1. R EVIEW ON Q - D ROP M ECHANISM B ernard V ISENTIN International Workshop on Thin Films 9 th - 12 th October 2006

2. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 2 Definition of Used Parameters Q 0 Quality Factor (figure of merit) G Geometry Factor R S Surface Resistance E acc Accelerating 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. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 3 Thin Film Cavities & Q-Drop Wuppertal – Nb 3 Sn / Nb – 1.5 GHz Vapor Deposition Technique G. Müller et al.- EPAC (1996) Saclay – Nb / Cu – 1.5 GHz Magnetron Sputtering in Ar P. Bosland et al.- ASC (1998) ( no field emission, no quench only RF power limitation ) 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 T c & H sh ) severe Q-drop limits High Gradient Performances E acc < 25 MV/m E acc =15 MV/m

4. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 4 Thin Film & Bulk Cavities very steep Q-drop exists on Bulk Cavities (BCP or EP)

5. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 5 very steep Q-drop exists on Bulk Cavities (BCP or EP) Thin Film & Bulk Cavities It can be cured by baking : limitations in Q 0 and E acc can be exceeded reason why R&D has been more extended for Nb bulk cavities B. Visentin et al.- EPAC (1998) & SRF (1999)

6. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 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  –droplet) J. Langner - CARE Report (2005) JLab – Nb / Cu – 500 MHz E-beam evaporation + ECR plasma (Nb ionization) G. Wu - Argonne Workshop (2004) & SRF (2005)

7. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 7 Application of Thin Film Cavities CERN Technology Large Size (< 700 MHz) with Thick Wall ( 6 mm ) Specifications for Low Gradients ( < 15 MV/m) LHC : 400 MHz S 3 rd H C : 1500 MHz SOLEIL : 352 MHz : 200 MHz - fact.  - coll. SLS

8. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 8 Not a fundamental limitation : improve cleanness during process (substrate, sputtering,…) Granular Superconductor Theory : Josephson fluxon penetration in weak links (grain boundaries  oxidized sputter island) Thermal resistance at superconductor-substrate interface Energy Gap dependence  (H) 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 ??? J. Halbritter - Workshop of the Eloisatron Project (1999) B. Bonin - Supercond. Sci. Technol. 4,257 (1991) V. Arbet - Engels et al.- NIMA (2001) not enough data on Thin Film Cavities Q-Drop Origin ( Thin Film ) V. Palmieri - SRF (2005)

9. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 9 Three different slopes in bulk Nb Cavity at Low Medium High Field Q-Drops for Bulk Cavity Associated Theories NbO X ClustersSurface Heating - R S (T)I.T.E, M.F.E. … etc. …

10. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 10 Theory : NbO x 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 Nb 2 O 5 + 10 HF → 2 H 2 NbOF 5 + 3 H 2 O Low Field Q-Drop J. Halbritter – SRF Workshop (2001) B. Visentin – Argonne Workshop (2004)

11. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 11 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) Medium Field Q-Drop G. Ciovati - Argonne Workshop (2004) J. Halbritter – 38th INFN Eloisatron Project Workshop (1999) & SRF (2001)

12. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 12 ( Medium + High ) Field Q-Drop Thermal Model Refinement non linear correction due to RF pair breaking Experimental Checking Thermal Feedback Model with linear or non linear R BCS before and after baking. P. Bauer et al. – SRF (2005) A. Gurevich – Argonne Workshop (2004) better fit with non linear model but not enough to explain the high field Q-drop

13. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 13 High Field Q-Drop ( 6 theories ) H. Safa - SRF (2001) J. Halbritter – Eloisatron Workshop (1999) 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, H peak ) :“ Thermal Feedback ” “ Energy Gap Dependence  H  ”

14. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 14 High Field Q-Drop ( 6 theories ) H. Safa - SRF (2001) J. Halbritter – Eloisatron Workshop (1999) 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, H P ) :“ Thermal Feedback ” “ Energy Gap Dependence  H  ”

15. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 15 J. Halbritter - SRF (2001) & IEEE Trans. on Appl. Supercond. 11, (2001) RF field on metallic surface Dielectric oxide layer on metal  enhancement of Z E by I.T.E. ( localized states of Nb 2 O 5-y and density of state of Nb ) with electron diffusion at NbO x - Nb 2 O 5-y interface I.T.E.  quantitative description of Q-slope ITE reduction by : smoothening surface ( EP ) (  *  and E°  ) baking : Nb 2 O 5-y vanished - better interface ( reduction of localised states ) RHRH RERE E° Interface Tunnel Exchange

16. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 16 Magnetic Field Enhancement J. Knobloch - SRF (1999) microstructure on RF surface ( surface roughness - step height 10  m ) magnetic field enhancement normal conducting region if factor ( BCP ) K. Saïto - PAC (2003) electromagnetic code + thermal simulation  Q 0 (E acc ) Q-slope origin the most dissipative G.B.  quench (equator) EP : ( H C /  m = 223 mT )  m =1 BCP : ( H C /  m = 95 mT )  m =2.34

17. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 17 HF Rinse - used to suppress field emission - does not affect baking benefit Experimental High Lights B. Visentin - Argonne (2004) & SRF(2005) 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 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. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 18 Eacc > 40 MV/m ( TESLA like shape - fine grains) BCP chemistry instead of EP chemistry Some Exceptional Occurrences P. Kneisel - SRF (1995) Baking Resistance B. Visentin - EPAC (2006) 48 h baking B. Visentin - EPAC (2002) T. Saeki – TTC Meeting @ KEK (2006) W. Singer - SRF (2001) Nb/Cu clad cavity (after baking)

19. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 19 Theories / Experiments Confrontation B. Visentin - SRF (2003) – updated at Argonne Workshop (2004) Y / N = theory in agreement / contradiction with experimental observation N+  = undisputable disagreement with experiment

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

21. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 21 Hot spots ( large grain cavity ) Reduced after baking Q-slope restored by 40 V anodization Large-grain single-cell Large Grain : Hot Spots G. Ciovati - LINAC (2006)

22. Bernard Visentin International Workshop on Thin Films - Legnaro - October 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 Cavity surface with hotspots (dark grey) caused by smaller defects of radius r 0 (black) Hot spots consequences: non linear effect reduce breakdown magnetic field H C increase high field Q-drop

23. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 23 Open Issue : Oxygen Role Correlated problem to the Q-drop existence, why baking suppress it ? Cavity Baking  Interstitial Oxygen diffusion  analytic solutions 2 nd Fick's law semi infinite solid : C(0,t) = C S Improved model with decomposition of oxide layer G. Ciovati - SRF (2005) S. Calatroni - SRF (2001) From oxide decomposition Initial interstitial oxygen minimum of O for 140°C at x=0 near of optimum baking parameter 120°C

24. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 24 Oxygen involved in Q-slope SIMS measurements on samples. After baking, O concentration is modified: increased for multiple grain reduced for large grain J. Kaufman, H. Padamsee - SRF (2005) 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) Some observations are in agreement…

25. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 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. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 26 Oxygen not involved in Q-slope 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 - EPAC (2006) B. Visentin - Argonne (2004) Q-slope not restored after formation of new oxide layer ( HF rinse ) Or after anodization 5 V – oxide thickness x2 ( 10 nm ) H. Padamsee - Argonne (2004) But controversy exists in experimental results arguing for the non involvement…

27. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 27 SIMS Analysis and Baking Efficient Q-slope improvement by Baking ( Q 0 vs. E acc ) if there is any noticeable O diffusion in Nb ( SIMS analyses ) - UHV or Argon atmosphere : oxygen free - no diffusion from surface - T /  (110 °C / 60 h ↔ 145 °C / 3 h) upper limit before diffusion from NbO x B. Visentin – TTC Meeting @ KEK (2006) Debate is still open : If oxygen is not involved, which is responsible ? Strong correlations between RF results and SIMS analyses

28. Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006 28CONCLUSION  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 )