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(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 ?

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Innovative processing of materials

(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
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 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,…

Damage layer 100 200 m m
Damage layer:100-200 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 @ ~ 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)…

Why do we need to process the cavities1

Ni particles

b~ 3

b~ 100-500

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 =>bE

Field emission is the main practical limitation in accelerator operation

Detail of the usual process 1 2
Detail of the usual process (1/2)




Nb = getter.

Degraded RRR @ weld => Q0/10

EB welding

Clean welding

Ti purification

Increase RRR

RRR 300-400 now commercially available



Remove damage layer (100-200 µm)

BCP limited to ~ 30MV/m; EP => >40 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

800°C annealing

Light etching

Remove diffusion layer (O, C, N)

Diffusion layer < ~1µm

Detail of the usual process 2 2
Detail of the usual process (2/2)

…Light etching



HF, H2O2, ethanol, degreasing,…

…Special rinse

Fight field emission gt rid of S (after EP)


Get rid of dust particles

Most convenient, but not sufficient

Ancillaries: couplers antennas…

In clean room. But re-contamination still possible


Get rid of the high field losses (Q-drop)

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

Baking, 120°C, 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 !

High pressure rinsing hpr 1 2


100 bars


vf ~ 160 m/s


Particles are displaced when Fe > Fad


High pressure rinsing (HPR) 1/2

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

High pressure rinsing hpr 2 2
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]

Rf post processing he processing hppp
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
High Peak Power processing (HPP)

HPP in a Cryomodule at ELBE, Rossendorf [1]

HPP at Cornell on multicell cavities [2]

  • 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

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

Other post processing
Other post processing

Advantage: applicable in situ, after assembly

  • Dry ice cleaning

  • Developed @ DESY

    • Carbonic snow => residuals = CO2

    • Mechanical effect, similar to HPR

    • Applicable on horizontal cavities

  • In situ ECR plasma cleaning

  • Developed @ 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]

Coating as a bulk niobium cavity treatment
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 => 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

Other possible processing methods
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 +


  • 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