Innovative Processing of materials

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(Innovative) Processing of materials

• SRF materials Workshop Fermilab May 23-24, 2007

Todays process is long, complex, expensive and
not very efficient
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Why do we need to process the cavities ?

• 1) Getting a good superconductor
• OOPS !? What is a good SC ?
• Empirically inferred with time
• Good thermal conductivity (need to use high RRR
  material)
• EB-welding, in very good vacuum (Nb good
  getter!)
• Low interstitials (dont anneal in poor vacuum,
  avoid hydrogen)
• No damage layer ? (need to chemically remove 100
  -200 mm of the surface before achieving good
  performances)
• No inclusion (metallic inclusion hot spot for
  sure !)
• Smooth surface ? (EP better than BCP)
• . ?
• Other suspects surface oxides, chemical
  residues, grain boundaries, adsorbed layers,

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Damage layer100-200 mm

• Origin previous mechanical history (rolling,
  deep drawing/spinning)
• Not controlled yet, batch to batch variations
• Various recipes tried
• Chemical etching (BCP)
• Quick, efficient, reproducible but rough
  surfaces
• But stuck _at_ 30 MV/m
• Problem roughness near the weld area ?
• Alternative solutions monoXstals, hydroforming
  (no welding seam, no roughness!)
• Electropolishing (EP)
• Slow, expensive, higher risk of H contamination
• Gives the best results?40mV/m
• Lack of reproducibility (aging of solution,
  chemical residues ?)
• Alternative EPs under study
• BCP EP
• need to remove 100 mm (EP) to achieve smooth
  surface
• Barrel polishing (mechanical) BCP/EP
• need to remove 100 mm (EP) to get rid of the
  damage layer

• Ideal surface processing
• removes 200 mm of internal surface
• no damage layer, no roughness
• no chemical contamination (e.g. hydrogen)

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Why do we need to process the cavities ?

• 2) Get a dust free surface to prevent filed
  emission
• (high electric field regions cavities irises)

• Emitting sites dusts, scratches
• Dust particles gather and weld together and to
  surface
• Local enhancement of E gt bE

Field emission is the main practical limitation
in accelerator operation
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Detail of the usual process (1/2)
Forming
WHY
COMMENT
Nb getter. Degraded RRR _at_ weld gt Q0/10
EB welding
Clean welding
Ti purification
Increase RRR
RRR 300-400 now commercially available
BCP EP
Remove damage layer (100-200 µm)
BCP limited to 30MV/m EP gt gt40 mV/m but
lack of reproducibility
Deep etching
hydrogen source wet processes Hydrogen
segregates at the surface and form hydrides (poor
SC)
Remove Hydrogen contamination
800C annealing
Light etching
Remove diffusion layer (O, C, N)
Diffusion layer lt 1µm
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Detail of the usual process (2/2)
Light etching
WHY
COMMENT
HF, H2O2, ethanol, degreasing,
Special rinse
Fight field emission gt rid of S (after EP)
HPR
Get rid of dust particles
Most convenient, but not sufficient
Ancillaries couplers antennas
In clean room. But re-contamination still possible
assembly
Get rid of the high field losses (Q-drop)
Mechanism not understood, concerns the first 10
nm of the material
Baking, 120C, 48h
Get rid of dust particles Due to assembly
Under development Ex dry ice cleaning, plasma
Post processing
RF test
He processing, HPP
Field emission
Field emission SRF accelerator plague !
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High pressure rinsing (HPR) 1/2

• ultra pure H2O, ultra filtered, 80-100 bars

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High pressure rinsing (HPR) 2/2

• HPR is due to mechanical effect of the droplets
• Fe is high enough to deform Nb (sl Nb
  150-200 MPa)
• post contamination after HPR is still possible
• HPR is not very efficient on S particles after
  EP (S embedded in organic material ?)

Before HPR
After HPR
M. Luong, PhD, 1998
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RF post processing He processing HPPP

• Helium processing
• Developed mainly _at_ CERN
• Helium gaz RF gt plasma
• Low efficiency, mainly low field
• High Peak Power processing (HPP)
• Concept developed _at_ Cornell burning out
  particles at high field
• Pulsed RF to prevent quench
• High power klystron or adjustable coupling
  (expensive)
• High risks limitations of the couplers, creation
  of stable emitters

Advantage in situ, after assembly
H.Padamsee et al., RF superconductivity for
accelerators, 1998
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High Peak Power processing (HPP)
HPP in a Cryomodule at ELBE, Rossendorf 1
HPP at Cornell on multicell cavities 2

• SCgtlong pulses to compensate filling time
• Need for high power or adjustable couplers
• Need for high power Klystron
• Was never tested for field higher than 25 MV/m
  (no power source available until recently)
• Reliability and thermal load issues

For ILC 10MW (1.565mS) klystron and 1MW power
coupler. Qext 3.5x10-6 Power could be available
but needs re-configuration of RF distribution
(expensive!!!)
1 A. Boechner et al., Proc. of EPAC06, p413,
2006 2 H.Padamsee et al., RF superconductivity
for accelerators, 1998 3 W-D. Moeller et al.,
Proc. of EPAC96, p2013, 1996
HPP power and field in Tesla 9-cell cavity
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Other post processing
Advantage applicable in situ, after assembly

• Dry ice cleaning
• Developed _at_ DESY
• Carbonic snow gt residuals CO2
• Mechanical effect, similar to HPR
• Applicable on horizontal cavities

• In situ ECR plasma cleaning
• Developed _at_ FNAL
• Applicable on equipped cavities usual antennas,
  RF source
• Need for a valve external magnet, no internal
  parts
• Cleaning of particles/surface layers by plasma
• Possible post/ (dry) oxidation to protect surfaces

courtesy of D.Reschke, DESY
ECR electron cyclotron resonance
courtesy of G. Wu, FNAL
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Coating as a bulk niobium cavity treatment

• Standard Nb coating methods

• Concept overlay bulk Nb defects by a good,
  very pure Nb layer, no wet process.
• Drawback thin layers are usually less good
  than bulk Nb
• Advantage substrate Nb gt annealing
  (recrystallization) possible
• Other drawback post contamination still
  possible (complex assembly/re-assembly process)

Vacuum Arc deposition 1
Biased magnetron sputtering 3

• M. J. Sadowski et al., The Andrzej Soltan
  Institute
• A-M. Valente et al., JLAB
• S. Calatroni, CERN

Electron cyclotron resonance plasma deposition 2
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Other possible processing methods

• Laser, electron or ion beam irradiation
• Recrystallization of the surface, vaporization of
  defects, particles
• Non-HF wet chemical etching, polishing, other
  recipes
• To replace EP
• Alternative rinsing (for S, organic
  contamination, EP specific)
• US degreasing
• Ethanol rinsing
• H2O2
• UV ozone
• Plasma processing/etching
• Electrohydrodynamic cleaning (corona plasma)
• Ion beam
• Ion cluster beam etching
• Ultrasonic, megasonic
• Better cleaning of sub micron particles

Field emission
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Conclusion

• Deep etching cannot be prevented, but better
  definition/specifications of the material could
  help to reduce it.
• Final treatment should produce smooth surface
  and be able to get rid of chemical residues as
  well as dust particles.
• In situ post processing should be developed
  since recontamination during assembly is still
  possible.
• Processing of ancillaries parts should also be
  addressed.
• New ideas are awaited