1. Progress of Nb/Cu Technology With 1.5 GHz Cavities S. Calatroni E. Barbero Soto C. Benvenuti L. Ferreira H. Neupert

2. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Progress with copper electropolishing Substrate studies –Angle of incidence –Thermal impedance –Aluminium cavity Hydrogen studies Conclusions

3. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Q  1x10 10 @ 15 MV/m Q  3x10 9 @ 20 MV/m No news good news?

4. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion The window of optimum polishing parameters is rather narrow. Reproducing these parameters over the entire surface of the cavity cell needs further R&D Production of Cu(OH) 2 on the surface! Production of O 2 bubbles!

5. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Polarisation curve for standard bath H 3 PO 4 (55%) + C 4 H 10 O (45%) Cu 15 g/litre, conductivity 30 mS/cm Resistance:~ 0.1  diffusion layer + ~ 0.1  bath volume Polarisation curve for aged bath std. + H 2 O (10%) Cu 25 g/litre, conductivity 110 mS/cm Resistance:~ 0.1  diffusion layer + ~ 0.03  bath volume

6. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Numerical modelling of the cathode by simulation of the entire EP process with the Elsy 2D/3D computer code (www.elsyca.be)www.elsyca.be Introduction Electropolishing Substrate Hydrogen Conclusion Cathode active region Current density is uniform over all the cell surface

7. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Iris Equator Cut-off

8. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Electropolishing is now performed vertically. No gas bubbles are developed since all the cell surface is polished at the plateau. No longer problems of stabilisation time like for horizontal polishing with the cavity rotating and half-filled. Easier operation Thanks to: V. Palmieri, G. Gemme, R. Parodi

9. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion

10. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion 1020304050 10 20 30 40 50 60 70 80 20406080 10 20 30 40 50 60 70 80 Average incidence angle of the Nb coating (0º means perpendicular). The abscissa is the length in mm along the cavity axis PACO cavityStandard 1.5 GHz cavity

11. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Cu (RRR=100) EP4300 ± 200 Wm -2 K - 1 Cu EP + 1.5 µm Nb4100 ± 200 Wm -2 K - 1 Nb (RRR=180) EP1200 ± 200 Wm -2 K - 1 Nb EP + 1.5 µm Nb1000 ± 200 Wm -2 K - 1 The overall thermal impedance has been measured for pure Nb and Cu, and for Nb/Cu and Nb/Nb films, on 2-mm thick disks. Nb (RRR=670)(extrap.)2500 ± 200 Wm -2 K - 1 (Still lower than Nb/Cu, but Nb cavities performs better at high field !!) The thermal impedance of the film (if any) has no effect Thanks to: G. Vandoni, J-M Rieubland, L. Dufay

12. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion 1p5.1 Nb bulk cavity 1p5.2 Nb/Nb (quench limited) Thanks to: V. Palmieri, D. Reschke, R. Losito

13. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Al 99.999% purity: 10X thermal conductivity of Cu at 4.2 K Spinning + chemical polishing + coating Thanks to: V. Palmieri, G. Lanza

14. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion The hydrogen content and its binding energy with the lattice can be measured with isosteric cycles and successive injections of known quantities of hydrogen. p H 2  (c H ) 2 exp(-E H /kT) Oxidised copper Oxidised copper:400±200 ppm340±10 meV/at Oxide free:1000±200 ppm320±10 meV/at

15. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion In order to reduce hydrogen, a NEG (TiVZr) – coated pump has been added to the coating system. Standard coatingNEG added

16. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion

17. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion p H 2  (c H ) 2 --> Hydrogen in Nb is 3 times less In order to reduce hydrogen, a Ti underlayer has been deposited onto copper, prior to Nb deposition. The H 2 equilibrium pressure has been measured, in order to verify that hydrogen is indeed reduced Nb/Cu Nb/Ti/Cu

18. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion

19. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Electropolishing is well understood, and provides a solid basis for studying the physical origin of the residual resistance increase at high field. The angle of incidence of the coating is not an issue Thermal barrier between Nb and Cu is not a limitation Hydrogen has been decreased: no improvement of RF performance Intrinsic film roughness? NbO at the surface (preferentially at grain boundaries)? Nano-defects in the RF penetration depth?

20. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities

21. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion Electropolished copper surface average roughness: 0.02 µm A few defects still appearing Cross section of a copper cavity Defects are present inside ! Not an artifact of the preparation Thanks to: G. Arnau-Izquierdo

22. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities A further test has been carried out by coating in static conditions: no turbo-pump, only NEG pumping The amount of Kr implanted in the film is so small that only a negligible amount of gas must be supplied during coating, in order to keep the pressure constant No significant change has been seen in the residual gas spectra during coating… Introduction Electropolishing Substrate Hydrogen Conclusion

23. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion

24. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Introduction Electropolishing Substrate Hydrogen Conclusion The hydrogen content and has been measured by thermal outgassing to 350 ºC, for passivated and non-passivated Nb films (i.e. Nb x O y at the surface) Oxidised copper:2200±200 ppm800±200 ppm Oxide free:2000±200 ppm980±200 ppm Bare Cu

25. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Measurement of pinholes Irregularities on the substrate surface shadowing effect film inhomogeneities film p1p1 p2p2 Substrate disk Machining and cleaning Film deposition Substrate removal He leak rate experiment ‹  inc › fraction of leaky film surface - equator 9° 4.4 ppm - (~iris 50° 25 ppm) - equator 9° 0.1 ppm CP EP

26. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Grain size with FIB micrographs Standard filmsOxide-free films 0.5 µm Courtesy: P. Jacob - EMPA

27. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Roughness & incidence angle of Nb mm 50 mm 0 mm

28. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Incidence angle and residual resistance An issue which is of great interest also for low-ß cavities, is the correlation between the incidence angle of the film and the residual resistance

29. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Digression: new ideas for low-beta cavities coating Problem: coating incidence angle in low-beta cavities Solution: coat at favourable angle before welding Cu half-cell Nb ring

30. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Properties of the coatings II: purity, H c1, texture, etc... Standard Oxide-free RRR11.5 ± 0.1 28.9 ± 0.9 T C 9.51 ± 0.01 K 9.36 ± 0.04 K Ar cont. 435 ± 70 ppm 286 ± 43 ppm Texture(110) (110), (211), (200) Hc 1 85 ± 3 mT 31 ± 5 mT Hc 2 1.150 ± 0.1 T 0.73 ± 0.05 T a 0 3.3240(10)Å 3.3184(6) Å  a  /a  0.636 ± 0.096 %0.466 ± 0.093 % Stress -706 ± 56 MPa -565 ± 78 MPa Grain size 110 ± 20 nm > 1 µm

31. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Residual resistance fro OF and OX OX: oxidised copper substrate OF: oxide-free copper substrate CP: chemical polishing EP: electropolishing Oxide-free films (larger grains, better purity, less stress, etc...) have on average a larger residual resistance (measured at low field) In contrast with models linking the residual resistance with Josephson dissipation at grain boundaries

32. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Outgassing of hydroformed Question: why were coatings on oxide-free hydroformed cavities consistently worse than for spun cavities? (Legnaro 1997) Possible answer: a larger quantity of hydrogen was migrating into the film from the hydroformed cavity

33. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities HPWR reduces Rs Nb/Cu film on CP copper HPWR effectively reduces R res 0 (1999 result)

34. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities HPWR smoothens Nb surface Before HPWRAfter HPWR Bulk Nb - EP with KEK bath HPWR smoothens the grain boundaries (2001 results) 10 µm

35. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Standard and oxide-free coatings methods

36. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities PACO electropolishing

37. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Annealed Cu grain size 50 µm Meridian cut Equator cut Standard CuAnnealed at 350 ºC

38. SRF2003 - 10 Sept. 2003Sergio Calatroni – CERN Progress Of Nb/Cu Technology With 1.5 GHz Cavities Annealed Cu grain size Oxide-free coatings on standard copper substrate Oxide-free coatings on 350 ºC annealed copper substrate