review of srf materials workshop - cernreview of srf materials workshop, may 2007 1 review of srf...

October 16, 2007 SRF2007, Beijing

Review of SRF Materials Workshop, May 2007

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Review of SRF Materials Workshop

Genfa Wu, Lance Cooley, Claire Antoine, and Helen Edwards

Fermilab



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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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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


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


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


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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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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Near-field scanning microwave microscopy, Wu et al., Kansas State Univ



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Laser Scanning MicroscopeAnlage et al., University of Maryland




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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




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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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Plasma etching, M. Raskovic et al.

Moving mechanism risks contamination



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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


Salt 1

Salt 2

Salt 3



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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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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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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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Thank you



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Back up slides



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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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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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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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Z. H. Sung

Analytical electron microscopy studies and transport characteristics of large grain niobium for SRF cavity



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Z. H. Sung




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Z. H. Sung




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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




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Tsuyoshi Tajima




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Tsuyoshi Tajima




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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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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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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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65

Zeke Insepov




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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




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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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Session 5: Niobium production

Chris Compton



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Session 5: Niobium production

Chris Compton

review of srf materials workshop - cernreview of srf materials workshop, may 2007 1 review of srf...

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