wu - surface topography investigation for niobium cavities and its implication for thin film
DESCRIPTION
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.TRANSCRIPT
1October 4, 2010
Surface topography investigation for niobium cavities and its implication for
thin film SRFGenfa 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.
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Outlines
9-cell cavity yieldANL/FNAL facility and diagnostic capability
Quench localization, optical inspection and surface replicaCavity processing/testing summary
Cavity performances and surface featuresCavity performances and surface roughnessImpact of surface topology for thin film/new material and Fermilab’s effort in cavity RF surface preparationSummary
October 4, 2010
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 preparationAll 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
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Argonne/Fermilab Cavity Processing Facility
Electro-PolishingUltrasonic Degreasing
High-Pressure Rinsing Assembly & Vacuum Leak TestingCourtesy of M. Champion
October 4, 2010
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Fermilab Cavity diagnostics – optical inspection
Camera inspection system by Kyoto University/KEK
Questar QM-1, originated at Cornell USAF-1951 seen by Questar
Kyoto/KEK: standard ILC cavities. Questar: All style cavities
Resolution (µm)
Questar ~10
Kyoto/KEK ~25
Image of TE1ACC006
Image of 3C9FER001
October 4, 2010
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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
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 roughnessAchieved resolution at the micron detail (to resolve features ~<10µm in diameter; ~1µm in depth).
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Fermilab Cavity diagnostics – Surface replica
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
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
A man-made pit 125µm deep, 300µm in diameter Pit replicaµm
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Length = 0.8 mm Pt = 128 µm Scale = 200 µm µm
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Length = 0.8 mm Pt = 128 µm Scale = 200 µm
profile of a pit on the Nb coupon profile of pit’s replica
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Surface replica - resolution
Overall, 1 µm detail can be replicatedOctober 4, 2010
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A discussion about surface detail
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
Does 1 micron resolution sufficiently characterize the RF surface?
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).
At this time, the defect can be divided into one lossy corner and a remaining flux-free body.
The effective corner curvature is in micron scale as in the case of normal conducting niobium.
Surface replica resolution of 1 µm sufficient to characterize the RF surfaces
High resolution profilometer (KLA Tencor Model P-16)
Probe tip with 0.8 µm diameter Horizontal resolution < 1 µm Vertical resolution ~1 Å
October 4, 2010
Epoxy after casting on silicone
Ribbon
SiliconeFeature on silicone
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Feature’s 3D shape after profilometer scanning
Silicone pouring into an open half cell with a string embedded
Visually indentify the features
Surface replica – extraction and surface scan
October 4, 2010
Ethanol rinsing and HPR after molding
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Surface replica - cavity RF performance verification
0 10 20 30 401.000E+08
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TE1AES004 - Q vs EComparison before and after applying molding material
2/9/20094/10/2009
Eacc [MV/m]
Q0
No performance degradation
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
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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
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Cavity performances and surface features
Cavities represent six categoriesFine grain BCPLarge grain BCPSingle crystal BCPFine grain light EPFine grain heavy EPFine grain Tumble polishing plus light EP
Cavities have apparent geometric defectsBumpsPitsScratches or grain boundaries
Cavities have no visible defects Surface roughness in
Weld seamHeat Affected Zone (HAZ)Normal area
October 4, 2010
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Length = 0.623 mm Pt = 148 µm Scale = 160 µm
1.3GHz 9-cell cavity TB9ACC017
EB-Welding seam
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.
Surface defect limits cavity performance at low field
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Length = 0.645 mm Pt = 143 µm Scale = 160 µm
October 4, 2010
Spot (a)
Spot (b)
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Length = 0.756 mm Pt = 159 µm Scale = 170 µm
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Length = 1 mm Pt = 155 µm Scale = 160 µm
AES001 quenched at 15.6~21MV/m, Twin peaks were found at Cell #3 equator 169 deg region
Spot A height 160µm, diameter 700µm on bottom
Spot B height 155µm, diameter 1000µm on bottom
Surface defect limits cavity performance at low field
15October 4, 2010
Diameter: 400µm,Depth: 60µm
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Length = 1.49 mm Pt = 81.9 µm Scale = 100 µm
Diameter: 1300µm, Depth: 70µm A 15µm high tiny bump in the center.
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Length = 0.5 mm Pt = 63.9 µm Scale = 70 µm
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40.2 MV/m quenched at pit region
39MV/m quenched NOT at pit region
TE1ACC003
TE1AES004
Equator welding seam
Equator
welding seam
HAZ
Surface defect limits cavity performance at high field
16October 4, 2010
Field enhancement factor with r/R model
r/R h simulation h meas.
TB9ACC017 ≈0.14 ≈2.2 ≈3.4
TE1ACC003 ≈0.23 ≈1.8 ≈1.17
TE1AES004 ≈0.96 ≈1.2 ≈1.08
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).
17October 4, 2010
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1-cell cavity: PKU-LG1
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.
43 MV/m. 189 mT
NingXia large grain RRR niobium
Post purified BCP processed Low temperature baking
Features found at multiple locations: BCP stains BCP etching pits Weld Pits Steep grain boundaries
October 4, 2010
A-A’
Joint of grain boundary
Eacc reached 43MV/m
The height of step on A-A’: 60µmThe height of step on B-B’: 25µm
Joint of grain boundary
Welding seam
B-B’
The measurement of grain boundary step
A-A’
B-B’
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Profile on A-A’
Profile on B-B’
Optical image of PKU-LG1
October 4, 2010
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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?
October 4, 2010
Cavity Computed field enhancement
factor
Measured maximum H field
[mT]
PKU-LG1 1.6 185
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
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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
Cavity Feature Computed field enhancement factor
Measured maximum H field [mT]
Max. H field from field enhancement
factor [mT]TB9ACC017 Pit 2.2 54.1 119
AES001 Bump 1.5 96.8 145.2TE1ACC003 Pit 1.8 154 277 ?TE1AES004 non-quench pit 1.2 168.0 201.6
PKU-LG1 Grain boundary 1.6 185 296 ?
Cavity surface roughnessWelding seam HAZ Normal area
BCP'd 70µmNR-6(3 similar)
EP'd 40µmTE1ACC005
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22October 4, 2010
Cavity surface roughnessWelding seam HAZ Normal area
EP'd 120µmTE1ACC003(8 similar)
EP'd 150µmTB9ACC017
Tumbling 100-120µmTE1ACC004(4 similar)
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23October 4, 2010
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Brand newTE1ACC006
BCP'd 70µmNR-6
EP'd 40µmTE1ACC005 EP'd 120µm
TE1ACC003
Tumbling 100-120µmTE1ACC004
Brand newTE1PAV001
PKU Large grain BCP'd cavity
PKU Single crys-tal BCP'd cavity
Surface roughness vs. Gradient
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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
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Investigation of intrinsic quench limits
36.83 MV/m
Geometric Defect
Courtesy of G. Ciovati
Cavity was cut open, samples are being analyzed – A. Romanenko
Quench location
A work in progress !
October 4, 2010
Influence of Cavity Topography on Thin-Film SRF Performance
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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
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Fermilab’s effort in cavity processing
Tumble polishingLight EPChemical mechanical polishing
BCP 150 µm
Tumble 120 µm
EP 40 µm
October 4, 2010C. Cooper, Tumble polishing, TTC 2010, Fermilab
Chemical mechanical polishing (before)
Courtesy of CMPC Surface FinishesFermilab collaborator
Courtesy of CMPC Surface Finishes
Chemical mechanical polishing (after)
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Conclusions
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
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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/MDTLTom Reid, R. Murphy, D. Bice, C. Baker at ANLJ. 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