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Second Major Open Physics Topic in RF Superconductivity H. Padamsee

Second Major Open Physics Topic in RF Superconductivity H. Padamsee

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Second Major Open Physics Topic in RF Superconductivity H. Padamsee

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  1. Second Major Open Physics Topic in RF SuperconductivityH. Padamsee • Why does the surface resistance of niobium increase sharply at high RF magnetic field? • Why does the high-field Q-slope become less (or disappear) on mild baking • (120 C – 2 days) • Hopefully this review will stimulate ideas for further surface studies

  2. Topic 2: High-Field Q-Slope

  3. Possible Mechanisms and Non-mechanisms Discussed in the Past for High-Field Q-slope Possible mechanisms • Surface roughness • Varies due to chemical treatment (BCP, EP) • Density of grain boundaries (grain size) • Reduced by high temp (>1200 C) heat treatment, • Very few grain boundaries for large grain and • none for single grain niobium material • Pollution at metal-oxide interface • Q-slope changes by baking at 85 - 120 C for 48 hours • Some supporting evidence from surface analysis

  4. Tools for Research on High Field Q-Slope • Cavities • Vary surface treatment • Q vs E curves, • preferably with thermometry • Surface analysis • SEM/EDX, AES, SIMS, XPS, 3DAP, AFM, Optical Profilometry, EBSD (OIM) …

  5. Thermometry System for Single Cell Cavities

  6. High Field Q-slope Losses Originate in Magnetic Field Region of Cavity Slope ≈ 18 Slope ≈ 2 - 3 Cornell Data

  7. Magnetic Field Region Strong High Field Q-Slope ≈ 800 Oe at 2 K Cornell Data

  8. Electric Field Region No High Field Q-Slope Cornell Data

  9. Role of Surface Roughness ? High Field Q-Slope Occurs in Cavities with BCP and EP BCP: Chemical Etching DESY/CERN Results EP: Electropolishing • Slight difference before bake • Big difference after bake 800 Oe

  10. Big Difference Between BCP and EP After Bake BCP + Bake • Bake= 85-120C, 48 hours EP + Bake Saclay Results

  11. Obvious Major Difference Between BCP and EP Is Surface Roughness EP: Average surface roughness < 0.5 um BCP, average surface roughness 1 - 5 um DESY Results

  12. More BCP (roughness?) Lowers Onset of High-Field Q-Slope Increase roughness by frequent BCP lowers onset field of Q-slopeDecrease roughness by EP raises onset field of Q-slope(all after mild bake) DESY Results, also confirmed by similar studies at KEK (not shown)

  13. Possible Effects of Roughness (Cornell, Knobloch…) • Magnetic field enhancement at grain boundary steps • Quenching at the edge of the steepest grain boundary

  14. Why Does Mild Baking Reduce Q-Slope: Pollution Model (Safa, Ciovati…) • A “pollution layer” (1- 10 nm) of high O concentration resides below the oxide • some spotty evidence for this from surface analysis (next slide) • This layer weakens the superconducting properties of the thin layer, e.g., by lowering Hc1 and Hsh • Magnetic flux begins to penetrate at lower field, and causes RF losses. • At 100 C, 48 hours, O diffusion (50 – 100 nm) comparable to penetration depth (50 nm) • Baking dilutes the pollution layer, raising Hc1 and Hsh.

  15. Example Surface Analysis Results • There are many others

  16. Oxide ~4.9nm Possible Sub-oxide NW: 3DAP NC-State/Jlab : TEM Cornell-SIMS: O/Nb Before and After Bake Angstroms

  17. Evidence For O-Diffusion Role Changing Baking Temp and Time, but Preserving O-Diffusion Length, Yields Similar Q-slope Benefits,e.g BCP surface 110 C – 48 hours 145C, 3 hours Saclay Results

  18. Complementary Evidence for O-Diffusion Diffusion of Oxygen into RF Layer lowers electron mfp, and BCS Rs • Bake at 120 C for 48 hours to reduce BCS resistance • Remove layers of Nb sequentially and measure progressive increase of BCS resistance • Estimate electron mfp from BCS resistance • Estimate O concentration responsible for mfp • Compare with diffusion coefficient • Assumption: surface always has maximum amount of O (at solubility limit)

  19. Weakness of Pollution Model: Oxide Layer and Oxide Growth Are Not Responsible for Q-Slope Start with no-Q-slope by baking Remove oxide layer by HF and grow a new oxide layer Q-slope does not return Saclay Results

  20. Increase Oxide Thickness by Anodizing A Cavity With Reduced Q-Slope Q-slope Returns by increasing oxide thickness to 60nm Start with Reduced Q-slope by baking 3 1 • Nb thickness converted to oxide ≈ 20 nm • => Baking benefit extends to • < 20 nm, • Not the full 50 nm penetration depth Q-slope does not return by increasing oxide thickness to 10nm 2 Cornell Results

  21. Grain Boundaries Play A Weak to No Role Single Crystal Nb cavity, and roughness < 50 nm Jlab Results

  22. Single Crystal Results • Q-slope is still present, and heals after baking, characteristic signature • But, onset field is higher : • 1300 Oe instead of 1200 Oe for a polycrystalline cavity at the same RF frequency (2.2 GHz) • => Grain boundaries are not the only cause of Q-slope, but may be one of several contributing factors • Pollution layer still plays a role? Jlab Results

  23. Small Grain Cavity Grain Boundaries are Not Prominent Heat Sources Both Show Q-Slope Large Grain Cavity Cornell Results

  24. Mechanisms Likely NOT Relevant to High Field Q-slope • Monolayers of adsorbates (e.g., water, hydrocarbon) on surface • Heal Q-slope by baking • Expose surface to dust-free air + water, • Re-test : Q-slope does not return • Hydrogen • Cavity with Q-slope baked at 800 C and just rinsed with water shows Q-slope • Thermal feedback • Predicted field dependence is too weak compared to high field Q-slope • One of the mechanisms for medium field Q-slope

  25. Baking Effect Preserved with Air and Water Exposure

  26. Bottom Line • We don’t really understand the Q-slope and why baking cures it. • There may be more than one mechanism • Roughness, Pollution layer, O-doping… • Oxygen may not be the relevant impurity • Q-slope in BCP and EP unbaked cavities may have different causes. • Are there physical links between low field, medium field and high field Q-slopes? • We need your help !

  27. H is Not Responsible • In the first test after BCP and no bake the cavity showed a strong Q-slope starting at Eacc = 20 and remained field emission free to the maximum field. • Baking at 150 C for 43 hours made the Q-slope stronger, as with other cavities baked at too high temperatures. • Furnace treatment at 880 C for 2 hours removed H. • HPR to remove particulate contaminants • By avoiding acid treatment no additional H was introduced. • The Q-slope returned to nearly the starting test case. To emphasize the point, • The removal of H by 880 C heat treatment did not eliminate the Q-slope.

  28. H : Heat treatment at 880 C is well known to remove H from bulk Nb But 1) Does not get rid of Q-slope 2) Recovers original Q-slope after degrading by baking at 150 C A. BCP surface D. Heat 880, C 2 hrs B.C. Baked 150 C, 43 hrs

  29. End