1. CERN Studies on Niobium-Coated 1.5 GHz Copper Cavities
Sergio Calatroni
C. Benvenuti
M. Hakovirta
H. Neupert
M. Prada
A.-M. Valente

2. Motivation for the study: the high-field residual resistance
Outline of the talk
Motivation for the study: the high-field residual resistance
Thermal effects
Surface defects and roughness
Role of the grain size and purity
Hydrogen studies:
RF measurement
Thermodynamics
Conclusions
Sergio Calatroni - CERN

3. Motivation: the high-field residual resistance
Coatings performed using krypton on electropolished spun copper cavities
(Santa Fe 1999)
Limited by RF power
Sergio Calatroni - CERN

4. Testing: higher field operation
The performance can be extended straightforwardly at higher fields
Limited by RF power (250 W)
(Thermal instability of the standard small cryostat is not a limiting issue)
Sergio Calatroni - CERN

5. Testing: thermal effects
In Nb/Cu cavities we must cope with several thermal impedances: Cu/He (~6000 W/m2K), across Cu (negligible), Nb/Cu interface (?), across Nb film (negligible)
If the Rs increase would only be due to some heating of the film -> RBCS
It is then possible to estimate the temperature increase that corresponds to the measured Rs increase.
The dissipated power is also easily calculated
A Kapitza-like mechanism would give a straight line in the plot
Sergio Calatroni - CERN

6. Surface defects & roughness
2.0 µm
2.0 µm
Chemically polished copper
average roughness: 0.2 mm
pinholes of 0.3 mm
Electropolished copper
average roughness: 0.02 mm
nearly no defects
Sergio Calatroni - CERN

7. Surface defects: measurement of pinholes
Irregularities on the substrate surface shadowing effect
film inhomogeneities
He leak rate experiment
‹inc› fraction of leaky film surface
- equator 9° ppm
- (~iris ° 25 ppm)
- equator 9° ppm CP EP
Substrate disk
Machining and cleaning
Film deposition
Substrate removal p1 p2
film
Sergio Calatroni - CERN

8. Copper roughness I: electropolishing
The window of optimum polishing parameters is rather narrow.
Reproducing these parameters over the entire surface of the cavity cell needs further R&D
Production of O2 bubbles!
Production of Cu(OH)2 on the surface!
Sergio Calatroni - CERN

9. Copper roughness II: electropolishing
Electropolishing parameters:
55% vol. H3PO4
45% vol. butanol
Current density: 200A/m2
HP water rinsing
Electropolishing with a quasi-homotetic cathode has replaced chemical polishing (LEP standard) for surface preparation.
Sergio Calatroni - CERN

10. Copper roughness III: electropolishing
current density is 5 times larger at iris
current density is identical
The electropolishing cathode is being optimised by means of a sophisticated computer simulation code. The current density can be made uniform
Sergio Calatroni - CERN

11. Intrinsic film roughness & incidence angle of the niobium atoms10 20 30 40 50 60 70 80
degrees mm
50 mm
0 mm
Sergio Calatroni - CERN

12. Intrinsic film roughness: incidence angle and residual resistance
An issue which is of great interest also for low-ß cavities, is the correlation between the incidence angle of the film and the residual resistance
Sergio Calatroni - CERN

13. Digression: new ideas for low-beta cavities coating
Nb ring
Cu half-cell
Problem: coating incidence angle in low-beta cavities
Solution: coat at favourable angle before welding
Sergio Calatroni - CERN

14. Recall: standard and oxide-free coatings
Sergio Calatroni - CERN

15. Properties of the coatings I : grain size with FIB micrographs
Standard films
Oxide-free films
0.5 µm
0.5 µm
Courtesy: P. Jacob - EMPA
Sergio Calatroni - CERN

16. Properties of the coatings II: purity, Hc1, texture, etc...
Standard Oxide-free
RRR ± ± 0.9
TC ± 0.01 K ± 0.04 K
Ar cont ± 70 ppm ± 43 ppm
Texture (110) (110), (211), (200)
Hc ± 3 mT ± 5 mT
Hc ± 0.1 T ± 0.05 T
a (10)Å (6) Å
Da/a ± % ± %
Stress ± 56 MPa ± 78 MPa
Grain size 110 ± 20 nm > 1 µm
Sergio Calatroni - CERN

17. Properties of the coatings III: effect on residual resistance
OX: oxidised copper substrate
OF: oxide-free copper substrate
CP: chemical polishing
EP: electropolishing
Oxide-free films (larger grains, better purity, less stress, etc...) have on average a larger residual resistance (measured at low field)
In contrast with models linking the residual resistance with Josephson dissipation at grain boundaries
Sergio Calatroni - CERN

18. Hydrogen studies I: film loading
Spun cavities coated using argon as sputter gas loaded with 1.4 at.% of hydrogen
Hydrogen loading induces a much larger degradation for oxide-free films
Sergio Calatroni - CERN

19. Hydrogen studies II: outgassings
Question: why were coatings on oxide-free hydroformed cavities consistently worse than for spun cavities? (Legnaro 1997)
Possible answer: a larger quantity of hydrogen was migrating into the film from the hydroformed cavity
Sergio Calatroni - CERN

20. Hydrogen studies III: outgassings
Bare Cu
The hydrogen content and has been measured also by thermal outgassing to 350 ºC, for passivated and non-passivated Nb films (i.e. NbxOy at the surface)
Oxidised copper: 2200±200 ppm 800±200 ppm
Oxide free: 2000±200 ppm 980±200 ppm
Sergio Calatroni - CERN

21. Hydrogen studies IV: isosteres
Oxidised copper
pH2(cH)2exp(-EH/kT)
The hydrogen content and its binding energy with the lattice can be measured with isosteric cycles and successive injections of known quantities of hydrogen.
Oxidised copper: 400±200 ppm 340±10 meV/at
Oxide free: 1000±200 ppm 320±10 meV/at
Sergio Calatroni - CERN

22. HPWR effectively reduces Rres0 (1999 result)
Digression: HPWR I
Nb/Cu film on CP copper
HPWR effectively reduces Rres0 (1999 result)
Sergio Calatroni - CERN

23. Digression: HPWR II Before HPWR After HPWR 10 µm 10 µm
Bulk Nb - EP with KEK bath
HPWR smoothens the grain boundaries (2001 results)
Sergio Calatroni - CERN

24. Conclusions
The maximum field of Nb/Cu cavities is not limited by any intrinsic phenomena.
However, the residual resistance suffers from a steep increase at high field (threshold ~15 MV/m).
Several possible causes have been investigated. The most probable sources are: surface defects, hydrogen content.
Possible cures are: better electropolishing (under way), reducing hydrogen content (difficult).
Coating for low-beta cavities are being addressed with a suitable modification of the sputtering technique.
Sergio Calatroni - CERN