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R EVIEW ON Q - D ROP M ECHANISM B ernard V ISENTIN International Workshop on Thin Films 9 th - 12 th

R EVIEW ON Q - D ROP M ECHANISM B ernard V ISENTIN International Workshop on Thin Films 9 th - 12 th October 2006. Definition of Used Parameters. Q 0 Quality Factor (figure of merit) G Geometry Factor R S Surface Resistance.

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R EVIEW ON Q - D ROP M ECHANISM B ernard V ISENTIN International Workshop on Thin Films 9 th - 12 th

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  1. REVIEW ON Q-DROP MECHANISM Bernard VISENTIN International Workshop on Thin Films 9th - 12th October 2006

  2. Definition of Used Parameters Q0Quality Factor (figure of merit) G Geometry Factor RS Surface Resistance EaccAccelerating Field, average electric field seen by particle crossing cavity gap L Baking : ~ 110 -120 °C / 2 days ( under UHV ) Annealing : ~ 800 °C - remove Hydrogen from Nb bulk ~ 1350 °C ( +Ti ) - remove Oxygen and improve thermal conductivity (bulk)

  3. Thin Film Cavities & Q-Drop Eacc=15 MV/m Saclay – Nb/ Cu – 1.5 GHz Magnetron Sputtering in Ar P. Bosland et al.- ASC (1998) CERN – Nb/ Cu – 1.5 GHz Magnetron Sputtering in Kr V. Arbet-Engels et al.- NIMA (2001) Advantages to use Thin Film Technology for SRF Cavities : Reduced Cost – New Superconducting Material (higher Tc & Hsh) severe Q-drop limits High Gradient Performances Eacc < 25 MV/m Wuppertal – Nb3Sn/ Nb – 1.5 GHz Vapor Deposition Technique G. Müller et al.- EPAC (1996) ( no field emission, no quench only RF power limitation )

  4. Thin Film & Bulk Cavities very steep Q-drop exists on Bulk Cavities (BCP or EP)

  5. Thin Film & Bulk Cavities It can be cured by baking : limitations in Q0 and Eacc can be exceeded very steep Q-drop exists on Bulk Cavities (BCP or EP) B. Visentin et al .- EPAC (1998) & SRF (1999) reason why R&D has been more extended for Nb bulk cavities

  6. Thin Film R & D on Cavities R&D gave up now at CERN and at Saclay since 2001 but still continues in Europe (CARE program) and USA (JLab, Cornell) IPJ Poland / INFN – Nb/ Cu – 1.3 GHz Cylindrical UHV Arc Discharge magnetic filter (m –droplet) J. Langner- CAREReport(2005) JLab – Nb/ Cu – 500 MHz E-beam evaporation + ECR plasma (Nb ionization) G. Wu - Argonne Workshop (2004) & SRF (2005)

  7. Application of Thin Film Cavities SLS n - fact. m - coll. : 200 MHz SOLEIL : 352 MHz CERN Technology Large Size (< 700 MHz) with Thick Wall ( 6 mm ) Specifications for Low Gradients ( < 15 MV/m) LHC : 400 MHz S 3rd H C : 1500 MHz

  8. Q-Drop Origin ( Thin Film ) • Granular Superconductor Theory : Josephson fluxon penetration • in weak links (grain boundaries oxidized sputter island) • Thermal resistance at superconductor-substrate interface • Energy Gap dependence D(H) V. Palmieri - SRF (2005) B. Bonin - Supercond. Sci. Technol. 4,257(1991) J. Halbritter - Workshop of the Eloisatron Project (1999) • Not a fundamental limitation : improve cleanness during process • (substrate, sputtering,…) V. Arbet - Engels et al.- NIMA (2001) Lot of Theories and Experiments have been performed on Nb Bulk cavities Situation Review in bulk case ( past + latest results ) Where do we stand to understand Q-Drop origin ? Hope to clear up the Thin Film issue ??? not enough data on Thin Film Cavities

  9. Q-Drops for Bulk Cavity Three different slopes in bulk Nb Cavity at LowMediumHigh Field ?

  10. Low Field Q-Drop J. Halbritter – SRFWorkshop (2001) Theory : NbOx Clusters in Nb localized states inside energy gap (Rs) Baking : Q-Slope enhancement  Additional Clusters ( O Diffusion ) HF Rinse (10%) : initial Q-slope restoredPhenomenon localized at Ox./Nb Interface Nb2O5 + 10 HF → 2 H2NbOF5 + 3 H2O B. Visentin – ArgonneWorkshop (2004)

  11. Medium Field Q-Drop Theory quadratic dependence : linear dependence : hysteresis losses due to Josephson fluxons in weak links (oxidation of grain boundaries) Experimental Checking:quadratic and linear dependence at JLab & DESY (x-cells) only quadratic dependence at Saclay (1-cell) J. Halbritter – 38th INFNEloisatron Project Workshop (1999) & SRF (2001) G. Ciovati - ArgonneWorkshop (2004)

  12. ( Medium + High ) Field Q-Drop ThermalModelRefinement non linear correction due to RF pair breaking ExperimentalChecking Thermal Feedback Model with linear ornon linear RBCS before and after baking. A. Gurevich – ArgonneWorkshop (2004) P. Bauer et al. – SRF (2005) better fit with non linear model but not enough to explain the high field Q-drop

  13. High Field Q-Drop ( 6 theories ) J. Halbritter – Eloisatron Workshop (1999) H. Safa - SRF (2001) B. Bonin - SRF (1995) J. Knobloch - SRF (1999) E. Haebel – TTF Meeting (1998) A. Didenko – EPAC (1996) • Diffusion (O, Imp.): “ Interface Tunnel Exchange” “ Bad Superconducting Layer” “ Granular Superconductivity” • Surface Roughness : “ Magnetic Field Enhancement” • High Field (T, Hpeak) : “ Thermal Feedback ” “ Energy Gap Dependence D (H)”

  14. High Field Q-Drop ( 6 theories ) J. Halbritter – Eloisatron Workshop (1999) H. Safa - SRF (2001) B. Bonin - SRF (1995) J. Knobloch - SRF (1999) E. Haebel – TTF Meeting (1998) A. Didenko – EPAC (1996) • Diffusion (O, Imp.): “ Interface Tunnel Exchange” “ Bad Superconducting Layer” “ Granular Superconductivity” • Surface Roughness : “ Magnetic Field Enhancement” • High Field (T, HP) : “ Thermal Feedback ” “ Energy Gap Dependence D (H)”

  15. Interface Tunnel Exchange RH E° • ITE reduction by : • smoothening surface ( EP ) • ( b*  and E°  ) • baking : Nb2O5-y vanished - better interface • ( reduction of localised states ) RE RF field on metallic surface • Dielectric oxide layer on metal  enhancement of ZE by I.T.E. • ( localized states of Nb2O5-y and density of state of Nb ) • with electron diffusion at NbOx - Nb2O5-y interface I.T.E.  quantitative description of Q-slope J. Halbritter - SRF (2001) & IEEE Trans. on Appl. Supercond. 11, (2001)

  16. Magnetic Field Enhancement microstructure on RF surface ( surface roughness - step height 10 mm ) magnetic field enhancement normal conducting region if factor ( BCP ) J. Knobloch - SRF (1999) Q-slope origin the most dissipative G.B. quench (equator) K. Saïto - PAC (2003) EP : ( HC/bm= 223 mT ) bm=1 BCP : ( HC/bm = 95 mT ) bm=2.34 electromagnetic code + thermal simulation  Q0(Eacc)

  17. Experimental High Lights Baking: Definitive Treatment air exposure for 4 years - HPR High Field Q-drop Similarity between BCP and EP cavities (before baking) In contradiction with M.F.E. theory B. Visentin - Argonne(2004) & SRF(2005) HF Rinse - used to suppress field emission - does not affect baking benefit Baking: Universal Treatment fine, large, single crystal, clad, shape w / wo annealing @ 800 or 1350 °C EP (>40 MV/m) or BCP chemistry In contradiction with I.T.E. theory

  18. Some Exceptional Occurrences 48 h baking Baking Resistance Eacc > 40 MV/m ( TESLA like shape - fine grains) BCP chemistry instead of EP chemistry P. Kneisel - SRF (1995) T. Saeki – TTC Meeting @ KEK (2006) W. Singer - SRF (2001) Nb/Cu clad cavity (after baking) B. Visentin - EPAC (2002) B. Visentin - EPAC (2006)

  19. Theories / Experiments Confrontation B. Visentin - SRF (2003) – updated at ArgonneWorkshop (2004) Y / N = theory in agreement / contradiction with experimental observation N+  = undisputable disagreement with experiment

  20. Where do we stand now ? Fine and large grain cavities @ 1.5 GHz / BCP G. Eremeev, H. Padamsee - EPAC (2006) L. Lilje - SRF (1999) Fine grain Fine grain Global heating - Large spread out ( fine grain ) Hot spots for large grain cavity Grain boundaries not involved in Q-drop Large grain (G.B.= white lines)

  21. Large Grain : Hot Spots Large-grain single-cell Hot spots ( large grain cavity ) Reduced after baking Q-slope restored by 40 V anodization G. Ciovati - LINAC (2006)

  22. Hot Spot Theory A. Gurevich - SRF (2005) • Localized sources of dissipation • caused by defects: • grain boundaries (vortex penetration) • precipitates • non uniform surface oxide layer • Hot spots consequences: • non linear effect • reduce breakdown magnetic field HC • increase high field Q-drop Cavity surface with hotspots (dark grey) caused by smaller defects of radius r0 (black)

  23. Open Issue : Oxygen Role 2nd Fick's law  analytic solutions Initial interstitial oxygen From oxide decomposition Correlated problem to the Q-drop existence , why baking suppress it ? Cavity Baking  Interstitial Oxygen diffusion Improved model with decomposition of oxide layer semi infinite solid : C(0,t) = CS minimum of O for 140°C at x=0 near of optimum baking parameter 120°C G. Ciovati - SRF (2005) S. Calatroni - SRF (2001)

  24. Oxygen involved in Q-slope Some observations are in agreement… • SIMS measurements on samples. • After baking, • O concentration is modified: • increased for multiple grain • reduced for large grain Q-slope restored after oxide layer thickening ( anodization 40 V) G. Ciovati - LINAC (2006) Already observed at Cornell (30 V – 60 V) H. Padamsee - Argonne (2004) J. Kaufman, H. Padamsee - SRF (2005)

  25. Oxygen involved in Q-slope ( cont. ) Fast Baking : Based on a equivalence in terms of interstitial oxygen diffusion: 110 °C / 60 h↔ 145 °C / 3 h B. Visentin - SRF (2005)

  26. Oxygen not involved in Q-slope But controversy exists in experimental results arguing for the non involvement… Q-slope not restored after formation of new oxide layer ( HF rinse ) SIMS measurements any noticeable difference before (A) and after UHV baking (8 & 3) ( multiple grain samples) (only for baking at high temperature in air) B. Visentin - Argonne (2004) Or after anodization 5 V – oxide thickness x2 ( 10 nm ) B. Visentin - EPAC (2006) H. Padamsee - Argonne (2004)

  27. SIMS Analysis and Baking • Efficient Q-slope improvement by Baking ( Q0 vs. Eacc ) • if there is any noticeable O diffusion in Nb ( SIMS analyses ) • UHV or Argon atmosphere : oxygen free - no diffusion from surface • - T / t (110 °C / 60 h ↔ 145 °C / 3 h) upper limit before diffusion from NbOx B. Visentin – TTC Meeting @ KEK (2006) Strong correlations between RF results and SIMS analyses Debate is still open : If oxygen is not involved, which is responsible ?

  28. CONCLUSION • Bulk Cavity: • no substrate (only Nb) - BCP (reproducibility) – (EP) - Baking (cure) • lot of experimental data (worldwide) and theories since 1998 • not sufficient to understand HF Q-Drop origin • noose is tightening (theoretical explanations rejected) - in progress • Thin Film (Nb/Cu) : more difficult • Substrate + Thin Film • Lot of parameters to adjust or different process for coating (pressure and gas, bias, atomic Nb, ionic assistance, Nb ions… ) • Very important for the SRF future (new superconducting material) • Split-up between substrate and thin film issues is necessary • Nb substrate (BCP) - no matter what the absolute RF performances are - • Coating parameters Optimization in terms of relative RF performances. RF tests on Nb substrate before coating  same substrate Annealing possible ( hydrogen, oxygen contributions to Q-drop )

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