Material issues for SRF applications

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Material issues for SRF applications
2
Cavity Figures of merit
G geometric factor Eacc depends on the shape

• Superconductivity ! RS(Nb) some nO (_at_ 2K)
  ( RS(Cu) some 100 µO)
• Esurf 106-107 MV/m
• Isurf 109-1010 A/m2
• lL 50 nm

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RF Superconductivity RS
RS(Nb) qqs nO (_at_ 2K) ? 0 (because RF !)

• ? RBCSgt ? D, ? l, ? Tc (High Tc SC ?)
• ? RResgt ? magnetic shielding, ? GB losses (High
  Tc oups !), ? purity (?)

4
Purity high RRR material required , why ?

• not for superconductivity ! (in theory ? rn, ? ?
  RBCS Rs, but 2nd order effect)
• ? thermal conductivity kT
• but
• ? grain size
• ? mechanical resistance
• ? cost
• ? ? Optimum ? .

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RF SC Superheating field
intrinsic limitation phase transition
Pb
Nb
2.5
Normal state
2.0

• RF period10-9 s
• bulk vortices nucleation 10-6 s

Mixed state
1.5
Superheated state
1.0

• No mixed state in RF
• RF critical field doesnt depend on Hc2

Meissner state
0.5
Type II
Type I
0.6
1.2
1.6
2.0
0
0.4
GL parameter k ( l/x)
but it is close to it for Nb
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Cavities limitations dissipation, Hsh
Nb Limits _at_ 2 K, 1.3 GHz Eacc 55 MV/m (? 5
MV/m ?) Jc 109-1010 A/m2 (105-106 A/cm2) (Tesla
Shape Hc/Eacc 42 Oe/(MV/m))
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Practical limitations

• Before being limited by superconductivity, there
  are several trivial limits
• Mechanical properties
• Welding
• Magnetic shielding of cryostats
• Field emission

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Cold mechanical properties

• Mechanical Properties
• Recent breaking of an antenna (3.9 GHz coupler)

• Diagnostic brittle fracture, but precursor
  cracks during processing ?
• Recommendation we need to know better cold and
  room temperature mechanical properties of Nb

after cold RF test
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Mechanical properties

• Mechanical Properties 2
• Recent Forming Problems at AES Nb too hard,
  spring back, 6 passes vs 1, ovalization.

Deviation from the specified shape

• Microstructure Mechanical Properties studied
  (MSU)
• Diagnostic non fully recrystallized material
• Recommendations
• Re-annealing of the batch ( 200 sheets)
• QA delivered material should meet tightly
  specifications
• We must work with the suppliers to help them to
  meet specification

Non equiaxe grains gt recrystallization not
completed
High yield strength gt still some strain
hardening gt poor formability
before HT
after HT
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15 years of evolution in SRF cavities
or why it is effective/ necessary/useful to
do basic RD along with technological developments
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15 years Q-disease
750-800C annealing
Annealed _at_ 800C, 2h
Not annealed
Hydrogen !?
Saclay - B. Bonin, et al. RFSC ' 91
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ERDA /HFS hydrogen profiling
some 0.1 to 1 at. (10000 x Hbulk !!!)

• H up-take machining, heavy BCP, (EP!), H2O
• H Surface segregation
• Hydride forming (NbHx) _at_ 100 K
• Annealing gt purification re-crystallization
• Still an issue sometimes gt process or unknown
  j?

oxide-metal interface

• Hydrogen up take with time
• up annealed sample
• down same sample after 18 month in air

• Hydrogen up take with HCl (10 days)
• up normal
• down recrystallized sample

13
15 years Field emission
High Pressure Rinsing
With HPR
Without HPR
Quench, no e-
Limitation / e-
CERN - P. Bernard et al. EPAC ' 92
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Field Emission
Emitting sites dusts, scratches
Scratch edge
Scratch end
Before HPR After
Dust particles gather and weld together and to
surface ? local enhancement of E

• HPR suppress emitting sites up to Epeak
  80-100 MV/m
• Its a mechanical effect

4 PhDs _at_ Saclay (A. Curtoni, J. Jimenez, J.
Tan, M. Luong) ? role of dust particles,
scratches role efficiency of cures.
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15 years titanization
Optimization of purification annealing
Cornell - H. Padamsee et al. SRF 89 Saclay Safa
H. et al., J. Al. Comp. 232 (1996) 281.
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Thermal behavior lT ? RRR ? purity

• Purification annealing with a getter (Ti)
• DG (TiO2) lt DG (Nb2O5)
• Moderate vacuum, temperature
• Diffusion limited ? Practical for macroscopic
  objects

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15 years Q-slope
Baking 110C/60 h
Baked
Not baked
Saclay - B. Visentin et al. EPAC ' 98
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15 years EP (baking!)
BCP !
EP
baking
KEK - K. Saito et al. RFSC ' 89 DESY-CERN-Saclay
- L.Lilje et al, TTF report, 1999
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Comparison BCP/EP Thats a surface effect
BCP degrades EP results

• Issues
• Eacc gt 40 MV/m cannot be reached without
  baking
• High field dissipation ? quench mechanism

KEK - K. Saito et al. RFSC ' 97
Buffer Chemical Polishing / Electro-Polishing
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Baking optimization
B. Visentin et al, Saclay

• 120 C, 48 h, UHV baking is long and costfull
• 120 C, 48 h, in situ baking (clean air) has
  proven as effective
• 145 C, 3 h, UHV baking (or Ar) has proven
  effective
• But recently at JLab 120 C, 12 h, UHV baking
  OK ?
• and 120 C, 3 h on monocrystal cavity OK ?
• There is room for optimization !

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Todays issues

• Superconductivity
• High field dissipation
• Quench
• Material issues
• Monocrystal/ large grain cavities
• Shape optimization (field emission)
• Mechanical properties

Real Surface
Theoretical Surface
lL
lL depth penetration where dissipation
occurs. For Nb 50-100 nm
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High field dissipation general

• Baking effect preserved after
• Air exposure (up to 46 months)
• HF rinsing and/or HPR
• anodization
• Slope comes back after any (light) etching,
  250nm oxipolishing
• Bulk properties of the SC are not changed (except
  l)
• Similar results baking 110C/60 h vacuum and
  air, 145C/3 h vacuum only
• Only 3-6 hours for monocrystalline cavities
  !!!????

• Results consistent with
• thermal diffusion of light impurities
• very superficial effect inside the SC (not
  oxide)
• no fundamental ? EP/BCP

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High field dissipation what diffusion species?

• Glow Discharge Luminescence (GDL)

• Elastic Recoil Detection Analysis (ERDA)

Results consistent with thermal diffusion of O
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High field dissipation chasing interstitial O
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High field dissipation chasing interstitial O
XPS Principal Components Analysis
aNb2O5 bNbO2 gNbO dNb4O eNb6O hNb ?
At least 6 statistically significant components
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Interstitials oxygens (Oi)

• Segregation near metal-oxyde interface
• Oi ranging from 10 At Arfaoui to 70 At
  Hellwig (5/70.714)
• Origin
• Upon oxydation competition between oxidation/Oi
  injection TB, Halbritter, Arfaoui, Hellwig...
• Thermal diffusion (upon cooling) TB, ...

Local distorsion in the close neighborhood of
randomly distributed defects (1 O) BCC ?
trigonal w phase, seen on Nb monocrystal by
diffuse scattering Dosch (bulk),
Delheusy(surface,)
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Depth-sensitive diffuse scattering in the near
surface region
Nb oxide
Scattering depth ? f(af,ai)
Inside the oxide no signature of the distorsion
Diffuse scattering intensity evolution for
different scattering depths.
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High field dissipation chasing O or C ?

• No large observable diffusion of O (on that
  scale) !

SIMS

• Some C diffusion!!????
• Short baking time for monocrystalline cavities
  !????

• Need for sensitivity and depth resolution

Atom-probe tomography (APT)
Collbn NUniversity
Cleaner interface gt ? SC behavior !?
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Influence of interstitial on superconducting gap ?
10 decrease in D gt Rs x10
Photoemission
Point contact spectroscopy
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Quench mechanisms

• thermal/magnetic instability
• local defect
• not affected by
• Baking
• Rinsing (H2O, HF, HPR)
• Affected by
• Purification annealing
• Surface etching (EP/BCP)

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Replica _at_ the quench site
Size of the defect ? 550 µm x ? 15 µm
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Replica _at_ the quench site
a) first quench site, b) same area after 20 µm
(quench site _at_ a new location ) c) new quench
location.

• Morphological features not observed on EP
  cavities
• gt quench origin is ? ???

Local morphology is consistent for explaining the
quench
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Grain boundaries and roughness

• Annealed material with grain ? 1-2 mm
• Small grain material with ? 70 mm

EP ?
BCP ?
b 1.4
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Grain boundaries evidence of preferential
diffusion
apparently depending on GB orientation
1) Purification annealing Nb 1µm Ti
2) Chemical etching 15 µm
2) PIXE imaging ? (probe size 1µm) ?some
1000 Atppm !!!
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Morphological effect or depleted SC ?
What is the problem with GB ?
Flux penetration _at_ GB _at_ artificial notch
Collabo. WU/FSU
A. Polyanskii et al, WU/FSU
There is a local field enhancement due to
roughness
Saturation-field H0 gives information on
de-pairing Jd of SC GB
Sung Hawn
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Metallurgy issues- mono Xstal/large grain material

• GB Issues
• Segregation can play an important role in the
  normal state GB conductivity
• Depends strongly on disorientation
• gt work on monocrystals !?

Welding _at_ triple point ?
collabn, MSU
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CONCLUSION

• RD is a way to confirm/ optimize (/choose
  among) empiric improvements
• We need to master the supply of adequate
  material (purity, mechanical properties, )
• We need to master surface processing and field
  emission
• The physics behind ultimate limitations is
  still not well understood. We need explanation
  for
• Baking effect
• Quench
• Nice fundamental physics AND practical
  applications!

And Some Perspectives
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Surface processing

• Field emission (particle contamination) is still
  the major practical limitation source

• What are the possible sources?
• Bad control of the wet process particle
  counting is not enough
• Bad control of the ancillaries e.g. cleaning of
  couplers
• Contamination during assembly long, complex,
  manually made
• Absence of post processing solution
• Recommendations
• Develop new designs/tooling to ease assembling
• Collabn Jlab
• Develop post processing applicable to
  assembled cavities
• e.g. Plasma cleaning w ECR plasma
• New designs of flanges, couplers?

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Beyond bulk niobium nano-composite
superconductors

• Theory (FSU - National High Magnetic Field Lab)
• MgB2 (ANL, Penn State), NbN (ANL, JLAB), Nb3Sn
  (ANL, JLAB)

Higher-Hc SC
Nb (has the highest Hc1)
A. Gurevich, APL 2005
Higher Tc thin layers provide magnetic screening
of Nb
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Superconductivity limits

• Superconductivity limits
• The theoretical limits for RF superconductivity
  arent well known

• What causes the high field losses/ hot spots ?
• Morphology ?
• Grain boundaries
• Surface contamination (O)
• Recommendations
• Basic RD on superconductivity
• e.g. Hot Spot Model A. Gurevich (Colln w FSU)

Heat affected zone

• Effect of trapped vortices
• Heat source can be very small (nm )
• Thermally affected zone 5 mm and growing with
  B!

Trapped vortices