# Presentation: 488482

## 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)
- Hopefully this review will stimulate ideas for further surface studies

## Topic 2:  High-Field Q-Slope

## Possible Mechanisms and Non-mechanisms Discussed in the Past for High-Field Q-slope

- 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

- Possible mechanisms

## 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) ...

## Thermometry System for Single Cell Cavities 

- Thermometry System for Single Cell Cavities

## High Field Q-slope Losses Originate in Magnetic Field Region of Cavity

- Slope ≈ 2 - 3

- Slope ≈ 18

- Cornell Data

## Magnetic Field Region

- ≈ 800 Oe at 2 K

- Strong High Field Q-Slope

- Cornell Data

- Electric Field Region
- No High Field Q-Slope

- Cornell Data

## Role of Surface Roughness ? High Field Q-Slope Occurs in Cavities with BCP and EP

- BCP: Chemical Etching

- EP: Electropolishing

- 800 Oe

- Slight difference before bake
- Big difference after bake

- Role of Surface Roughness ? High Field Q-Slope Occurs in Cavities with BCP _and_ EP

- DESY/CERN Results

## BCP + Bake

- EP + Bake

- Bake= 85-120C, 48 hours

- Saclay Results

- Big Difference Between BCP and EP After Bake

## Obvious Major Difference Between BCP and EP Is Surface Roughness

- BCP, average surface roughness
-  1 - 5 um

- EP: Average surface roughness <  0.5 um

- DESY Results

## Increase roughness by  frequent BCP lowers onset field of Q-slopeDecrease roughness by EP  raises onset field of Q-slope(all after mild bake)

- DESY Results, also confirmed by similar studies at KEK (not shown)

- More BCP (roughness?) Lowers Onset of High-Field Q-Slope

## Possible Effects of Roughness (Cornell, Knobloch...)

- Magnetic field enhancement at grain boundary steps
- Quenching at the edge of the steepest grain boundary

## Slide 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.

## Example Surface Analysis Results

- There are many others

## Angstroms

- Cornell-SIMS: O/Nb
- Before and After Bake

- NC-State/Jlab : TEM

- NW: 3DAP

## Evidence For O-Diffusion Role

- Changing Baking Temp and Time, but Preserving O-Diffusion Length, Yields Similar Q-slope Benefits, e.g BCP surface

- 145C, 3 hours

- 110 C – 48 hours

- Saclay Results

## Diffusion of Oxygen into RF Layer lowers electron mfp, and BCS Rs

- Complementary Evidence for O-Diffusion

- 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)

## 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

## Increase Oxide Thickness by Anodizing A Cavity With Reduced Q-Slope 

- Increase Oxide Thickness by Anodizing A Cavity With Reduced Q-Slope

- Start with Reduced Q-slope by baking

- Q-slope does not return by increasing oxide thickness to 10nm

- Cornell Results

- Q-slope Returns by increasing oxide thickness to 60nm

- Nb thickness converted to oxide ≈  20 nm
- => Baking benefit extends to
- < 20 nm,
-  Not the full 50 nm penetration depth

- 1

- 2

- 3

## Grain Boundaries Play A Weak to No Role 

- Single Crystal Nb cavity, and roughness < 50 nm

- Jlab Results

## 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

## Grain Boundaries are Not Prominent Heat Sources

- Small Grain Cavity

- Large Grain Cavity

- Cornell Results

- Both Show Q-Slope

## 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

## Baking Effect Preserved with Air  and Water Exposure 

## 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 !

## 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.

## H : Heat treatment at 880 C is well known to remove H from bulk Nb 

- A. BCP surface

- B.C.  Baked 150 C, 43 hrs

- D. Heat 880, C 2 hrs

- 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

## End