Thin films: a mandatory path for the future of SRF ?

1. Thin films: a mandatory path for the future of SRF ?

2. What kind of thin films ? • Niobium on copper (µm) • After ~ 20 years stagnation : new revolutionary deposition techniques • Great expectations in cost reduction • No improved performances/ bulk Nb • Higher Tc material (µm) • Based on superheating model. • Higher field and lower Q0 expected • Superheating model questioned for type II SC • Higher Tc material (nm), multilayer • Based on trapped vortices model (Gurevich) • Higher field and lower Q0 expected • Recent experimental evidences • Specific characterization tools needed • Better understanding of SRF physics needed Subtask 2 Subtask 3 Subtask 4 Subtask 1

3. Subtask 2

4. Limits in a RF cavity Pb Nb 2.5 Normal state 2.0 Mixed state 1.5 Superheated state 1.0 Meissner state 0.5 Type II Type I 1.2 1.6 2.0 0 0.4 0.6 GL parameter k (= l/x) Classical theory BCS + RF : • Magnetic RF field limits Eacc :Eacc  HRF • Phase transition when magnetic HRF ~> HSH (superheating field) • For Nb HSH ~1,2.HC(thermodynamic) • Higher Tc => higher Hc => higher Eacc But… • BCS valid only near TC , clean limit • we work at 2 K + rather dirty limit. BCS model needs to be completed Subtask 1 TC ↑ and HC ↑ Hypothesis : HSH ~1,2.HC for k ~1 HSH ~0,75.HC for k>>1 Note : increase of Q0 also very important

5. Nb3Sn development at CERN • Development of a thermal deposition set up. • Comparison between : • thermal diffusion of tin into niobium • co-deposition by sputtering of Nb and Sn followed by a thermal reaction Subtask 3

6. High field dissipations : due to vortices ? 1.5 GHz Nb3Sn cavity (Wuppertal, 1985) 1.3 GHz Nb cavity (Saclay, 1999) Theoretical Work from Gurevich : temperature correction • Non linear BCS resistance at high field : quadratic variation of RBCS • Vortices : normal area ~ some nm can cause “hot spots” ~ 1 cm (comparable to what is observed on cavities) • At high field vortices => thermal dissipation => Quench • Nb is the best for SRF because it has the highest HC1, (prevents vortex penetration) HC1 Nb3Sn (0.05 T) HC1 Nb (0.17 T) Subtask 1 Nb3Sn Nb • Nb is close to its ultimate limits (normal state transition) • avoiding vortex penetration => keep below HC1 • increasing the field => increase HC1 • “invent” new superconductors with HC1> HC1Nb A. Gurevich, "Multiscale mechanisms of SRF breakdown". Physica C, 2006. 441(1-2): p. 38-43 A. Gurevich, "Enhancement of RF breakdown field of SC by multilayer coating". Appl. Phys.Lett., 2006. 88: p. 12511. P. Bauer, et al., "Evidence for non-linear BCS resistance in SRF cavities ". Physica C, 2006. 441: p. 51–56

7. Breaking Niobium monopoly x 200 • Overcoming niobium limits (A.Gurevich, 2006) : • Keep niobium but shield its surface from RF field to prevent vortex penetration • Usenanometric films (w. d < l) of higher Tc SC : => HC1enhancement • Example : • NbN , x = 5 nm, l = 200 nm • HC1 = 0,02 T • 20 nmfilm => H’C1 = 4,2 T Subtask 3 (similar improvement with MgB2 or Nb3Sn) Happlied • high HC1 => no transition, no vortex in the layer • applied field is damped by each layer • insulating layer prevents Josephson coupling between layers • applied field, i.e. accelerating field can be increased without high field dissipation • thin film w. high Tc => low RBCS at low field => higher Q0 HNb Cavity's internal surface → Outside wall 

8. High Tc nanometric SC films : low RS, high HC1 Magnetic field B (mT) • In summary : take a Nb cavity… • deposit composite nanometric SC (multilayers) insideNb / insulator/ superconductor / insulator /superconductor… Quality coefficient Q0 Accelerating Field Eacc (MV/m) Best candidates Nb3Sn MgB2 Increasing of Eacc AND Q0 !!!

9. Multilayers optimization Nb NbN Al2O3 MgO Cu Metallic substrates more realistic): • SC structure optimization • Deposition techniques optimization • Magnetron sputtering Inac (Grenoble), • Atomic Layer Deposition INP (Grenoble) • From samples to cavities : • ALD involves the use of a pair of reagents • Application of this AB Scheme • Reforms a new surface • Adds precisely 1 monolayer • Viscous flow (~1 Torr) allows rapid growth • No line of site requirements • => uniform layers, larges surfaces, well adapted to complex shapes : cavities! • up grade of existing cavities ?

10. Need for specific characterization • Development (optimization) of thin films easier on flat samples • allows to scan multiple parameters with standard fabrication devices => good fabrication parameter • then optimization of the deposition technique to cavity geometry • Need to develop specific RF cavities for flat samples • usually higher frequency • gives only low field behavior (Rs, then Q0 estimation) • In between we need a tool to evaluate maximum achievable field • HC1DC ≤ HC1RF ≤ HC ~HSH : measure of HC1 • SQUID or Local magnetometry Subtask 1

11. Sample RF cavity • CERN Thin film test stand • Quadrupole resonator: focus magnetic fields on sample • operation in higher order mode • Calorimetric measurement • B-field focused on sample by resonator loops operated in quadrupole mode • Plans at HZB • Build complementary apparatus at HZB with sub-nanoohm resolution • Reduce sample size to 1-2 cm diameter • (build insert for CERN apparatus) Quadrupole resonator at CERN

12. Local magnetometry (1) Differential Locking Amplifier Excitation/Detection coil (small/sample) = T/Tc • 3rd harmonic measurement, local measurement : sample size >> excitation/measuring coil • b0cos (wt) applied in the coil • temperature ramp • third harmonic signal appears @ BC1 increasing I = increasing B TCi at Bi Sample SL : third harmonic signal for various b0

13. Local magnetometry (3) • SL sample : 250 nm Nb + 14 nm MgO + 25 nm NbN • 8.90K < Tp° < 16K : behavior ~ NbN alone • Tp°< 8.90K, i.e. when Nb substrate is SC , => BC1SL >> BC1Nb

14. Local magnetometry + SQUID measurement @ 4,5K • SL sample : 250 nm Nb + 14 nm MgO + 25 nm NbN • At 4.5 K HPSL~ 96 mT (+ 78 mT /Nb alone) !!! (~ 20 MV/m) • Need to extend measure @ higher field and lower temperature

15. Structure of the task

16. Conclusions & perspectives: • New techniques combined with theoretical (serious!) work open new technological pathways for SRF performances • Improvement expected for EaccAND Q0 : all accelerator applications can benefit from these approaches ! • It is a truly innovative topic with promising exploratory results : opens the way to Eacc > 100 MV/m and Q0 x 10… • It is difficult to have it granted by one particular machine project or laboratory. • It is right in the scope of the EC support : basic R&D for a whole scientific community

17. Compléments

18. Adjustment of coil distance glue glue + copperpowder glass bead wedge : 60 µm coil

19. First exp. results on highquality model samples ~ 12 nm NbN x 4 14 nm insulator (MgO) 250 nm Nb “bulk” Monocrystalline sapphire Test sample ML • Choice of NbN: • ML structure = close to Josephson junction preparation (SC/insulator compatibility) • Use of asserted techniques for superconducting electronics circuits preparation: • Magnetron sputtering • Flat monocrystalline substrates ~ 25 nm NbN ~ 15 nm insulator (MgO) Reference sample R 250 nm Nb “bulk” Test sample SL Monocrystalline sapphire • Collaboration with J.C. Villégier, CEA-Inac / Grenoble

20. Local magnetometry (4) • Sample SL : small Nb signal @ ~TcNb : Nb is sensed through the NbN layer ! • Since the Nb layer feels a field attenuated by the NbN layer, the apparent transition field is higher. • This curve provides a direct measurement of the attenuation of the field due to the NbN layer BC1 curves for Niobium in the reference (direct measurement) and in SL (under the NbN layer).

21. Bulk Nb ultimate limits : not far from here ! • Cavité 1DE3 : • EP @ Saclay • T- map @ DESY • Film : courtoisie • Gössel + • D. Reschke • (DESY, • Début 2008) The hot spot is not localized : the material is ~ equivalent at each location => cavity not limited /local defect, but by material properties ?

22. Rappels sur les principaux supras