Nb 3 Sn conductor development in Europe for high field accelerator magnets

1. Nb3Sn conductor development in Europe for high field accelerator magnets L. Oberli Thierry Boutboul, Christian Scheuerlein, Jean-Louis Servais, Zinur Charifoulline, Daniel Leroy, Arnaud Devred CARE06

2. OUTLINE • Introduction • NED Conductor Specification • Conductor development plan • Status of strand development • Conclusion CARE06

3. INTRODUCTION MOTIVATION • To promote the development of high performance Nb3Sn wires and cables in collaboration with European industry • To get ready in Europe for the next generation of accelerator magnets GOAL • To develop a conductor for high-field dipoles and high-field gradient quadrupoles needed for LHC luminosity upgrade in the Insertion Regions. In the frame of the CARE (Coordinated Accelerator Research in Europe) project, the NED activity has started with a preliminary design of a large aperture, high field Nb3Sn dipole aimed at deriving meaningful Nb3Sn conductor specification. CARE06

4. NED strand specification The main NED strand characteristics are: • Diameter 1.250 mm • Effective filament f < 50 mm • Cu to non-Cu ratio 1.25 ± 0.10 • Minimum critical current 818 A at 15 T & 4.2 K • non-Cu Jc at 4.2 K 1500 A/mm2 at 15 T 3000 A/mm2 at 12 T • 2 m0M < 300 mT at 2 T & 4.2 K • RRR (after heat treatment) > 200 a large number of filaments => The main cable characteristics are: Trapezoidal Rutherford cable with a width of 26 mm 40 strands Minimum critical current 29440 A at 15 T & 4.2 K CARE06

5. Why NED strand is innovative ? OST strand of 0.7 mm 54 Nb3Sn filaments considered as the State of the Art with 3000 A/mm2 at 12 T & 4.2 K ~ 80 mm filament diameter NED strand of 1.25 mm 288 Nb3Sn filaments 50 mm filament diameter CARE06

6. Why filament diameter is important ? 1. To limit field distortions in accelerator magnets: induce multipoles errors are indeed proportional to the magnetization and are especially significant at low field. M a Jcdeff 2. To limit flux jumps to achieve good quench performances: large filament diameter produces flux jumps which are more likely to occur at low fields where Jc is the larger. Flux jumps are accompanied by power dissipation which can lead to a quench. Magnetization curves of OST strand with ~ 80 mm filament diameter Flux jump in the range of 10 mT which can give aDb3~ 2 units at Binj = 1T - CARE06

7. Conductor Development Strategy Based on the NED specification, a call for tender was issued by CERN 2 contracts were awarded late September 2004 in the frame of the NED/CARE project to : • ShapeMetal Innovation (SMI) for the production of 290 m of cable by the Powder In Tube (PIT) technology. • Alstom Magnet and Superconductors for the production of 580 m of cable by the Internal Tin Diffusion (ITD) technology. The goal is to develop both technologies with the objective to have an industrial process suitable for large-scale applications. CARE06

8. Conductor Development Plan A development plan was established with each company to reach step by step the NED goals (1636 A at 12 T & 4.2 K in a strand of 1.25 mm with 50 mm effective filament diameter). The potentiality of the billet design and the manufacturing process were discussed with each company in view of its industrialization • Step1 : Qualification of initial strand design Fabrication and test of at least 10 kg of strand • Step2 : Qualification of final strand design Fabrication and test of at least 10 kg of strand and relevant cabling tests to demonstrate that the strand does not degrade at cabling • Total strand and cable production CARE06

9. SMI : Status of strand development • As part of step 1, SMI started a development based on an existing 1 mm diameter strand having 192 filaments that achieved a Jc non-Cu value larger than 2280 A/mm2 at 12 T and 4.2 K with the target to achieve 2500 A/mm2. For a given PIT strand layout, an increase of the Jc calls for reacting more fully the Nb tubes, which request a higher Sn content in the powder (mixture Sn + NbSn2) or which calls for the use of a Ta Barrier around the Nb tubes to prevent from Sn leakage. B 207 Powder after reaction Un-reacted Nb Nb3Sn CARE06

10. SMI - Step 1 • Based on the existing layout with 192 (NbTa)3Sn filaments, SMI produced four 3 kg billets: - 2 billets with Ta tubes - 2 billets with an increased Sn powder composition => too many breakages B 207 B179 B201 f 1.00 mm, 192 filaments of f50 mm Cu/non-Cu = 0.93, Sn leak due to a too high free Sn content in the powder a relatively low non-Cu Jc achieved Good behavior under deformation f 1.00 mm, 192 filaments of f50 mm Cu/non-Cu = 0.73, Ic = 1105 A at 12T with 84 h at 675 C Jc non-Cu = 2410 A/mm2 at 12 T CARE06

11. SMI : Strand deformation by rolling To evaluate if the strands are capable to sustain cabling, the strands were deformed by rolling to investigate the filament layout behaviour under different level of deformation. B179 B201 t def =25 % The deformation by rolling is intended for selecting the strand design most suitable for cabling. CARE06

12. SMI : Strand deformation by rolling B 201 B 179 Deformation of 25 %, i.e. d0 - t = 0.25 mm. First result: Distribution of Cu within the strand important in order the strand could sustained heavy mechanical deformation as in cabling. CARE06

13. SMI - Step 2 Based on the results obtained in Step 1, the decision was taken to continue the development with a strand design for NED. • A strand of 1.25 mm in diameter with 288 (NbTa)3Sn filaments to get 50 mm filament diameter, • Keeping the same NbTa tube and same powder composition as for billet 179, • By adjusting the filament layout to have more copper around the filaments. B 207 B201 B215 CARE06

14. SMI - Step 2 : Strand characterization For step 2, SMI has produced a 10 kg billet (B215) drawn without breakage to 1.25 mm in diameter (strand of ~ 900 m). The critical current density goal of 2500 A/mm2at 4.2 K and 12 T has been achieved in a strand with 50 mm filament diameter. Diameter: 1.257 mm Cu to non-Cu ratio = 1.217 Ic ≈ 1400 A at 12 T & 4.2 K Jc ≈ 2500 A/mm2 RRR = 113 HT = 84 hours at 675 0C CARE06

15. SMI – Deformation by rolling on B215 Samples from B215 were rolled at different levels of deformation Def = 28 % The samples deformed at a level of 28 % sustained well the mechanical deformation. The SMI-NED strand has passed successfully all the deformation tests and has to be qualified by cabling test. CARE06

16. ALSTOM : Status of Strand development The manufacturing process of Alstom is based on the Internal Tin Diffusion technology. Technological goals for step 1 were : • Develop the ITD process based on a double stacking billet design and on cold drawing • Optimize sub-element composition to have the highest possible Nb fraction and to provide enough Sn to react part of the Nb barrier and to be at the stoichiometry • Optimize strand design to get a good workability. Nb filament Sn core Sub-element with the Sn core Nb barrier CARE06

17. ALSTOM : Status of Strand development During step 1, Alstom tried few sub-element designs with different filament layouts. Only one sub-element design was qualified for its excellent workability. A billet assembled with 78 sub-elements was drawn to 1.25 mm with only few breakages. => effective sub-element diameter of 85 mm. Part of the billet was drawn to 0.8 mm to reach 50 mm for the sub-element diameter. Drawing done successfully with only one breakage. All these results indicate that a good design has been achieved for the sub-element. CARE06

18. ALSTOM : Status of Strand development The first Alstom/NED wire was characterized by Alstom/MSA and CERN. Diameter: 1.25 mm Sub-elements: 78 (~ 85 mm) Cu to non-Cu ratio = 1.54 Ic ≈ 740 A at 12 T & 4.2 K Jc ≈ 1500 A/mm2 RRR = 100 HT = 120 hours at 660 0C The critical current density achieved for the wire corresponds to the calculated value, as for this sub-element, there was a large amount of copper within the sub-element. CARE06

19. ALSTOM : Status of Strand development A sound sub-element design has been achieved by Alstom during step 1. The sub-element design is being used for step 2 keeping a very similar filament layout but increasing the amount of Nb and decreasing the amount of Cu which should gives at least a non-Cu Jc of 2500 A/mm2 at 12 T. For step 2, Alstom has launched in fabrication few billets with mainly 2 different sub-element designs. Alstom is now focusing the development on the manufacturing process of the final billet to switch from 78 to 246 sub-elements in order to get 50 mm sub-element diameter. CARE06

20. Conclusion Significant progresses have been achieved by the NED program . • Vigorous effort were carried out by Alstom to develop the NED strand and very encouraging results have been obtained. The Jc was already doubled as compared to the value achieved by Alstom before starting the NED program and a sound sub-element design has been obtained. • SMI has developed a superconducting strand made of 288 Nb3Sn filaments which achieved 2500 A/mm2 at 12 T and 4.2 K. CARE06