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

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

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, 2015 Slide 3

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, 2015 Slide 4

Nb3Sn: Challenges Material is brittle Films avoid these Low thermal conductivity Small coherence length ξ ~ 3 - 4 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, 2015 Slide 5

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

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, 2015 Slide 7

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, 2015 Slide 8

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

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, 2015 Slide 10

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

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, 2015 Slide 12

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

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

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

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, 2015 Slide 16

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, 2015 Slide 17

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, 2015 Slide 18

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, 2015 Slide 19

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, 2015 Slide 20

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

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”: 24-25 % Sn – “2”: 16-18 % Sn M. Liepe FCC Week, 2015, Washington March 24, 2015 Slide 22

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, 2015 Slide 23

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, 2015 Slide 24

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, 2015 Slide 25

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, 2015 Slide 26

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, 2015 Slide 27

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, 2015 Slide 28

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). http://dx.doi.org/10.1063/1.4913247 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). http://dx.doi.org/10.1063/1.4913617 Advances in development of Nb3Sn superconducting radio-frequency cavities, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams 15, 112001 (2014). http://dx.doi.org/10.1103/PhysRevSTAB.17.112001 M. Liepe FCC Week, 2015, Washington March 24, 2015 Slide 29

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

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