Wu - Surface topography investigation for niobium cavities and its implication for thin film

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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.

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  • Oscillating Superleak Transducer – OST. Also mention the procurement of Cornell/JLAB carbon T-map.
  • 22MV/m curve?
  • T-map was not used in this cavity. Quench location unknown
  • Change the another point about tehe model.
  • Change the another point about tehe model.
  • Add captions, and test results
  • Add results
  • Wu - Surface topography investigation for niobium cavities and its implication for thin film

    1. 1. October 4, 2010<br />1<br />Surface topology investigation for niobium cavities and its implication for thin film SRF<br />Genfa Wu<br />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 topology 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. <br />
    2. 2. 2<br />Outlines<br />9-cell cavity yield<br />ANL/FNAL facility and diagnostic capability<br />Quench localization, optical inspection and surface replica<br />Cavity processing/testing summary<br />Cavity performances and surface features<br />Cavity performances and surface roughness<br />Impact of surface topology for thin film/new material and Fermilab’s effort in cavity RF surface preparation<br />Summary<br />October 4, 2010<br />
    3. 3. 9-cell Cavity Performance & Stats<br />For cavities from established vendors, using optimized EP processing and handling procedures:<br />1st-pass cavity yield >35 MV/m is (29 +- 8) %<br />2nd-pass cavity yield >35 MV/m is (56 +- 1) %, where 2nd pass refers to any surface preparation<br />All cavities passing gradient requirement also pass Q0 requirement<br />Data from C. Ginsburg<br />Even after 2nd pass, less than 56% cavities yield at 35MV/m.<br />Surface defects affects SRF cavity superconductivity dramatically.<br />October 4, 2010<br />
    4. 4. 4<br />Argonne/Fermilab Cavity Processing Facility<br />Electro-Polishing<br />Ultrasonic Degreasing<br />High-Pressure Rinsing<br />Assembly & Vacuum Leak Testing<br />Courtesy of M. Champion<br />October 4, 2010<br />
    5. 5. 5<br />Fermilab Cavity diagnostics – optical inspection<br />Kyoto/KEK: standard ILC cavities. Questar: All style cavities<br />Image of TE1ACC006<br />Camera inspection system by Kyoto University/KEK<br />Image of 3C9FER001<br />Questar QM-1, originated at Cornell USAF-1951 seen by Questar <br />October 4, 2010<br />
    6. 6. 6<br />Fermilab Cavity diagnostics – T-map/OST<br />Cernox based fast thermometry (FNAL T&I Dept. ) and diode based T-map (A. Mukherjee et al.)<br />Oscillating Superleak Transducer (“second-sound” detectors from Cornell University – Z. Conway<br />October 4, 2010<br />
    7. 7. 7<br />Fermilab Cavity diagnostics – Surface replica<br />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.<br />Useful to assess surface preparation effectiveness by extracting surface roughness<br />Achieved resolution at the micron detail (to resolve features ~<10µm in diameter; ~1µm in depth).<br />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<br />M Ge, G Wu, D Burk, J Ozelis, E Harms, D Sergatskov, D Hicks, and L D Cooley, <br />Routine characterization of 3-D profiles of SRF cavity defects using replica techniques,<br />Manuscript submitted to Journal of Superconducting Science and Technology.<br />October 4, 2010<br />
    8. 8. Surface replica - resolution<br />Pit replica<br />A man-made pit 125µm deep, 300µm in diameter<br />profile of pit’s replica<br />profile of a pit on the Nb coupon<br />8<br />Overall, 1 µm detail can be replicated<br />October 4, 2010<br />
    9. 9. A discussion about surface detail<br />9<br />Does 1 micron resolution sufficiently characterize the RF surface?<br />As magnetic field at the edge of the defect approaches the thermal critical magnetic field, the magnetic flux then penetrates the corner area deeper, depending on the field enhancement factor (corner radius). <br />At this time, the defect can be divided into one lossy corner and a remaining flux-free body.<br />The effective corner curvature is in micron scale as in the case of normal conducting niobium.<br /><ul><li>High resolution profilometer (KLA Tencor Model P-16)
    10. 10. Probe tip with 0.8 µm diameter
    11. 11. Horizontal resolution < 1 µm
    12. 12. Vertical resolution ~1 Å </li></ul>Surface replica resolution of 1 µm sufficient to characterize the RF surfaces<br /> 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<br />October 4, 2010<br />
    13. 13. Surface replica – extraction and surface scan<br />Ribbon<br />Silicone<br />Feature on silicone<br />Silicone pouring into an open half cell with a string embedded<br />Visually indentify the features<br />Epoxy after casting on silicone<br />Feature’s 3D shape after profilometer scanning<br />October 4, 2010<br />
    14. 14. 11<br />Surface replica - cavity RF performance verification<br />No performance degradation<br />Ethanol rinsing and HPR after molding<br />M Ge, G Wu, D Burk, J Ozelis, E Harms, D Sergatskov, D Hicks, and L D Cooley, <br />Routine characterization of 3-D profiles of SRF cavity defects using replica techniques,<br />Manuscript submitted to Journal of Superconducting Science and Technology.<br />October 4, 2010<br />
    15. 15. 12<br />Cavity processing/testing summary<br />1-cell cavity processing highly optimized to be free of field emission.<br />It allows “clean” studies of 1-cell cavity performance.<br />October 4, 2010<br />
    16. 16. 13<br />Cavity performances and surface features<br />Cavities represent six categories<br />Fine grain BCP<br />Large grain BCP<br />Single crystal BCP<br />Fine grain light EP<br />Fine grain heavy EP<br />Fine grain Tumble polishing plus light EP<br />Cavities have apparent geometric defects<br />Bumps<br />Pits<br />Scratches or grain boundaries<br />Cavities have no visible defects<br />Surface roughness in <br />Weld seam<br />Heat Affected Zone (HAZ)<br />Normal area <br />October 4, 2010<br />
    17. 17. EB-Welding seam<br />Surface defect limits cavity performance at low field<br />1.3GHz 9-cell cavity TB9ACC017<br />Y-Y’<br />X-X’<br />X-X’<br />Y-Y’<br />TB9ACC017 quenched at 12.3MV/m, <br />Pit was found at Cell #4 equator 180 deg region (quench location), <br />the pit is 150 µm deep and 200 µm wide on the top.<br />October 4, 2010<br />14<br />
    18. 18. Surface defect limits cavity performance at low field<br />Spot (a)<br />Spot (b)<br />Spot A height 160µm, diameter 700µm on bottom <br />AES001 quenched at 15.6~21MV/m, <br />Twin peaks were found at Cell #3 equator 169 deg region<br />Spot B height 155µm, diameter 1000µm on bottom <br />October 4, 2010<br />15<br />
    19. 19. Surface defect limits cavity performance at high field<br />Equator<br />welding seam<br />HAZ<br />Equator welding seam<br />40.2 MV/m quenched at pit region<br />TE1ACC003<br />Diameter: 400µm,Depth: 60µm<br />39MV/m quenched NOT at pit region<br />TE1AES004<br />Diameter: 1300µm, Depth: 70µm <br />A 15µm high tiny bump in the center. <br />16<br />October 4, 2010<br />
    20. 20. Field enhancement factor with r/R model<br />Hrf,critical = 180mT, Hp/Eacc=4.26 mT/(MV/m)<br />V. Shemelin, H. Padamsee, “Magnetic field enhancement at pits and bumps on the surface of superconducting cavities”, TTC-Report 2008-07, (2008).<br />17<br />October 4, 2010<br />
    21. 21. 1-cell cavity: PKU-LG1<br />18<br /><ul><li>43 MV/m. 189 mT
    22. 22. NingXia large grain RRR niobium
    23. 23. Post purified
    24. 24. BCP processed
    25. 25. Low temperature baking</li></ul>Features found at multiple locations:<br /><ul><li>BCP stains
    26. 26. BCP etching pits
    27. 27. Weld Pits
    28. 28. Steep grain boundaries</li></ul>P. 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. <br />October 4, 2010<br />
    29. 29. The measurement of grain boundary step<br />A-A’<br />B-B’<br />Joint of grain boundary<br />A-A’<br />Joint of grain boundary<br />Welding seam<br />Eacc reached 43MV/m<br />The height of step on A-A’: 60µm<br />The height of step on B-B’: 25µm<br />B-B’<br />19<br />Profile on A-A’<br />Optical image of PKU-LG1<br />Profile on B-B’<br />October 4, 2010<br />
    30. 30. 20<br />Field enhancement factor with step model<br />Severe grain boundary do not cause material imperfection in this cavity.<br />Phonon peak in thermal conductivity helps to reduce the effect of the normal conducting region? <br /> 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<br />October 4, 2010<br />
    31. 31. 21<br />Field enhancement factor<br />Many cavity quenches are found that cannot be explained by magnetic field enhancement.<br />Surface topology plays as secondary role in cavity performance limitation. <br />Intrinsic material imperfections remain as primary cause for poor performance in SRF cavities.<br />Severe grain boundary is not a concern for large grain cavities.<br />October 4, 2010<br />
    32. 32. Cavity surface roughness<br />HAZ<br />Welding seam<br />Normal area<br />Brand new<br />TE1PAV001<br />(5 similar cavities)<br />PKU single crystal BCP‘d cavity<br />BCP'd 70µm<br />NR-6<br />(3 similar)<br />EP'd 40µm<br />TE1ACC005<br />22<br />October 4, 2010<br />
    33. 33. Cavity surface roughness<br />Welding seam<br />HAZ<br />Normal area<br />EP'd 120µm<br />TE1ACC003<br />(8 similar)<br />EP'd 150µm<br />TB9ACC017<br />Tumbling <br />100-120µm<br />TE1ACC004<br />(4 similar)<br />PKU Large grain BCP‘d cavity<br />23<br />October 4, 2010<br />
    34. 34. Surface roughness vs. Gradient<br />24<br />TESLA shape ,quenched but no obvious defects<br />Cavity performance becomes less dependent on the surface roughness as the maximum magnetic field reached a plateau<br />October 4, 2010<br />
    35. 35. Investigation of intrinsic quench limits <br />25<br />Quench location<br />Geometric Defect<br />Cavity was cut open, samples are being analyzed – A. Romanenko<br />36.83 MV/m<br />A work in progress !<br />Courtesy of G. Ciovati<br />October 4, 2010<br />
    36. 36. Influence of Cavity Topology on Thin-Film SRF Performance <br />26<br />For thin film RF surface, the top layer is vulnerable to magnetic field enhancement. The performance of such cavity will depend on the surface topology. Increasing the film thickness can help, but brings other unforeseen problems. <br />The substrate cavity preparation is very important. <br />October 4, 2010<br />
    37. 37. Fermilab’s effort in cavity processing<br />Tumble polishing<br />Light EP<br />Chemical mechanical polishing<br />27<br />BCP 150 µm<br />Tumble 120 µm<br />EP 40 µm<br />C. Cooper, Tumble polishing, TTC 2010, Fermilab<br />October 4, 2010<br />
    38. 38. Chemical mechanical polishing (before)<br />Fermilab collaborator<br />Courtesy of CMPC Surface Finishes<br />
    39. 39. Chemical mechanical polishing (after)<br />Courtesy of CMPC Surface Finishes<br />
    40. 40. Conclusions<br />30<br />Cavity replica can be a great technique to identify the quench site <br />The defects can be classified as geometric imperfections or intrinsic to the material. <br />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. <br />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.<br />Material study (cavity cut out) is a must to understand quench behavior further.<br />Perfect substrate preparation remains a challenge for thin film SRF.<br />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.<br />October 4, 2010<br />
    41. 41. 31<br />Acknowledgements<br />M. Ge, C. Ginsburg, A. Romanenko, L. Cooley, P. Kneisel, G. Ciovati, M. Morrone.<br />R. Schuessler, D. Hicks, C. Thompson, D. Burke at Fermilab/MDTL<br />Tom Reid, R. Murphy, D. Bice, C. Baker at ANL<br />J. Ozelis, M. Carter, D. Marks, G. Kirschbaum, R. Ward at Fermilab/IB1. <br />We also thank Mark Champion for his useful discussion and support.<br />October 4, 2010<br />
    42. 42. Extra slides<br />32<br />
    43. 43. Silicone<br />Balloon<br />Surface replica – Introduction of molding material <br />Cup<br />Molding iris region<br />Head<br />Rail<br />Stage<br />Molding equator region<br />October 4, 2010<br />
    44. 44. Reveal pits profile on iris region<br />Diameter: 400µm,Depth: 70µm<br />TB9RI026<br />Spot A<br />Spot A<br />Spot B<br />TB9jl002<br />Spot A: Diameter: 800µm,Depth: 180µm<br />Spot B<br />Spot B: Diameter: 550µm,Depth: 110µm<br />A 20µm deep crevice in center<br />34<br />
    45. 45. Pits geometry and cavity RF performance<br />12.3MV/m<br />39MV/m<br />36MV/m<br />35<br />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 <br />
    46. 46. Reveal pits profile on iris region (Hollow structure)<br />36<br />0.19mm<br />4mm<br />0.8mm<br />0.18mm<br />TB9jl002 Iris cell #6 & #7, 66deg<br />Cavity Wall<br />
    47. 47. Quench location variation due to frozen flux<br />37<br />Defect<br />Previous quench site<br />37.72 MV/m<br />
    48. 48. Roughness comparison<br />Fine grain BCP’d cavity<br />Eacc reached 26MV/m!<br />Fine grain EP’d cavity<br />Eacc reached 40MV/m!<br />Large grain BCP’d cavity<br />Eacc reached 43MV/m!<br />38<br />
    49. 49. Optical features of PKU-LG1<br />June 1, 2009<br />39<br /><ul><li>BCP acid stains</li></ul>This picture covers 10mm in length<br />
    50. 50. Optical features of PKU-LG1<br />June 1, 2009<br />40<br /><ul><li>BCP etching pits</li></li></ul><li>Optical features of PKU-LG1<br />June 1, 2009<br />41<br /><ul><li>Pits in weld and HAZ</li></li></ul><li>Optical features of PKU-LG1<br />June 1, 2009<br />42<br /><ul><li>BCP enhanced grain boundary</li></li></ul><li>The measurement of intra grain etching pits<br />43<br />Optical image of PKU-LG1<br />A result from surface scanning profilometer<br />Cross section view<br />BCP etching pits, while interesting, do not affect cavity performance<br />

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