1 / 30

Second Major Open Physics Topic in RF Superconductivity H. Padamsee

Second Major Open Physics Topic in RF Superconductivity H. 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)

villette
Download Presentation

Second Major Open Physics Topic in RF Superconductivity H. Padamsee

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related