1. Advances in Development of Diffused Nb3Sn Cavities at Cornell
Matthias Liepe, Cornell University
M. Liepe FCC Week, 2015, Washington March 24, Slide 1

2. Outline Why Nb3Sn? Challenges Cornell Nb3Sn Program Recent Advances
Potential for the FCC
Conclusions
M. Liepe FCC Week, 2015, Washington March 24, Slide 2

3. Why Nb3Sn? η4.2 K / η2.0 K ≈ 3.6, simpler cryoplant
Increase in Q0 via N-doping
Niobium
Nb3Sn
Critical Temperature Tc
9 K
18 K
Q0 at 4.2 K
6 x 108
6 x 1010
Q0 at 2.0 K
2.5 x 1010
>1011
Max. gradient Eacc (theory)
50 MV/m
100 MV/m
5x 1010
Approximate Eacc and Q0 given for 1.3 GHz TeSLA cavities with Rres small
Halve # of cavities to reach energy?
M. Liepe FCC Week, 2015, Washington March 24, Slide 3

4. Why Nb3Sn? Nb3Sn has large Δ and large Tc
Very small RBCS(T)
Can have high Q0 even at relatively high T
Rres dominates even near 4.5 K
M. Liepe FCC Week, 2015, Washington March 24, Slide 4

5. Nb3Sn: Challenges Material is brittle Films avoid these
Low thermal conductivity
Small coherence length ξ ~ nm
More sensitive to small defects
Low first critical field Hc1=> need to operate in the flux free metastable Meissner state
=> Need high quality Nb3Sn film
Films avoid these
Nb3Sn Nb
Niobium
Meissner state
Meissner state (metastable)
Vortex state
Nb3Sn ?
Hc1
Hsh
M. Liepe FCC Week, 2015, Washington March 24, Slide 5

6. Stoichiometry Nb3Sn Phase Diagram Tc vs. Tin Content 24% target
A. Godeke, Supercond. Sci. Tech, 2006
M. Liepe FCC Week, 2015, Washington March 24, Slide 6

7. Nb3Sn SRF History
Pioneering work at Siemens AG, U. Wuppertal, K.F. Karlsruhe, SLAC, Cornell, JLab, and CERN
U. Wuppertal:
Very high Q0 values in Nb3Sn cavities
Strong Q-slope, cause uncertain
Q-slope: fundamental?
Approximate range at which Hwall reaches for Hc1 of Nb3Sn
Nb at 2.0 K
Nb at 4.2 K
M. Liepe FCC Week, 2015, Washington March 24, Slide 7

8. Cornell Nb3Sn Program Where we started:
Limited R&D effort in the past did not result in a usable high performance cavity
Central question: Is Hc1 a limit for the flux free state of small-ξ superconductors in RF magnetic fields?
YES: No hope for Nb3Sn (or any other strongly type II superconductor for SRF)
No: Great potential for Nb3Sn to transform SRF cavities to much higher performance
Boldly gave Nb3Sn another chance…
M. Liepe FCC Week, 2015, Washington March 24, Slide 8

9. Coating Mechanism: Vapor Diffusion
M. Liepe FCC Week, 2015, Washington March 24, Slide 9

10. Cornell Nb3Sn Cavity Coating Chamber
Cavity Temp Thermo-couples
Heat Shields
Heater Temp Thermo-couples
UHV Furnace
Tin Heater
Tin Container
M. Liepe FCC Week, 2015, Washington March 24, Slide 10

11. Nb3Sn Coating Procedure
Cavity
Tin source
2. Nucleation
1. Degas
~ 1 day
4. Surface diffusion
M. Liepe FCC Week, 2015, Washington March 24, Slide 11

12. Nb3Sn Sample Coatings and Studies
Anodization
SEM
EDX / XPS
Not anodized
Anodized
Grains ~1 μm. Appearance similar to Nb3Sn from other studies
24.2 ± 0.5 atomic % Sn, uniform over surface; 2 μm deep
Pink -> Nb3Sn
RRR Measurement
Tc Measurement
FIB
Sample prep for TEM, view of coating cross section
Minimal degradation of thermal conductivity from coating process
20 µm
M. Liepe FCC Week, 2015, Washington March 24, Slide 12

13. Preparation of Cavity for Coating
Start out with single-cell Niobium 1.3 GHz cavity
M. Liepe FCC Week, 2015, Washington March 24, Slide 13

14. Coated Cornell 1.3 GHz Nb3Sn Cavity
Before Coating
Standard Nb cavity
Nb3Sn-Coated
After Coating
M. Liepe FCC Week, 2015, Washington March 24, Slide 14

15. Vertical Performance Testing
M. Liepe FCC Week, 2015, Washington March 24, Slide 15

16. Cornell Nb3Sn Results After improving of the coating process
The first accelerator cavity made with an alternative superconductor that outperforms Nb at usable gradients!
No strong Q-slope
Excellent Q0 at 4.2K at medium fields
Prove that Hc1 is not a fundamental limit for SRF
Nb3Sn shows great promise for even higher performance with continued R&D
M. Liepe FCC Week, 2015, Washington March 24, Slide 16

17. Breakthrough Nb3Sn Cavity
Single-cell 1.3 GHz cavity at 4.2K
Low Rres ~ 10 nΩ, similar to most Wuppertal cavities
Huge (factor of ~10) Q0 improvement at 4.2 K at medium fields compared to Wuppertal
~ 20x more efficient than Nb at 4.2 K!
M. Liepe FCC Week, 2015, Washington March 24, Slide 17

18. Hc1 is NOT the Limit for Small-ξ Materials!
Bc1 range: 27 ± 5 mT for Cornell cavity Q0
Well above Bc1 without strong Q slope!
=> Energy barrier keeps Meissner state metastable, even with small ξ of Nb3Sn.
M. Liepe FCC Week, 2015, Washington March 24, Slide 18

19. Repeatability Current quench field: 12 to 17 MV/m
1.3 GHz single-cell cavities at 4.2K
Current quench field: 12 to 17 MV/m
M. Liepe FCC Week, 2015, Washington March 24, Slide 19

20. Field Limitation – Quench
Localized pre-heating just below first quench
Defect – not a fundamental limit
Can reach higher fields by fixing defect
Before quench, Eacc = 13 MV/m, Q0 = 1x1010
M. Liepe FCC Week, 2015, Washington March 24, Slide 20

21. Pulsed Quench Field >20 MV/m
Shorter pulses => higher quench fields => defect limited
M. Liepe FCC Week, 2015, Washington March 24, Slide 21

22. Low Tin Content Nb3Sn Regions (I)
TEM/EDX and X-ray Diffraction (XRD) revealed tin-depleted regions (~0.5 μm size) Nb
Nb3Sn 2 1
Nb L
0.5 μm 2 1
Sn L
0.5 μm
“1”: % Sn – “2”: % Sn
M. Liepe FCC Week, 2015, Washington March 24, Slide 22

23. Low Tin Content Nb3Sn Regions (II)
Low tin content regions near surface are likely candidate for source of quench
Have much lower Tc, lower critical fields
A. Godeke, Supercond. Sci. Tech, 2006
M. Liepe FCC Week, 2015, Washington March 24, Slide 23

24. Nb3Sn: State of the Art With improved slow cool-down procedure:
Single-cell 1.3 GHz cavity at 4.2K
Q0 > 1.5x1010 at 4.2K
Residual resistance Rres = 11nΩ at max field
M. Liepe FCC Week, 2015, Washington March 24, Slide 24

25. Cooling-Power Impact for 1.3 GHz SRF
SRF operation at ~4K instead of 2K!
Greatly simplified cryo-system
Greatly reduced cryo AC power
- 60%
- 80%
M. Liepe FCC Week, 2015, Washington March 24, Slide 25

26. Cooling-Power Impact for 800 MHz SRF
SRF operation at ~4K instead of 2K!
Greatly simplified cryo-system
Greatly reduced cryo AC power
- 50%
- 75%
Similar cryo load reductions for 400 MHz to 800 MHz range.
M. Liepe FCC Week, 2015, Washington March 24, Slide 26

27. Nb3Sn: Next Steps
Reduce residual resistance further to fully utilize the very high 4.5K Q0 performance of Nb3Sn
Increase quench fields by improving fabrication parameters to eliminate tin-depleted regions
Extend coating to multicell cavities
Nb3Sn R&D
Strong, DOE supported Nb3Sn program at Cornell
Coating facility and R&D program at JLAB
Nb3Sn R&D starting up at FNAL …
M. Liepe FCC Week, 2015, Washington March 24, Slide 27

28. Nb3Sn: Conclusions Nb3Sn very promising for SRF
Potential for factor ~4 higher efficiency, twice energy gain per length
Cornell’s leading Nb3Sn program has resulted in significant Nb3Sn performance improvements
Strong Q-slope seen previously suppressed
Hc1 NOT a fundamental limit
High Q0 at useful fields, T = 4.2 K
First Nb3Sn cavities to outperform niobium
Low-Tc Nb-Sn alloys are likely cause of quench
Very promising for FCC-ee and many other future accelerators with continued R&D
M. Liepe FCC Week, 2015, Washington March 24, Slide 28

29. The end of this talk… …just the beginning of Nb3Sn for SRF
Thank you for your attention!
The end of this talk… …just the beginning of Nb3Sn for SRF
Recent Publications:
Proof-of-principle demonstration of Nb3Sn superconducting RF cavities for high Q0 applications, S. Posen, M. Liepe. D. Hall, Applied Physics Letters 106, Issue 8 (2015).
Analysis of Nb3Sn surface layers for superconducting RF cavity applications, C. Becker, S. Posen, N. Groll, R. Cook, C. M. Schlepuetz, D. Hall, M. Liepe, M. Pellin, J. Zasadzsinski, and T. Proslier, Applied Physics Letters 106, Issue 8 (2015).
Advances in development of Nb3Sn superconducting radio-frequency cavities, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams 15, (2014).
M. Liepe FCC Week, 2015, Washington March 24, Slide 29

30. Backup slide: Pcryo,Nb3Sn/Pcryo,Nb
For Rres,Nb3Sn = 11 nΩ
For Rres,Nb3Sn = Rres,Nb
M. Liepe FCC Week, 2015, Washington March 24, Slide 30

31. Material Parameters
M. Liepe FCC Week, 2015, Washington March 24, Slide 31