1. Recent Developments in Nb 3 Sn Dipole Technology at Texas A&M Peter McIntyre Dept. of Physics Texas A&M University p-mcintyre@physics.tamu.edu

2. The goal: Dipoles for future hadron colliders TAMU4: 14.1 T, 4 x 3 cm 2 aperture 28 cm 2 superconductor Collider-quality field, suppress p.c. multipoles LHC Tripler: 24 T, 56 mm aperture Windings = Bi-2212 inner, Nb 3 Sn outer

3. Designing dipoles with Nb 3 Sn The challenges The conductor is fragile – strain < 0.5% High field limit would be imposed by Lorentz stress Filaments are large – snap-back too large The solutions Block-coil geometry Stress management Hydraulic preload Flux-plate suppression of snap-back

4. Stress management

5. Offload stress from windings to structure stress (PSI) in structure @ 14 T stress (PSI) in coils only @ 14 T

6. Field strength decreases smoothly - conductor optimization Mixed-strand cable places Cu strands where they are needed for quench protection. Example: 12 T dipole – outer winding can even be NbTi!

7. Pancake coils are easy to build, control axial stress internally Center double pancaketop/bottom single pancakes

8. Provide overall preload using expansion bladders Flux return split vertically, serves as piston Bladders filled with low-melt Wood’s metal Bladders located between flux return and Al shell 2,000 psi pressure delivers full-field Lorentz load In cooldown, Al shell delivers additional preload

9. Suppression of multipoles from persistent current magnetization Persistent magnetization is generated from current loops within the filaments, Magnetization relaxes via BIC’s, then snap-back

10. The steel flux plate redistributes flux to suppress multipoles 0.5 T12 T

11. Multipoles with Persistent Currents 5x suppression of p.c. sextupole – compensates for larger filament size

12. The Texas A&M program TAMU1 (6.5 T) –evaluate block-coil geometry, winding and impregnation strategies using NbTi model - tested to short sample TAMU2 (5.2 T) –single-pancake mirror magnet with ITER Nb 3 Sn conductor - completed, ready for testing TAMU3 (13.5 T) –double-pancake model with 2.4 kA/mm 2 conductor - beginning fabrication TAMU4 (14.1 T ) –complete Nb 3 Sn dipole with 4x3 cm bore

13. TAMU1 Model dipole to study block coil geometry: cable preparation, winding techniques, impregnation: treat exactly according to the design for Nb 3 Sn.

14. Testing of TAMU1 Winding voltages during quench

15. AC losses TAMU1 is the first fully impregnated NbTi dipole made in modern times. It operated to short sample without training and exhibits good AC performance. This result demonstrates that the helium access thought essential for NbTi stability is not necessary, provided that stress is managed so as to prevent conductor motion and friction heat. 1 T/s 1.5 T/s

16. TAMU2: our entry into Nb 3 Sn technology TAMU2: 1 single-pancake winding mirror geometry, ITER superconductor 5.6 T short-sample bore field

17. Coil winding Ti mandrel to preserve preload through cooldown. Inconel ribs, laminar springs transfer stress between windings.

18. Reaction bake @ 650 C Argon atmosphere purge manifolded throughout coil. Same furnace can bake 875 C in O 2 purge for Bi-2212 and maintain separate purges of Ar in Nb3Sn, O 2 in Bi-2212 windings. We can react a 3 m long dipole in this furnace.

19. Splice leads Nb 3 Sn to NbTi Lead is supported in rigid frame anchored into winding superstructure, spliced to a pair of NbTi leads.

20. Preload, weld pancake subassembly Preload side bars and end shoes. Weld cover skin to stabilize coil subassembly. Note: For stress management, we do not apply large preload, only ~3 MPa, just enough to remove soft modulus from coil. After impreg and dipole assembly, we will apply larger preload to the structure to provide stiff walls.

21. Vacuum impregnation Horizontal orientation (with tilt), multiple flow paths assure full impregnation We can impregnate a 3 m long dipole in this retort!

22. Bladder preload Entire dipole heated to 80 C. Bladders preloaded to 2,000 psi using hand pumps. Pressure sustained while magnet is cooled using water jacket.

23. TAMU3: going to high field and testing stress management TAMU3: 2 single-pancake windings 3 kA/mm 2 superconductor 13.5 T short-sample bore field TAMU4: 2 single-pancakes, one double-pancake 3 kA/mm 2 superconductor 14.1 T short-sample bore field

24. Magnets are getting more efficient! NbTi Nb 3 Sn Bi-2212

25. Inject to LHC from SuperSPS For luminosity upgrade of LHC, one option is to replace the SPS and PS with a rapid-cycling superconducting injector chain. 1 TeV in SPS tunnel  1.25 T in hybrid dipole: flux plate is unsaturated, suppression of snap- back multipoles at injection. SuperSPS needs 6 T field, ~10 s cycle time for filling Tripler  >1 T/s ramp rate

26. In block-coil dipole, cables are oriented vertically: Result: minimum induced current loop, minimum AC losses Again block-coil geometry is optimum! In cos  dipole, cables are oriented on an azimuthal arch: Result: maximum induced current loop, maximum AC losses

27. Preliminary design for Super-SPS dipole 6 T short-sample field (to allow for AC loss degradation) LHC NbTi strand (wider cable to optimize geometry, minimize inductance) We are modeling AC losses, expect to be low. Flux plate suppresses multipoles from persistent currents, AC-induced currents (flux plate must be laminated)