1. Block-coil NbTi dipoles for 6 Tesla Rapid-cycling SuperSPS
Peter McIntyre
Dept. of Physics
Texas A&M University

2. The goal of our R&D: Dipoles for future hadron colliders
TAMU4: 14.1 T, 4 x 3 cm2 aperture
28 cm2 superconductor
Collider-quality field, suppress p.c. multipoles
LHC Tripler: 24 T, 56 mm aperture
Windings = Bi-2212 inner, Nb3Sn outer

3. Designing dipoles with Nb3Sn
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 14 T
stress (PSI) in coils 14 T

6. 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

7. 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

8. The steel flux plate redistributes flux to suppress multipoles
0.5 T T

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

10. The Texas A&M program TAMU1 (6.5 T) TAMU2 (5.2 T) TAMU3 (13.5 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 Nb3Sn conductor - completed, ready for testing
TAMU3 (13.5 T)
double-pancake model with 2.4 kA/mm2 conductor - beginning fabrication
TAMU4 (14.1 T )
complete Nb3Sn dipole with 4x3 cm bore

11. TAMU1
Model dipole to study block coil geometry: cable preparation, winding techniques, impregnation: treat exactly according to the design for Nb3Sn.

12. Testing of TAMU1
Winding voltages during quench

13. AC losses
1 T/s
1.5 T/s
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.

14. TAMU2: our entry into Nb3Sn technology
TAMU2: 1 single-pancake winding
mirror geometry, ITER superconductor
5.6 T short-sample bore field

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

16. 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

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

18. Preliminary design for Super-SPS dipole
Vertical cable orientation to suppress AC losses
Flux plate to suppress magnetization multipoles at injection
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)