1. Conductor Requirements for Magnet Designers DOE- Conductor Development Program Daniel R. Dietderich Superconducting Magnet Program Office of Science ICFA Mini Workshop on High Field Magnets for Future pp Colliders Shanghai Jiao Tong University, Xuhui Campus June 14-17, 2015

2. ATAP SMP is uniquely qualified to address these challenges. A focused set of Grand Challenge questions addressing the P5/HEPAP subpanel report concerns have been identified Achieve a field of 16T in a bore of at least 50mm by focusing on simple, manufacturable designs Understand training of Nb 3 Sn magnets and develop ways to reduce or eliminate it Produce an HTS (Bi-2212/YBCO) insert with a self-field of > 4T and measure the field quality Reduce cost and improve performance of Nb 3 Sn Increase the current density by 30% with a scalable sub-element structure Aim for a cost per kg the same as NbTi Develop HTS conductor Reduce silver (i.e. increase Bi-2212 content) or replace silver Increase J c /J e Focus on magnets as technology drivers to generate larger market for cost reduction (Stewardship activity) as a collateral benefit to the program. Dr. X. Wang will address technology development in next talk June 15, 2015 D.R. Dietderich, LBNL 2

3. Conductor for Hi-Lumi LHC ITER strand: Non-Cu Jc  800 A/mm 2 (12 T, 4.2 K) Hi-Lumi strand: Non-Cu Jc  2500 A/mm 2 (12 T, 4.2 K) RRP 132/169 PIT 192 Critical current density versus field 2450 A/mm 2 1340 A/mm 2 ITER A. Ballarino June 15, 2015 D.R. Dietderich, LBNL 3

4. The 150 mm aperture QXF magnet program in LARP is initially using the RRP 108/127 Ti-Ternary strand. Strand specification Strand Diameter, mm0.85 J c (12 T) at 4.2 K, A/mm 2 > 2650 I c, A> 684 J c (15 T) at 4.2 K, A/mm 2 > 1400 I c, A> 361 d s, µm (nominal)< 60 Cu-fraction, %> 53 Cu/non-Cu> 1.13 RRR > 150 Piece length> 750 m LARP Nb 3 Sn Specification 4 June 15, 2015 D.R. Dietderich, LBNL

5. FCC Nb 3 Sn Requirements June 15, 2015 D.R. Dietderich, LBNL 5 CDP is now focusing on conductor needs beyond High Luminosity LHC

6. Conductor Priorities for 20T Reduce cost –Need to reduce cost of Nb 3 Sn and Bi-2212 –Can billets be scale-up ~ 200 kg billets Nb 3 Sn, perhaps 50 kg for Bi-2212 Need to establish process early Increase piece length Reduce subelement size (filament ) –Single re-stack or Double re-stack Increase Critical Current Density (J c ) –Need to understand the origin of J c variation between identical wire –Need to understand Nb 3 Sn formation –Need to understand loss of J c as the wire diameter decreases –Shift pinning curve June 15, 2015 D.R. Dietderich, LBNL 6

7. June 15, 2015 D.R. Dietderich, LBNL 7 Rounder Nb Filament Better piece length Perhaps better performance (Higher J c )

8. June 15, 2015 D.R. Dietderich, LBNL 8 ATI - Nb Grain Structure Heat AHeat C Edge of billet Center of billet Rod Micrographs Nb Type 1 Microstructure can be vary across a billet ATI developing processing to improve uniformity Courtesy of Phil Olarey OST working with ATI To improve Nb rods

9. June 15, 2015 D.R. Dietderich, LBNL 9 RRP ® with more sub-elements Smaller Filament Size 721 to 919 ~ 20  m RRP ® 192/217RRP ® 54/61

10. June 15, 2015 D.R. Dietderich, LBNL 10 CDP -- RRP ® 192/217 Properties RRR, J c, and n-value decrease with decreasing wire diameter Can reduce Sn to increase RRR Reduced Sn may permit the 30% thicker barrier to be used in a 252/271 stack or greater – 37 µm sub-elements at 0.85 mm No SF correction CDP billet 14832 650C/50 hrs 30  m

11. June 15, 2015 D.R. Dietderich, LBNL 11  m Sub-element Diameter,  m R=Cu/Non-Cu=1.15, 46.5% SC, 53.5 % Cu L. Cooley & A. Ghosh Formalization LARP and OST Standard

12. Subelement Number June 15, 2015 D.R. Dietderich, LBNL 12 What sub-element size is needed for HE-LHC and future machines? For a wire diameter of 0.85 mm Stack = Subelement size  m 271 = 42  m 271 = 37  m. 721 to 919 ~ 20  m Can this be produced? If billet starting diameter is ~100 mm (4”) Subelement diameter during billet fabrication is 3-4 mm At this size they are difficult to handle 54/61 RRP ® Perhaps cold extrusion for 6-8” diameter billet

13. Formation of Nb 3 Sn – Re-design of RRP ® Sub-element June 15, 2015 D.R. Dietderich, LBNL 13 Re-visit the design of the RRP ® sub- element –Cu channels between filaments –Nb filament diameter –Barrier thickness and uniformity Subelement will start to approach a tubular process J c decreases significantly for D s below ~50  m –Need better understanding of tin / bronze diffusion and the interaction with Nb-Sn-Cu ternary as D s ↓ Collaborations with Sanabria (FSU), Pong (LBL) 75  m 20  m RRP reacted 210 ° C/48hr + 400 ° C/48hr, Courtesy of FSU (Peter Lee, Charlie Sanabria)

14. Methods to Reduce Cost and Magnetization Single barrier approach ITER style conductor –Single barrier approach –Has lower magnetization and cost than RRP ® –However, Jc is low This would require an increase of the layer J c –Increasing flux pinning –Recent work by OSU and Hyper Tech No clear path to final conductor! June 15, 2015 D.R. Dietderich, LBNL 14

15. Double Re-stack A “double restack” – restacking the restack – can access the smaller D s at the desired wire size range –Need subelements tolerant to a high degree of deformation –Difficult to keep the Non-Cu fraction and I c up, Double restacking adds in extra copper June 15, 2015 D.R. Dietderich, LBNL 15 7 stack X 61 stack = 324 D s ~ 20  m @ 0.8 mm Perhaps one could cold extrusion a 6”-8” diameter billet

16. June 15, 2015 D.R. Dietderich, LBNL 16 Conductor Performance Improvement Need to increase current density at higher fields Nb 3 Sn - Shift pinning curve in wires to higher fields – Develop conductor with smaller grain Nb 3 Sn Bi-2212 – Need to reduce porosity – Improve filament continuity Nb 3 Sn Bi-2212 A. Godeke Target J E ~ 600 A/mm 2

17. June 15, 2015 D.R. Dietderich, LBNL 17 J of wire for Pinning Curve Shift Godeke, et al., unpublished New work Xingchen Xu of OSU and Hyper Tech on wires may be shifting the curve H c2 ? Target J E ~ 600 A/mm 2 Nb 3 Sn

18. June 15, 2015 D.R. Dietderich, LBNL 18 Shift Pinning Curve of Nb 3 Sn Films of Nb 3 Sn with “Engineered Microstructures” Dietderich and Godeke, Cryogenics 48, 331 (2008 ) Grain size ~ 20 nm Inclusions ~ 5 nm Grain size ~ 75 nm

19. June 15, 2015 D.R. Dietderich, LBNL 19 Bi-2212 With Over Pressure Bi-2212 Target J E ~ 600 A/mm 2 Godeke, et al., unpublished Magnet target has been achieved with overpressure processing

20. June 15, 2015 D.R. Dietderich, LBNL 20 Material Properties Conductor Development Program of HEP Nb 3 Sn: –Understand strain behavior RRP Nb 3 Sn Bi-2212 –Develop a high quality powder supplier –Increase critical current by exploring other powder compositions. –Improve strain tolerance of wire –Cable stress I c drops at 60 MPa A. Godeke

21. Bi-2212 Goals Reduce Cost –Bi-2212 needs a market other than HEP –Scale up billet size Can the amount of superconductor in wire cross section be increased? –Improve stress and strain tolerance –Area is now ~25%, need ~50% Would this lead to processing issues since Bi-2212 powder has not strength? –This would double critical current of a wire for the approximately the same cost –Past wires with 50% Ag showed NO increase in I c June 15, 2015 D.R. Dietderich, LBNL 21

22. June 15, 2015 D.R. Dietderich, LBNL 22 Bi-2212 Effort in CDP For a 1.2 mm diameter wire with a stack of 85 x 18 –No loss of critical current for twists lengths down to 12 mm Increase strength of wire –Increase Ag alloy fraction in wire from ~24% to ~42 % Could lead to more Bi-2212 in cross section With over-pressure the wire J c (4.2K, 12T) is about the same as that of a std. wire Ag alloy (blue)

23. June 15, 2015 D.R. Dietderich, LBNL 23 Bi-2212 Future Directions Concern about supply of high quality Bi-2212 powder –Supporting a Task to fabricate wires from the different powder suppliers “521” composition has been the standard for many years –Can the critical current be increased by obtaining a better understanding of why it seems to be the best? –Or composition developed before? 522, 523, & 524 developed by OST and Nexans Try new compositions –Over pressure processing may require different composition Improve filament continuity The effective filament size of a wire is large: How can it be reduced?

24. June 15, 2015 D.R. Dietderich, LBNL 24 End