http://www.surfacetreatments.it/thinfilms Surface topography investigation for thin film SRF (Genfa Wu - 20') Speaker: Genfa Wu - Fermilab | Duration: 20 min. Abstract The general surface topography will be discussed for various niobium cavities. The field enhancement factor for both niobium or thin film cavities will have different effects. Their implications for thin film based cavities will require investment in extensive surface preparations for cavity substrate cavities. The surface preparation effort at Fermilab will be discussed for future new material effort.

1. October 4, 2010 1 Surface topography investigation for niobium cavities and its implication for thin film SRF Genfa Wu Fermilab developed highly successful processing techniques for 1-cell cavities . Field emission in 1-cell cavity was completely under control. Magnetic field quench became dominant niobium cavity performance limitation. Optical inspection and surface replication of cavity equators revealed large number of surface defects in both 1-cell and 9-cell ILC cavities. The analysis indicated the surface topography and roughness played secondary role in cavity performance limitations in modern niobium cavity processing. Fermilab effort to perfect the niobium RF surface is progressing well.

2. 2 Outlines 9-cell cavity yield ANL/FNAL facility and diagnostic capability Quench localization, optical inspection and surface replica Cavity processing/testing summary Cavity performances and surface features Cavity performances and surface roughness Impact of surface topology for thin film/new material and Fermilab’s effort in cavity RF surface preparation Summary October 4, 2010

3. 9-cell Cavity Performance & Stats For cavities from established vendors, using optimized EP processing and handling procedures: 1st-pass cavity yield >35 MV/m is (29 +- 8) % 2nd-pass cavity yield >35 MV/m is (56 +- 1) %, where 2nd pass refers to any surface preparation All cavities passing gradient requirement also pass Q0 requirement Data from C. Ginsburg Even after 2nd pass, less than 56% cavities yield at 35MV/m. Surface defects affects SRF cavity superconductivity dramatically. October 4, 2010

4. 4 Argonne/Fermilab Cavity Processing Facility Electro-Polishing Ultrasonic Degreasing High-Pressure Rinsing Assembly & Vacuum Leak Testing Courtesy of M. Champion October 4, 2010

5. 5 Fermilab Cavity diagnostics – optical inspection Kyoto/KEK: standard ILC cavities. Questar: All style cavities Image of TE1ACC006 Camera inspection system by Kyoto University/KEK Image of 3C9FER001 Questar QM-1, originated at Cornell USAF-1951 seen by Questar October 4, 2010

6. 6 Fermilab Cavity diagnostics – T-map/OST Cernox based fast thermometry (FNAL T&I Dept. ) and diode based T-map (A. Mukherjee et al.) Oscillating Superleak Transducer (“second-sound” detectors from Cornell University – Z. Conway October 4, 2010

7. 7 Fermilab Cavity diagnostics – Surface replica Capable to reveal the 3-D profile of geometric defects and to evaluate the mechanism leading to quench at the defects based on local magnetic field enhancement. Useful to assess surface preparation effectiveness by extracting surface roughness Achieved resolution at the micron detail (to resolve features ~<10µm in diameter; ~1µm in depth). Berry S, Antoine C, Aspart A, Charrier J P, Desmons M, and Margueritte L, Topologic analysis of samples and cavities: A new tool for morphologic inspection of quench site, Proc. 11th Workshop on RF Superconductivity, 2003 M Ge, G Wu, D Burk, J Ozelis, E Harms, D Sergatskov, D Hicks, and L D Cooley, Routine characterization of 3-D profiles of SRF cavity defects using replica techniques, Manuscript submitted to Journal of Superconducting Science and Technology. October 4, 2010

8. Surface replica - resolution Pit replica A man-made pit 125µm deep, 300µm in diameter profile of pit’s replica profile of a pit on the Nb coupon 8 Overall, 1 µm detail can be replicated October 4, 2010

10. Probe tip with 0.8 µm diameter

11. Horizontal resolution < 1 µm

12. Vertical resolution ~1 Å Surface replica resolution of 1 µm sufficient to characterize the RF surfaces J. Knobloch, et al., “High Field Q Slope in Superconducting Cavities Due to Magnetic Field Enhancement at Grain Boundaries,” in Proc. of 9th Workshop on RF Superconductivity, Santa Fe, New Mexico,1999, pp. 77-91 October 4, 2010

13. Surface replica – extraction and surface scan Ribbon Silicone Feature on silicone Silicone pouring into an open half cell with a string embedded Visually indentify the features Epoxy after casting on silicone Feature’s 3D shape after profilometer scanning October 4, 2010

14. 11 Surface replica - cavity RF performance verification No performance degradation Ethanol rinsing and HPR after molding M Ge, G Wu, D Burk, J Ozelis, E Harms, D Sergatskov, D Hicks, and L D Cooley, Routine characterization of 3-D profiles of SRF cavity defects using replica techniques, Manuscript submitted to Journal of Superconducting Science and Technology. October 4, 2010

15. 12 Cavity processing/testing summary 1-cell cavity processing highly optimized to be free of field emission. It allows “clean” studies of 1-cell cavity performance. October 4, 2010

16. 13 Cavity performances and surface features Cavities represent six categories Fine grain BCP Large grain BCP Single crystal BCP Fine grain light EP Fine grain heavy EP Fine grain Tumble polishing plus light EP Cavities have apparent geometric defects Bumps Pits Scratches or grain boundaries Cavities have no visible defects Surface roughness in Weld seam Heat Affected Zone (HAZ) Normal area October 4, 2010

17. EB-Welding seam Surface defect limits cavity performance at low field 1.3GHz 9-cell cavity TB9ACC017 Y-Y’ X-X’ X-X’ Y-Y’ TB9ACC017 quenched at 12.3MV/m, Pit was found at Cell #4 equator 180 deg region (quench location), the pit is 150 µm deep and 200 µm wide on the top. October 4, 2010 14

18. Surface defect limits cavity performance at low field Spot (a) Spot (b) Spot A height 160µm, diameter 700µm on bottom AES001 quenched at 15.6~21MV/m, Twin peaks were found at Cell #3 equator 169 deg region Spot B height 155µm, diameter 1000µm on bottom October 4, 2010 15

19. Surface defect limits cavity performance at high field Equator welding seam HAZ Equator welding seam 40.2 MV/m quenched at pit region TE1ACC003 Diameter: 400µm,Depth: 60µm 39MV/m quenched NOT at pit region TE1AES004 Diameter: 1300µm, Depth: 70µm A 15µm high tiny bump in the center. 16 October 4, 2010

20. Field enhancement factor with r/R model Hrf,critical = 180mT, Hp/Eacc=4.26 mT/(MV/m) V. Shemelin, H. Padamsee, “Magnetic field enhancement at pits and bumps on the surface of superconducting cavities”, TTC-Report 2008-07, (2008). 17 October 4, 2010

22. NingXia large grain RRR niobium

23. Post purified

24. BCP processed

26. BCP etching pits

27. Weld Pits

28. Steep grain boundariesP. Kneisel, “Progress on Large Grain and Single Grain Niobium – Ingots and Sheet and Review of Progress on Large Grain and Single Grain Niobium,” in Proc. of 13th Workshop on RF Superconductivity, Beijing, China, 2007. October 4, 2010

29. The measurement of grain boundary step A-A’ B-B’ Joint of grain boundary A-A’ Joint of grain boundary Welding seam Eacc reached 43MV/m The height of step on A-A’: 60µm The height of step on B-B’: 25µm B-B’ 19 Profile on A-A’ Optical image of PKU-LG1 Profile on B-B’ October 4, 2010

30. 20 Field enhancement factor with step model Severe grain boundary do not cause material imperfection in this cavity. Phonon peak in thermal conductivity helps to reduce the effect of the normal conducting region? J. Knobloch, et al., “High Field Q Slope in Superconducting Cavities Due to Magnetic Field Enhancement at Grain Boundaries,” in Proc. of 9th Workshop on RF Superconductivity, Santa Fe, New Mexico,1999, pp. 77-91 October 4, 2010

31. 21 Field enhancement factor Many cavity quenches are found that cannot be explained by magnetic field enhancement. Surface topology plays as secondary role in cavity performance limitation. Intrinsic material imperfections remain as primary cause for poor performance in SRF cavities. Severe grain boundary is not a concern for large grain cavities. October 4, 2010

32. Cavity surface roughness HAZ Welding seam Normal area Brand new TE1PAV001 (5 similar cavities) PKU single crystal BCP‘d cavity BCP'd 70µm NR-6 (3 similar) EP'd 40µm TE1ACC005 22 October 4, 2010

33. Cavity surface roughness Welding seam HAZ Normal area EP'd 120µm TE1ACC003 (8 similar) EP'd 150µm TB9ACC017 Tumbling 100-120µm TE1ACC004 (4 similar) PKU Large grain BCP‘d cavity 23 October 4, 2010

34. Surface roughness vs. Gradient 24 TESLA shape ,quenched but no obvious defects Cavity performance becomes less dependent on the surface roughness as the maximum magnetic field reached a plateau October 4, 2010

35. Investigation of intrinsic quench limits 25 Quench location Geometric Defect Cavity was cut open, samples are being analyzed – A. Romanenko 36.83 MV/m A work in progress ! Courtesy of G. Ciovati October 4, 2010

36. Influence of Cavity Topography on Thin-Film SRF Performance 26 For thin film RF surface, the top layer is vulnerable to magnetic field enhancement. The performance of such cavity will depend on the surface topography. Increasing the film thickness can help, but brings other unforeseen problems. The substrate cavity preparation is very important. October 4, 2010

37. Fermilab’s effort in cavity processing Tumble polishing Light EP Chemical mechanical polishing 27 BCP 150 µm Tumble 120 µm EP 40 µm C. Cooper, Tumble polishing, TTC 2010, Fermilab October 4, 2010

38. Chemical mechanical polishing (before) Fermilab collaborator Courtesy of CMPC Surface Finishes

39. Chemical mechanical polishing (after) Courtesy of CMPC Surface Finishes

40. Conclusions 30 Cavity replica can be a great technique to identify the quench site The defects can be classified as geometric imperfections or intrinsic to the material. Geometric imperfections may only play a secondary role in limiting cavity performance. Not all defects are necessarily harmful to cavity performance, other than bearing the risk of trapping acidic water during processing. Existing processing techniques can easily reduce the magnetic field enhancement (by reducing surface roughness) and enable cavities to reach high gradients in the absence of intrinsic material imperfections. Material study (cavity cut out) is a must to understand quench behavior further. Perfect substrate preparation remains a challenge for thin film SRF. G. Wu, M. Ge, P. Kneisel, K. Zhao, L. Cooley, J. Ozelis, D. Sergatskov and C. Cooper, “Investigations of Surface Quality and SRF Cavity Performance”, manuscript submitted to IEEE transactions on Applied Superconductivity. October 4, 2010

41. 31 Acknowledgements M. Ge, C. Ginsburg, A. Romanenko, L. Cooley, P. Kneisel, G. Ciovati, M. Morrone. R. Schuessler, D. Hicks, C. Thompson, D. Burke at Fermilab/MDTL Tom Reid, R. Murphy, D. Bice, C. Baker at ANL J. Ozelis, M. Carter, D. Marks, G. Kirschbaum, R. Ward at Fermilab/IB1. We also thank Mark Champion for his useful discussion and support. October 4, 2010

42. Extra slides 32

43. Silicone Balloon Surface replica – Introduction of molding material Cup Molding iris region Head Rail Stage Molding equator region October 4, 2010

44. Reveal pits profile on iris region Diameter: 400µm,Depth: 70µm TB9RI026 Spot A Spot A Spot B TB9jl002 Spot A: Diameter: 800µm,Depth: 180µm Spot B Spot B: Diameter: 550µm,Depth: 110µm A 20µm deep crevice in center 34

45. Pits geometry and cavity RF performance 12.3MV/m 39MV/m 36MV/m 35 When the pit is very deep, the risk of trapping acid water is very high. Smoothening out these geometric defects can have strong benefit for electropolishing

46. Reveal pits profile on iris region (Hollow structure) 36 0.19mm 4mm 0.8mm 0.18mm TB9jl002 Iris cell #6 & #7, 66deg Cavity Wall

47. Quench location variation due to frozen flux 37 Defect Previous quench site 37.72 MV/m

48. Roughness comparison Fine grain BCP’d cavity Eacc reached 26MV/m! Fine grain EP’d cavity Eacc reached 40MV/m! Large grain BCP’d cavity Eacc reached 43MV/m! 38