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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 ?. 1) Getting a “good” superconductor OOPS !? What is a good SC ?

justine-noel
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(Innovative) Processing of materials

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  1. (Innovative) Processing of materials SRF materials Workshop Fermilab May 23-24, 2007 Today’s process is long, complex, expensive … and not very efficient

  2. 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,…

  3. 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)…

  4. 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

  5. Detail of the usual process (1/2) Forming WHY COMMENT Nb = getter. Degraded RRR @ weld => 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 => >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

  6. 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, 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 !

  7. Fe 100 bars (Droplets) vf ~ 160 m/s Fe Particles are displaced when Fe > Fad (Flow) High pressure rinsing (HPR) 1/2 • ultra pure H2O, ultra filtered, 80-100 bars

  8. 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]

  9. 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]

  10. 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

  11. 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]

  12. 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

  13. 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 +

  14. 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

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