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SUPERCONDUCTING RF CAVITIES

Last Advances. SUPERCONDUCTING RF CAVITIES. for e- LINEAR ACCELERATORS. Bernard V ISENTIN. Eventful Year for SRF R & D. Single Crystal & Large Grain Cavities. Manufacturing multi-cell Cavities ( LL shape). 2005. E acc Records mono-cell Cavities new shape

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SUPERCONDUCTING RF CAVITIES

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  1. Last Advances SUPERCONDUCTING RF CAVITIES for e- LINEAR ACCELERATORS Bernard VISENTIN

  2. Eventful Year for SRF R&D Single Crystal & Large Grain Cavities Manufacturing multi-cell Cavities ( LL shape) 2005 Eacc Records mono-cell Cavities new shape ( LL & RE ) Fast Baking : experimental show of O role

  3. High Gradients  S-LINAC Cost Relative Cost ( €/$/¥ ) C. Adolphsen 1st ILC Workshop Eacc (MV/m) Baseline Configuration Document for ILC (500 GeV) operating @ 31.5 MV/m recommendation Eacc 35 MV/m (including fabrication scatter 5%)  37 MV/m

  4. Recipe for High Gradients Electropolishing + Baking ( EQuench ) ( Q-slope ) B. Visentin - EPAC 1998 K. Saito - SRF Workshop 1997 40 MV/m

  5. where do we stand with ? ELECTROPOLISHING

  6. Electropolishing Oxide attack by HydroFluoric acid : re-oxidation Chemistry : Nb Surface Renewal Electrochemistry Standard Chemistry (BCP) Rotating Anode (+) : Nb cavity Cathode (-) : Al FS ( 1:9 ) V ( 17 volt ) – J ( 60 mA/cm2 ) plateau Nb oxidation : Dunked cavity FNP ( 1:1:2 ) Nb oxidation : Hypothesis: Surface roughness ( better water rinse – lower H field enhancement )

  7. Electropolishing Electropolishing at DESY Faced to this superiority  To set up an EP facility for 9-cell cavity L. Lilje - SRF Workshop 2005

  8. Electropolishing Electropolishing at DESY RF tests results on 9-cell cavities TTF Linac 1 GeV 400 MeV 800 MeV 4 MeV 120 MeV ACC6 modulewith 8 EP cavities @ 35 MV/m ( Summer 2006 )

  9. Electropolishing Reproducibility - Field Emission Unfortunately, in spite of the EP success a large spread exists in the cavity production compare to standard chemistry

  10. Electropolishing Optimization of EP Treatment Nb sample  mono-cell cavity 9-cell JRA1 - SRF • EP bath impurities • Cathode corrosion (Al) and Sulphur deposition • solution  alcohol rinse • EP bath ageing ( Nb dissolved ):  HF concentration • F:S concentration 1:9 (9 g/l)  4:9 (23 g/l) • Optimize parameters ( V-I ) • Explore different working zones ( plateau, …) EP test bench for mono-cell F. Eozénou - SRF Workshop 2005

  11. Electropolishing • RMS surface roughness: • fine grain (BCP) : 1274 nm • fine grain (EP) : 251 nm • large grain (BCP) : 27 nm Large Grain Cavity High Gradient shape 1.5 GHz with Standard Chemistry « BCP» Very smooth surface ( EP not necessary ) P. Kneisel PAC 2005 & SRF Workshop 2005

  12. Electropolishing BCP large grain cavity  BCP fine grain cavity ( 30 MV/m ) with better surface roughness than EP fine grain cavity @ 40 MV/m Open Question High Gradients @ 40 MV/m are they linked to Electro-Polishing (surface roughness) or just to Electro-Chemistry ? Potential Advantages of Large Grain Nb • Niobium discs sliced directly from ingots • ( saw cut or electrical discharge machining : no forging , no rolling ) • Reduced cost • Less impurities • Quality control more easy adopted as : Alternative Option for ILC 500 GeV WG 5 Report - 2nd ILC Workshop ( Snowmass 2005 )

  13. Electropolishing EP Large Grain Cavities at DESY TESLA Shape (1.3 GHz) – Electropolishing first test (before baking) : Q-slope - 28 MV/m

  14. Where do we stand with ? BAKING

  15. Baking Whatever the niobium structure… ( Single or Poly-crystal,) & Cavity Shape ( HG,LL,RE ) Whatever the fabrication method…( EB Welding or Hydroforming, bulk Nb or clad Nb/Cu) Whatever the thermal treatment… (nothing, 800°C, 1300°C/Ti ) Whatever the chemical treatment… ( Electropolishing or Standard Chemistry ) Baking Wide Application Area in-situ (UHV) T = 110 - 120 °C t = 1 – 2 days TTF 1.3 GHz - Saclay Polycrystalline 1300°C / Ti Chemical Etching 1:1:2 TTF 1.3 GHz - Saclay / KEK Polycrystalline No Thermal Treatment Electropolishing LL 2.2 GHz - JLab Single crystal 800°C – 1250°C / Ti Chemical Etching 1:1:1

  16. Baking Baking  Permanent Treatment • T = 120 °C / 2 days • + • Air exposure • for 4 years – • without any particular • precautions • Hydrofluoric Treatment • High Pressure Rinse

  17. Baking Baking  Diffusion interstitial oxygen RS Change through electron Mean Free Path ( l ) decrease 110 °C 60 hours in-situ ( UHV )

  18. Baking Explanation of Q-slope Improvement by Baking Doping Effect in Niobium Baking Q-slope Diffusion of Interstitial Oxygen B. Visentin SRF Workshop 2005

  19. Baking Baking Parameters ( T , t ) with Unchanged Oxygen Penetration 2nd Fick's law  analytic solutions 110 °C / 60 hours equivalence 145 °C / 3 hours semi infinite solid : C(0,t) = CS thin oxide layer : C(x,0) = Q d(x)

  20. Baking « Fast » Baking ( UHV ) • Infra-Red emitters (short T rise time) • Cavity pumped out ( Ultra High Vacuum ) 145 °C - 3 hours Similarities with Standard Baking

  21. Baking O % from interface Nb2O5 – NbOX / Nb  Control oxygen penetration in niobium

  22. Baking Fast Baking : Improvement for Cavity Mass Production Save time & Possibility to simplify the cavity preparation Electropolishing + Ultra Pure Water Rinse Electropolishing + Ultra Pure Water Rinse High Pressure Rinse - 85 bars (FE) High Pressure Rinse - 85 bars (FE) Clean Room ( Class 100 ) Air Baking : T °C /t hours Air Drying : 3 hours Assembly + Helium Test Assembly + Helium Test (RF Test ) RF Test Bed ( Cryostat ) Baking Ultra High Vacuum 110 °C / 2 days 145 °C / 3 hours RF Test RF Test

  23. « Air - Baking » in Clean Room FastAir-Baking on Wet Cavity under LaminarFlow (FE) OK (UHV) Temp. / TimeTurbulences adopted to substitute the “in-situ” baking Alternative Configuration Document ( ILC 500 GeV ) WG 5 Report - 2nd ILC Workshop ( Snowmass 2005 )

  24. Overcome ? 40 MV / m

  25. Nb Cavity Performances ( 1991 - 1999 ) Continuous improvements with Heat Treatments @ 800 and 1400°C, High Pressure Rinse, Baking & ElectroPolishing. 42 MV/m hard barrier (  2004 )

  26. Quench (SC Breakdown) : thermal effect on local defect if T >Tc (9.2K) or surface peak magnetic field Hp>Hc (2000 Oe) Experimental limitation @ 1750 0e Reduce Hp Ratio Hp/Eacc (determined by geometry) J. Sekutowicz SLAC Talk 2005 K. Saito - SRF 2005 -12% New Shape to Reduce Hp / Eacc TTF : Optimized to reduce field emission & multipacting ( iris rounded – cell rounded far from pill-box geometry) New shape : decrease surface H field  increase magnetic volume (for lower P dissipation  small Ø iris & maximize R/Q*G parameter) Critical Points : cleaning and preparation (Ep/Eacc increase) - stiffness

  27. Single Cell LL Cavity @ 45 MV/m Single Crystal ( 2.2 GHz ) Standard chemistry P. Kneisel – PAC 2005

  28. courtesy of Kenji Saito Single Cell RE Cavity @ 52 MV/m Fine Grain ( 1.3 GHz ) Electropolishing + Baking R.L. Geng – SRF Workshop 2005

  29. from Kenji Saito Single Cell LL or RE Cavities Eacc > 40 MV/m Obviously new step is crossed with cavity shape modification adopted as : Alternative option for ILC 500 GeV ( Baseline for 1 TeV upgrade ) Eacc > 40 MV/m ( including fabrication scatter )  42 MV/m WG 5 Report - 2nd ILC Workshop ( Snowmass 2005 )

  30. Multi Cell LL Cavities Four 7-Cell @ 1.5 GHz installed in “Renascence” 3rd prototype CW cryomodule - CEBAF upgrade 12 GeV Vertical tests – Q slope - 25 MV/m C. Reece – SRF Workshop 2005 “ICHIRO” cavities Goal : 51 MV/m T. Saeki – SRF Workshop 2005 Four 9-Cell ( 1.3 GHz )

  31. Conclusions • High gradients required for future e- linear accelerators : • within reach, at least for single cell cavities • Physical explanations ( EP and Baking ) are not well understood : • FundamentalR&D goes on • Main goal : • Technical R&D to optimize and simplify the preparation process • decrease in field emission • better reproducibility of EP ( scatter in performances ) • performing « fast air-baking » in clean room

  32. Acknowledgements • Thank you to my colleagues for providing • information and documents to prepare this presentation • DESY : L. Lilje – J. Sekutowicz • JLab : P. Kneisel – C. Reece • KEK : K. Saito – T. Saeki • Cornell : H. Padamsee – R.L. Geng

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