore field is the overall current density in the coil. The latter is dominated by the critical current density in the non-copper and the insulation thickness. Both have not been considered as a free parameter because they depend so much on the technology, which will be developed in the frame of the NED program. The insulation thickness of 0.2 mm on each side of the cable represents 17% of the total conductor surface. With a critical current density in the strand non-copper area of 1500 A/mm2 at 15 T, 4.2 K, the bore field as calculated is 14.4 T at 4.2 K for a maximum quenching field on the conductor of 15 T taking into account a cable degradation of 10 %. To reach 15 T in the aperture, the magnet has to be operated at 1.9.K as shown on the load lines represented in fig.2. The pressure on the broad face of the cable, only due to the magnetic forces amounts to 150MPa.   iron yokecollars1223 233315 300 25mm447096111411436mm217Fig.2.Load Lines 2.3 Overall characteristics of a 88 mm, two layer magnet design The number of conductors is 19 for the inner layer and 26 for the outer layer. The 2 layers of coil winding use the same cable of 40 strands having a diameter of 1.25 mm  2.3.1 Strand and Cable Characteristics  Table 1  Strand and Cable characteristics  strand diam. strand number Cu/ non_Cu height width inner width outer insulation Jc_str at 15T, 4.2K Jc_cab at 15T, 4.2K [mm]   [mm] [mm] [mm] [mm] [A/mm2] [A/mm2] 1.25 40 1.25 26 2.175 2.375 0.2 666.67 417.1  2.3.2 Magnet Characteristics Table 2 Magnet Characteristics       Calculations with a cable degradation of 10%. Field [T] / Current [A] L  [mH/m] Wm  [kJ/m] Max pressure [MPa] Fx, result. [MN/m] Overall diameter [mm] 14.42 / 28660  4.4  1810  148  15.8  1004 0500100015002000250030000 2 4 6 8 10 12 14 16 18 20B [T]J_Sc [A/mm2]JcSc(B)_4.2K JcSc(B)_1.9Kblock_4 block_1block_2 block_3block_5 block_6block_7 linear_fit_Jc(B)_4.2K2183. STRAND CHARACTERISTICS Figures 3 and 4 show cross-sections of the strands based on the two foreseen technologies. NED will not make use of a collective barrier in the ITD process.                             Fig. 3  I T D (ALSTOM)                                                       Fig. 4  P I T (SMI) 3.1 Strand Characteristics for NED The strand specification for NED will be based on a critical current density of 1500 A/mm2 at 15 T, 4.2 K and 3000 A/mm2 at 12 T, 4.2 K The effective filament size is smaller or equal to 50 µm. The strand diameter is determined by the maximum cabling capacity of 40 strands.  Table 3.Strand Characteristics Strand diameter  1.25 mm Effective Filament diameter according to billet design < 50.0 µm Copper to non-copper volume ratio  1.25 ± 0.10 Filament twist pitch after cabling 30 mm Filament twist direction right-handed screw Minimum critical current at 4.222 K 1636 A at 12 T 818 A at 15 T RRR          (after full reaction) > 200 n-value @ 15 T and 4.2 K > 30  2194.  CABLE CHARACTERISTICS(PRELIMINARY) Table 4 indicates the characteristics of the cable. The dimensions are preliminary and will be decided after cabling tests and measurements of the current degradation.   Table 4. Cable Characteristics Cable width 26 mm Cable mid-thickness at 50 MPa 2.275mm Keystone angle 0.22 degrees Cable thin edge thickness 2.175 mm Cable thick edge thickness 2.375 mm Cable transposition pitch ~180 mm Number of superconducting strands 40 Critical current at 4.222 K, with field normal to broad face 29440 A at 15 T 58880 A at 12 T Minimum critical current at 4.222 K of extracted strand 736 A at 15 T 1472 A at 12 T n-value @ 15 T and 4.222 K > 20 Residual resistance ratio before reaction                                         after reaction  70  120 Cable transposition direction left-handed screw thread Minimum unit length 145 m  5. PROGRAM FOR NED CONDUCTOR To obtain high a critical current density of 3000 A/mm2 in a strand of 1.25 mm is very difficult. The firms having a contract have to optimize the raw materials, the Sn and Nb content. The fabrication process must be suitable for large production. Billets of circa 70 kgs have to be produced and reach piece lengths of strands larger than 1000 m. Many metallurgical tests have to be performed on the strands.  A working group under the leadership of A. den Ouden will make the standardization of the electrical characterization. The follow-up of the contract will foresee several milestones related to payments. The indicative program of development is summarized in Table 5. The programme requests to make cabling tests with the strands under development to measure the effect of the billet design on cable degradation, if any. 220Table 5 .Indicative program of development, fabrication and milestones.  Steps Date Description Milestones  December 2004  Progress report 1 June 2005 Qualification of initial strand designs  Fabrication/tests of at least 10 kg of strand and relevant cabling tests Progress report 2 December 2005 Qualification of final strand design Fabrication/tests of at least 10 kg of strand and relevant cabling tests. Progress report 3 June 2006 Total strand production Fabrication/tests of total strand quantity and cabling tests. Progress report 4 December 2006 Total cable fabrication Total cable quantity delivery Final report  6. CONCLUSION The paper gives the general characteristics of the strand and the cable to be developed for the Next European Dipole (NED).The main characteristics are coming from a preliminary magnet design. The main features of the strand are a diameter of 1.25 mm and a current density in the non copper of 3000A/mm2 at 15 T, 4.2 K. This current density is very ambitious The strand fabrication will be based on Nb3Sn material following the two routes of Internal Tin and Powder in Tube. Orders will be placed in the European superconductor industries to develop the strand and cable within the next 3 years. ACKNOWLEDGEMENTS The author wishes to thank O.Vincent-Viry for the magnetic calculations of the preliminary magnet design and all the participants of the NED Collaboration lead by A. Devred  REFERENCES  [1] A. Devred, D.E. Baynham, L. Bottura, M. Chorowski, P. Fabbricatore, D. Leroy, A. den Ouden, J.M. Rifflet, L. Rossi, O. Vincent-Viry and G. Volpini High Field Accelerator Magnet R&D in Europe” Proceedings of the MT18 Conference, 20-24 October 2003, Morioka, Japan 221

Cable Design and R&D for NED

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