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October 16, 2007 SRF2007, Beijing
Review of SRF Materials Workshop, May 2007
1
Review of SRF Materials Workshop
Genfa Wu, Lance Cooley, Claire Antoine, and Helen Edwards
Fermilab
October 16, 2007 SRF2007, Beijing
Review of SRF Materials Workshop, May 2007
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Outline
Overview of the workshop
Highlights of new approaches, new results and new ideas
Highlights of the debate
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http://tdserver1.fnal.gov/project/workshops/RF_Materials/
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Overview of workshop
Workshop pushed by H. Edwards, D. Larbalestier, …
- H. Edwards
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Overview of workshop
73 registered participants from 5 national labs, 11 universities, 1 oversea national lab and 7 private businesses. 5 main topics.
Fundamentals of RF superconductivityMaterials properties and surface characterizationsNew materialsInnovative processing of materialsProduction of niobium
34 talks (9 review talks) in two days.
Workshop organized by H. Edwards, C. Antoine, G. Wu, supported by Technical Division, Fermilab
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Highlights of new things
Fundamental limitations Framework for superheated critical field by Sethna and PadamseeHigh field vortex dynamics by Gurevich
Multi-layer theory and experiment is converging Gurevich, Pellin et al., and Xi
Oxygen as a pollutant, and the role of baking Tian, Romenenko, and Eremeev
New approaches to surface characterizationMicrowave microscopy (Anlage et al., Wu et al.)Tunneling spectroscopy (Zasadzinski et al.)Studies of isolated grain boundaries (Sung et al.)
Alternative processing techniquesGas Cluster Ion Beam (GCIB) processing (Swenson et al.)Plasma processing for dry etching (Raskovic et al., Wu et al.)Acid-free surface polishing solutions (Crooks et al.)Ultra-smooth Chemical mechanical polishing (Muftu et al.)
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High light 1: Physics of the Ultimate RF critical magnetic field (superheating field)
Padamsee, Sethna
Operating RF field above Saito’s theoretical Hsh
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High light 1: Physics of the Ultimate RF critical magnetic field (superheating field)
Energy balance argument:Vortex line nucleation model gives an upper bound on the equilibrium critical field for vortex penetration, ∝Hc1
Energy balance does not discuss meta-stabilityModel is useful for inhomogeneities on the scale of the coherence length, but not as a fundamental limit for uniform, flat, pure superconductors
Line nucleation model inappropriate:Superheating is a prediction from the Abrikosov, G-L theory. It applies for T~Tc.Hysteresis (pinning) in magnetization gives unreliable answers for Hc1 and Hc
Hsh-rf = √2 Hsh-dc incorrect for phase transition field
Padamsee, Sethna
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Framework set to solve it
High light 1: Physics of the Ultimate RF critical magnetic field (superheating field)
Padamsee, Sethna
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A. Gurevich
Highlight 1: High field vortex dynamics
viscous vortex drag is not applicable for high fieldsDissipation is dominated by the jumps
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Gurevich, Pellin, Xi
Highlight 2: Multi-layer theory and experiment is converging
Atomic layer deposition is very attractive for multi-layer coating –ANL (TUP64)
H2, B2H6 Mg vapor
H0 = 2T
Hi = 50mT
d
Two step MgB2 coating possible –Penn State
0 50 100 150 200 250 3000
2
4
6
8
10
38 39 40 41 420.0
0.5
1.0
1.5
Tconset=41.0 K
Tczero =40.6 K
RRR=7.8
ρ (μ
Ω c
m)
Temperature (K)
ρ (μ
Ω c
m)
Tconset=41.0 K
Tczero =40.6 K
RRR=7.8
ρ (μ
Ω c
m)
T (K)
Multi-layer superconductor supports exceptionally high accelerating gradient with higher Q0
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Cut out studies by Romanenko
Highlight 3: Oxide, Baking and High field Q-slop
Nitrogen signal noticed
420 415 410 405 400 395 390
N 1s
Coun
ts, a
.u.
Binding energy, eV
Hot
Cold
Hot after bake
Nitrogen
23feb07b_2.dat
x 103
2
4
6
8
10
CPS
218 216 214 212 210 208 206 204 202 200 198Binding Energy (eV)22feb07c_2.dat
x 101
10
20
30
40
50
60
70
80
90
CPS
218 216 214 212 210 208 206 204 202 200 198Binding Energy (eV)
Hot
“Cold”
Nb0
Nb5+Oxide and metal/oxide interface do not contribute to the high field Q-slope losses
XPS
AES
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High T baking studies by Eremeev
Highlight 3: Oxide, Baking and High field Q-slop
Mainly Nb and a little NbO after HT baking
The bulk diffusion rates may not be applicable for diffusion near the surface
Remaining sub-oxides did not increase the residual resistance, Q-slope still present
Sample verification
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Baking studies and TEM, XPS by Tian
Highlight 3: Oxide, Baking and High field Q-slop
In-situ baked cavity remains good after air-exposure. Coupled with XPS data, one concludes sub-oxide not contributing High field Q-slope
Preliminary
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Highlight 4: New approaches for surface characterization
Low Temp. STM at Argonne:T=4.2 K, H=7 Te-beam for tip preparationIon gun for surface preparationCleaving stage in UHVin-situ transfer of sample and tipLEED/AugerMulti-source e-beam evaporator
E field configuration
Cu
Near-field scanning microwave microscopy, Wu et al., Kansas State Univ
FIB prepared sample for GB studiesEELS for Oxides or micro-chemical AnalysisSung et al. at Florida State University, WE105
ff0
|S21(f0)|2|S21(f0)|2
laser OFFlaser ON
resonator transmission
Δ|S12|2 ~ [JRF(x,y)]2
Laser Scanning MicroscopeAnlage et al., University of Maryland
Tunneling spectroscopyZasadzinski et al., ANL
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Highlight 4: New approaches for surface characterization
Studies on baked/unbaked samplePreliminary result show: change in gap was not evident. Inelastic behavior of interface changed.Oxide layer limited or no role?
-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14
0.2
0.4
0.6
0.8
1.0
1.2
1.4
T= 1.6 K
Nor
mal
ized
Con
duct
ance
Voltage [mV]
Unbaked : R = 24.25 kΩ, Δ = 1.55 meV, Γ = .41 meV. Baked : R = 12 kΩ , Δ = 1.55 meV, Γ = .31 meV
Nb NbOx NbO Nb2O5NbO2
Δ, ns
Tunneling spectroscopy
J. Zasadzinski
Gap expected to decrease
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Highlight 4: New approaches for surface characterization
Near-field scanning microwave microscopy, Wu et al., Kansas State Univ
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Laser Scanning MicroscopeAnlage et al., University of Maryland
Highlight 4: New approaches for surface characterization
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Highlight 5: Alternative processing techniques
Gas Cluster Ion Beams Alternative EP
Plasma Etching
Rotating platen
Polishing pad
Carrier
Wafer
Schematic of CMP operation
Material removal occurs due to particle abrasionof the chemically passivated wafer surface
Chemical mechanical polishing
D. Swenson et al., TEL E-pion Inc R. Crooks, Black Laboratory, LLC
M. Raskovic et al., Old Dominion University S. Muftu et al., Northeastern University
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Single cell 1K2 with Ti cage - Rs vs. 1/T
1
10
100
1000
0.23 0.28 0.33 0.38 0.43 0.48
1/T [1/K]
Rs
[nΩ
]
GCIB + HPR GCIB w ithout HPR BCS
5 nΩ
1.3GHz at Jlab
3.9GHz at Fermilab
Gas Cluster Ion Beams, D. Swenson
Highlight 5: Alternative processing techniques
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Highlight 5: Alternative processing techniques
Plasma etching, M. Raskovic (TUP73)
BCP-treated Plasma-treated (Sample No.1)
Morphology
Sample Wa (Å) WRMS (Å) Ra (μm) RRMS (μm) untreated 235 285 0.930 1.125
BCP 141 169 0.323 0.476 1 - plasma 20 26 0.246 0.309 2 - plasma 23 27 0.195 0.240 3 - plasma 11 16 0.295 0.353
Waviness (Wa) and roughness (Ra) of the samples’ surface determined by profilometer.
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Highlight 5: Alternative processing techniques
Plasma etching, M. Raskovic et al.
Moving mechanism risks contamination
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Highlight 5: Alternative processing techniques
Before
After
ECR plasma cleans away most of the sulfur in just 30 minutes
Marked contaminated witness sample inserted into the cavity
Plasma cleaning, G. Wu et al.
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Acid free EP, R. Crooks
Highlight 5: Alternative processing techniques
Salt 1
Salt 2
Salt 3
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Highlight 5: Alternative processing techniques
Acid free EP, R. Crooks
Salt 1 Salt 2
Salt 3 Salt 3
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Highlights of debate
Flux line penetration (AG) vs superheating (HP) GB problem or not?Heat transfer across interfaces?Are there ways to reduce the present requirements of near perfection?
1E+09
1E+10
1E+11
0 5 10 15 20 25 30 35 40
Eacc (MV/m)
Q0
Single crystal Large grain Fine grainT=2 K
Quench
G. Ciovati et al. Proc. of the 2006 LINAC, paper TUP033
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Highlights of debate
Medium field region (Linear) is still interesting subjectLack of broad understanding vs. interesting individual resultsProblem areas are local, but over what scale? … and what probe is best?Samples vs. cavities Will new material/coating ever deliver comparable results to solid niobium?
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Highlights of debate
Are oxides bad?Processing: show-and-tell, but no in-depth debateSingle crystal vs. very small grains Are there advantages for certain Nb texture?Are specifications for niobium adequate?
Does the specification determine a starting point from which imperfections in the process subtract?
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Thanks to C. Antoine, L. Cooley
New scientific results at SRF2007: ANL, FSU, MSU, W&M, FNAL, JLab, etc.
Next meeting will be in spring 2008
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Physics of the Ultimate RF critical magnetic field (superheating field)
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Physics of the Ultimate RF critical magnetic field (superheating field)
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A. Gurevich
Mechanisms for high field dissipation and multilayers for raising the ultimate fields
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A. Gurevich
Mechanisms for high field dissipation and multilayers for raising the ultimate fields
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Highlights of new things
XPS studies suggested oxide layer not a major role in low-T baking (by Tian)Oxide plays no significant role in hot spot (Eremeev, Romanenko)Tunneling Spectroscopy studies of niobium baking (Zasadzinski)Grain boundary studies by TEM,EELS and FIB prep (Sung)Microwave photo response by Laser Scanning microscope (Anlage)Mushroom cavity for high power measurement (Tajima)Near-field Scanning Microwave Microscopy (J.Wu)Phonon peak recovery can provide added insurance for unexpected cavity quenches. (Chandrasekaran)
Session 2: Material properties of superconductor & Surface Characterizations
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Surface oxide studies on solid Niobium for SRF Accelerators using variable photon energy XPS
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Surface oxide studies on solid Niobium for SRF Accelerators using variable photon energy XPS
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Surface Analysis of samples dissected from a cavity with high-field Q slope
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Surface Analysis of samples dissected from a cavity with high-field Q slope
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Surface Analysis of samples dissected from a cavity with high-field Q slope
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Correlation between XPS and temperature maps for nearly oxide-free niobium
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Correlation between XPS and temperature maps for nearly oxide-free niobium
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J.F. Zasadzinski, M. Iavarone, T. Proslier, K.E. Gray
Tunneling Spectroscopy and Surface Modification of Nb for SRF Cavity Development
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J.F. Zasadzinski, M. Iavarone, T. Proslier, K.E. Gray
Tunneling Spectroscopy and Surface Modification of Nb for SRF Cavity Development
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J.F. Zasadzinski, M. Iavarone, T. Proslier, K.E. Gray
Tunneling Spectroscopy and Surface Modification of Nb for SRF Cavity Development
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Z. H. Sung
Analytical electron microscopy studies and transport characteristics of large grain niobium for SRF cavity
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Z. H. Sung
Analytical electron microscopy studies and transport characteristics of large grain niobium for SRF cavity
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Z. H. Sung
Analytical electron microscopy studies and transport characteristics of large grain niobium for SRF cavity
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Imaging of Microwave Currents and Microscopic Sources of Nonlinearities in Superconducting Resonators
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Tsuyoshi Tajima
Critical magnetic field measurement of MgB2
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Tsuyoshi Tajima
Critical magnetic field measurement of MgB2
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Tsuyoshi Tajima
Critical magnetic field measurement of MgB2
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Tsuyoshi Tajima
Critical magnetic field measurement of MgB2
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S.K. Chandrasekaran
Heat transfer measurements of niobium for SRF cavities
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S.K. Chandrasekaran
Heat transfer measurements of niobium for SRF cavities
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Highlights of new things
Atomic layer deposition: an attractive method for surface engineering (Pellin)Highest quality MgB2 cavity coating is feasible (Xi)
Session 3: New Materials for the Future
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M. Pellin, J. Moore
Atomic layer deposition
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M. Pellin, J. Moore
Atomic layer deposition
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Xiaoxing Xi
MgB2 thin film and its application to RF cavities
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Xiaoxing Xi
MgB2 thin film and its application to RF cavities
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Highlights of new things
GCIB process was successful on other applications, can it be successful to SRF? (Swenson)Plasma etching, an alternative for surface etching (Raskovic, G.Wu)Alternative EP processes possible (Crooks)Chemical mechanical polishing as a pre-etching polishing (Muftu)
Session 4: (innovative) Processing of materials
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surface treatment of SRF cavities with Gas Cluster Ion Beams
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surface treatment of SRF cavities with Gas Cluster Ion Beams
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Zeke Insepov
surface treatment of SRF cavities with Gas Cluster Ion Beams
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Single cell 1K2 with Ti cage - Rs vs. 1/T
1
10
100
1000
0.23 0.28 0.33 0.38 0.43 0.48
1/T [1/K]
Rs
[nΩ
]
GCIB + HPR GCIB w ithout HPR BCS
5 nΩ
1.3GHz
3.9GHz
surface treatment of SRF cavities with Gas Cluster Ion Beams
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Plasma etching of Niobium surface
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Plasma etching of Niobium surface
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Plasma etching of Niobium surface
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ECR plasma: a possible in-situ cavity processing technique
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ECR plasma: a possible in-situ cavity processing technique
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Roy Crooks
Novel Surface Treatments for RRR Niobium
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Roy Crooks
Novel Surface Treatments for RRR Niobium
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Sinan Muftu
Chemical Mechanical Polishing for Obtaining Very Smooth Surfaces
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Sinan Muftu
Chemical Mechanical Polishing for Obtaining Very Smooth Surfaces
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Highlights of new things
Session 5: Niobium production
Chris Compton
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