1Claire AntoineCEA/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

2Claire AntoineCEA/Saclay - Fermilab 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  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,…

3Claire AntoineCEA/Saclay - Fermilab Damage layer:  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 : ~ 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)…

4Claire AntoineCEA/Saclay - Fermilab 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  ~ Ni particles

5Claire AntoineCEA/Saclay - Fermilab Detail of the usual process(1/2) Forming WHY COMMENT EB welding Clean welding Nb = getter. Degraded weld => Q 0 /10 Ti purification Deep etching Increase RRRRRR now commercially available BCP EP Remove damage layer ( µm) BCP limited to ~ 30MV/m; EP => >40 mV/m but lack of reproducibility 800°C annealing Remove Hydrogen contamination hydrogen source : wet processes Hydrogen segregates at the surface and form hydrides (poor SC) Diffusion layer < ~1µm Light etching Remove diffusion layer (O, C, N)

6Claire AntoineCEA/Saclay - Fermilab Detail of the usual process(2/2) WHY COMMENT HPR HF, H 2 O 2, ethanol, degreasing,… Fight field emission gt rid of S (after EP) …Special rinse …Light etching Get rid of dust particlesMost convenient, but not sufficient Ancillaries: couplers antennas… In clean room. But re-contamination still possible Baking, 120°C, 48h Get rid of the high field losses (Q-drop) Mechanism not understood, concerns the first 10 nm of the material assembly Post processing Get rid of dust particles Due to assembly Under development Ex: dry ice cleaning, plasma RF test He processing, HPP Field emissionField emission: SRF accelerator plague !

7Claire AntoineCEA/Saclay - Fermilab High pressure rinsing (HPR) 1/2 ultra pure H 2 O, ultra filtered, bars (Droplets) (Flow) FeFe v f ~ 160 m/s FeFe 100 bars Particles are displaced when F e > F ad

8Claire AntoineCEA/Saclay - Fermilab High pressure rinsing (HPR) 2/2 HPR is due to mechanical effect of the droplets F e is high enough to deform Nb (  l Nb ~ 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]

9Claire AntoineCEA/Saclay - Fermilab RF post processing : He processing & HPPP Helium processing Developed CERN Helium gaz + RF => plasma Low efficiency, mainly low field High Peak Power processing (HPP) Concept 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]

10Claire AntoineCEA/Saclay - Fermilab 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. Q ext = 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

11Claire AntoineCEA/Saclay - Fermilab Other post processing Advantage: applicable in situ, after assembly Dry ice cleaning DESY Carbonic snow => residuals = CO 2 Mechanical effect, similar to HPR Applicable on horizontal cavities In situ ECR plasma cleaning 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 ECR = electron cyclone resonance [courtesy of D.Reschke, DESY] [courtesy of G. Wu, FNAL]

12Claire AntoineCEA/Saclay - Fermilab Coating as a bulk niobium cavity treatment 1.M. J. Sadowski et al., The Andrzej Soltan Institute 2.A-M. Valente et al., JLAB 3.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

13Claire AntoineCEA/Saclay - Fermilab 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 H 2 O 2 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 +

14Claire AntoineCEA/Saclay - Fermilab 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