1. SRF Surface Studies and the High Field Q-slope Mystery
Alexander Romanenko
Cornell University
CLASSE

2. Outline High field Q-slope (HFQS) in niobium cavities
Sharp decrease in Q-factor with field
Empirically found “cure” – baking at C for hours
But explanation missing
Interesting physics
Understanding will significantly improve the current cavity treatment procedure
Ways to find the explanation
Cavity tests
Surface studies
Models studied
Oxygen diffusion
Magnetic field enhancement
Interface tunnel exchange
Newer ideas and experiments
11-Jan-19
A.Romanenko

3. Q-slope and baking HFQS is eliminated (EP) or improved (BCP) by vacuum
Real Nb cavities
Ideally
HFQS is eliminated
(EP) or improved
(BCP) by vacuum
for
24-48 hours
Adapted from B. Visentin (Saclay)
11-Jan-19
A.Romanenko

4. High Field Q-slope - History
Initially HFQS was called “European headache” since cavities chemically polished in Europe had it and electropolished and tested cavities at KEK (Japan) did not have it
Difference was due to the process KEK used to improve vacuum after high pressure rinsing – empirical HFQS cure was found by accident
C high vacuum annealing for hours
Removes HFQS in EP cavities
Improves HFQS in BCP cavities
11-Jan-19
A.Romanenko

5. Temperature Mapping Thermometry board T-map
Nb cavity on the test stand
Thermometry boards
mounted to the cavity
11-Jan-19
A.Romanenko

6. Non-uniformity of heating Some regions (hot) have higher losses [Why?]
Temperature Mapping
dT, mK
log(T)
Hot
“Cold”
log(Epk)
Epeak = 50 MV/m, Hpeak = 123 mT
Non-uniformity of heating
Some regions (hot) have higher losses [Why?]
11-Jan-19
A.Romanenko

7. Heating non-uniformity
Two main directions
Baking effect
Study Nb samples treated similarly to cavities
Heating non-uniformity
Cut tested Nb cavities and analyze hot/cold regions
11-Jan-19
A.Romanenko

8. Cavity Measurements vs. Surface Studies
Cavity measurements – macro characterization
Averaged over the whole surface characteristics
Not possible to pinpoint the physical entity responsible for the HFQS
Surface studies – micro characterization
Local probing of the surface
Depth resolution of a few nm is needed
11-Jan-19
A.Romanenko

9. Nb2O5 2-5 nm Nb ~40 nm London penetration depth @ 2K Nb Cavity Surface
Hydrocarbons
~A few monolayers
Nb2O5
2-5 nm
NbOx, x<2.5 Nb
~40 nm
London penetration 2K
11-Jan-19
A.Romanenko

10. Most popular model: O Adapted from G.Ciovati SRF’07 talk 11-Jan-19
A.Romanenko

11. Secondary Ion Mass Spectrometry
Very sensitive – ppb detection possible
Destructive depth profiling
BUT
Instrumental effects – preferential sputtering of oxygen, roughness effect on signal, chemical information not reliable due to sputtering-induced ion production
11-Jan-19
A.Romanenko

12. 5 nm 100 sec = 1 nm ToF-SIMS results G. Eremeev and J. Francis
11-Jan-19
A.Romanenko

13. Arguments against O model
No evidence of oxygen-enriched layer underneath oxide
No evidence of oxygen diffusion at C baking temperature
Oxygen depth profile does not change after baking
11-Jan-19
A.Romanenko

14. Magnetic field enhancement (MFE)
Chemically polished surface
Electropolished surface
Field enhancement region
with enhancement factor ,
if  H > Hc the region becomes
normal conducting
11-Jan-19
A.Romanenko

15. BCP and EP cavities behave similarly before baking
Arguments against
BCP and EP cavities behave similarly before baking
But roughness is different
No evidence of MFE at grain boundary steps from T-maps
Possibly overestimated the field enhancement – might be a negligible effect
No difference in roughness between hot/cold spots
11-Jan-19
A.Romanenko

16. One of the possible oxide-related loss mechanisms
Oxide losses
Adapted from G.Ciovati SRF’07 talk
One of the possible oxide-related loss mechanisms
11-Jan-19
A.Romanenko

17. X-Ray Photoelectron Spectroscopy
Conduction Band
Valence Band
L2,L3 L1 K
Fermi
Level
Free Electron
Incident X-ray
Ejected Photoelectron 1s 2s 2p
Elemental composition within first few nm (except for H and He)
Chemical state information
Sensitivity limited to about at.%
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A.Romanenko

18. No change in oxide structure
H.Tian, JLab,
FNAL Material Workshop, 2007
11-Jan-19
A.Romanenko

19. XPS: Oxide structure
Is non-uniformity of heating caused by oxide structure differences? – No!
Nb5+
Hot
Nb0
Valence band
Hot
Cold
Cold
11-Jan-19
A.Romanenko

20. Arguments against “oxide models”
Oxide structure change after baking is reversible
Namely cancelled by air exposure
Whereas cavity Q vs. E improvement is preserved
No difference in the oxide between “hot” and “cold” spots
11-Jan-19
A.Romanenko

21. Other aspects to explore
Crystalline orientation of individual niobium grains
Deformation (stress)
Vacancies and dislocations – defects of the crystalline lattice …
11-Jan-19
A.Romanenko

22. Electron Backscattered Diffraction
A tool to study crystalline
microstructure
Based on diffraction of backscattered electrons
Information depth – nm
Crystallographic orientation mapping
Information on crystal defects distribution
11-Jan-19
A.Romanenko

23. EBSD: Is grain orientation responsible? (Small grain BCP cavity)
Hot
Cold
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A.Romanenko

24. Recent ideas and methods
Scattering off magnetic impurities (i.e. Nb12O39) – [T.Proslier et al., App.Phys.Let. – in print]
Scanning Tunneling Microscopy (STM)
Role of dislocations, niobium vacancies and Vac-H complexes
Positron Annihilation Spectroscopy (PAS)
Electron Backscattered Diffraction (EBSD)
Detailed studies of the Nb/oxide interface
Transmission Electron Microscopy (TEM)
11-Jan-19
A.Romanenko

25. High field Q-slope is not yet understood
Conclusion
High field Q-slope is not yet understood
Set of possible causes is narrowing down
Several proposed mechanisms (roughness, grain orientation, oxide losses) have been eliminated
11-Jan-19
A.Romanenko