1claire antoinecea/saclay - fermilab (innovative) processing of materials srf materials workshop...

1Claire Antoine CEA/Saclay - Fermilab

(Innovative) Processing of materials

SRF materials Workshop Fermilab May 23-24, 2007

Today’s process is long, complex, expensive … and not very efficient


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 (don’t anneal in poor vacuum, avoid hydrogen…)

No damage layer ? (need to chemically remove 100 -200 m 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,…


Damage layer:100-200 m 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 @ ~ 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 m (EP) to achieve smooth surface

Barrel polishing (mechanical) + BCP/EP: need to remove ~ 100 m (EP) to get rid of the damage layer…

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


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 =>E

Field emission is the main practical limitation in accelerator operation

~ 3

~ 100-500

Ni particles


Detail of the usual process (1/2)

FormingFormingWHY COMMENT

EB weldingEB welding Clean weldingNb = getter. Degraded RRR @ weld => Q0/10

Ti purificationTi purification

Deep etchingDeep etching

Increase RRR RRR 300-400 now commercially available

BCP

EP

Remove damage layer (100-200 µm)

BCP limited to ~ 30MV/m; EP => >40 mV/m but lack of reproducibility

800°C annealing800°C annealing Remove Hydrogen contamination

hydrogen source : wet processesHydrogen segregates at the surface and form hydrides (poor SC)

Diffusion layer < ~1µmLight etchingLight etching Remove diffusion layer (O, C, N)


Detail of the usual process (2/2)

WHY COMMENT

HPRHPR

HF, H2O2, ethanol, degreasing,…

Fight field emission gt rid of S (after EP)……Special rinseSpecial rinse

……Light etchingLight etching

Get rid of dust particles Most convenient, but not sufficient

Ancillaries: couplers antennas…

In clean room. But re-contamination still possible

Baking, 120°C, 48hBaking, 120°C, 48h Get rid of the high field losses (Q-drop)

Mechanism not understood, concerns the first 10 nm of the material

assemblyassembly

Post processingPost processing Get rid of dust particlesDue to assembly

Under developmentEx: dry ice cleaning, plasma

RF testRF test

He processing, HPPHe processing, HPP Field emission Field emission: SRF accelerator plague !


High pressure rinsing (HPR) 1/2

ultra pure H2O, ultra filtered, 80-100 bars

MPa 14 2

v

2f

MPa 250

vu fs

(Droplets)

(Flow)

Fe

vf ~ 160 m/s

Fe

100 bars

Size

(m)

Fe

(N)

Fad

(N)

0.1 10-9 10-9

1 10-6 10-8

10 10-4 10-6

Particles are displaced when Fe > Fad


High pressure rinsing (HPR) 2/2

• HPR is due to mechanical effect of the droplets

• Fe is high enough to deform Nb (l 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]


RF post processing : He processing & HPPP

Helium processing

Developed mainly @ CERN

Helium gaz + RF => plasma

Low efficiency, mainly low field

High Peak Power processing (HPP)

Concept developed @ 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]


High Peak Power processing (HPP)

[1] A. Boechner et al., Proc. of EPAC06, p413, 2006[2] W-D. Moeller et al., Proc. of EPAC96, p2013, 1996

HPP in a Cryomodule at ELBE, Rossendorf [1] HPP for C19 at DESY [2]

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!!!)

HPP power and field in Tesla 9-cell cavity

SC=>long 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


Other post processingAdvantage: applicable in situ, after assembly

Dry ice cleaning

Developed @ DESYCarbonic snow => residuals = CO2

Mechanical effect, similar to HPR

Applicable on horizontal cavities

In situ ECR plasma cleaning

Developed @ FNALApplicable 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

ECR = electron cyclone resonance

m

eB

[courtesy of D.Reschke, DESY]

[courtesy of G. Wu, FNAL]


Coating as a bulk niobium cavity treatment

1. M. J. Sadowski et al., The Andrzej Soltan Institute2. A-M. Valente et al., JLAB3. S. Calatroni, CERN

Standard Nb coating methods:

Electron cyclotron resonance plasma deposition 2

Vacuum Arc deposition 1

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 => annealing (recrystallization) = possible

Other drawback : post contamination still possible (complex assembly/re-assembly process)

Biased magnetron sputtering 3


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/etchingElectrohydrodynamic cleaning (corona plasma)

Ion beam

Ion cluster beam etching…

Ultrasonic, megasonic Better cleaning of sub micron particles

Field emission +


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

1claire antoinecea/saclay - fermilab (innovative) processing of materials srf materials workshop...

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