1. High-Q, High Gradient Niobium-Coated Cavities at CERN
Sergio Calatroni
CERN, 1211 Geneva 23, Switzerland
C. Benvenuti
P. Darriulat
M.A. Peck
A.-M. Valente
C.A. Van’t Hof

2. Outline of the talk The Nb/Cu technology and motivation for the study
The coating procedure
RF and material characterization
Optimization of the residual residual resistance
High gradient operation
Conclusions
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3. Copper surface preparation 1
Copper surface preparation 1.5 GHz spun copper cavities have been used throughout the study. Hydroformed and electroformed cavities have also been tested.
Electropolishing parameters:
55% vol. H3PO4
45% vol. butanol
Current density: 0.2A/cm2
HP water rinsing
Electropolishing has replaced chemical polishing (LEP standard) for surface preparation. Advantages: lower roughness, absence of defects
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4. Niobium sputtering Sputtering parameters:
Discharge current stabilized at 3 A.
Sputter gas pressure of 1.5x10-3 mbar, corresponding to ~ 360 V.
Coating temperature is 150 °C.
Thickness: 1.5 µm
LEP “standard film” characteristics:
RRR: 11.5 ± 0.1
Argon content: 435 ± 70 ppm
Grain size: 110 ± 20 nm
Tc: 9.51 ± 0.01 K
Strain: a/a  = ± %
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5. Change of the sputter gas
Sputter gas ions are back-scattered by the cathode with an energy proportional to (MNb-Mgas)2/ (MNb+Mgas)2 (for normal incidence)
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6. Change of the sputter gas
Argon-Neon mixtures allow to fine-tune the RRR between 5 and 15
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7. Change of substrate
Various substrates have been explored:
Copper (covered with natural oxide)
Oxide free copper (by sputter etching prior to coating)
Niobium
Oxide free niobium
And various underlayers (sputter-coated):
Copper
Titanium
Gold
Aluminium
Oxidised aluminium
Niobium films on most substrates or underlayers behave as standard LEP films. However, some films belong to a second “family”.
In the case of oxide free copper:
- RRR: 28.9 ± Argon content: 286 ± 43 ppm
- Grain size: >1 µm - Tc: 9.27 ± 0.09 K
- Strain: a/a  = ± %
Sergio Calatroni - CERN

8. Standard films and films on oxide-free copper
Difference of the texture of the films
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9. The surface resistance
The surface resistance can be written in the form:
Rs (HRF, Hext, T) = RBCS (HRF, T) + Rfl (HRF, Hext, T) + Rres (HRF)
The dependence of RBCS (0,T) on  has been verified by changing the sputter gas
RBCS (HRF, T) has an intrinsic dependence of HRF
Rfl (HRF, Hext, T) has a dependence on  similar to RBCS (0,T)
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10. Theoretical and experimental BCS resistance at zero RF field1 10
350
400
450
500
550
600
650
700
750
800
850
900 2 5 20
RBCS (4.2K) [n]
1+0/2
RBCS at 4.2 K
Nb bulk: ~900 n
Nb films: ~400 n
RBCS at 1.7 K
Nb bulk: ~2.5 n
Nb films: ~1.5 n
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11. The RF field dependence of the BCS resistance
RBCS (4.2K, HRF) for film and bulk has a similar slope, and can be written as R0BCS (T)•G(HRF). The G(HRF) term can be approximated with a quadratic dependence on the RF field.
100
1000 10 20 30 40 50
RBCS (4.2 K) [n]
HRF [mT]
Squares: Nb bulk
Rounds: Nb films
G(HRF)( HRF2) or
G(ERF) (1+0.01ERF2)
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12. The RF field dependence of the BCS resistance
The value of RBCS (1.7K, HRF) for films can be calculated on the basis of the previous formulas. 10 9 11 5 15 20 25 30 35 40 Q
(1.7 K) E
acc
[MV/m]
RBCS  20 n at 35 MV/m
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13. Fluxon-induced losses
Fluxon-induced losses at 1.7 K are characterized as Rfl = (Rfl0 + Rfl1 HRF) Hext
The minimum values are obtained using krypton as sputter gas:
Rfl0 = 3n/G Rfl1 = 0.4 n/G/mT 1 10
100
1+0/2
Rfl0[n/G]
(a) 1 10
Rfl1[n/G/mT]
1+0/2
(b)
Triangles: bulk Nb
Squares: coatings on oxide-free copper
Circles: coatings on oxidized copper
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14. Fluxon-induced losses
Niobium films can be divided into two families, according to the substrate
Coatings using argon
Oxide-free copper family
Oxidized copper family
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15. Fluxon-induced losses
Fluxon-induced losses could be a limitation for high RF-field operation of film cavities, unshielded from the Earth’s magnetic field. 10 9 11 5 15 20 25 30 35 40 Q
(1.7 K) E
acc
[MV/m]
Fluxon losses for krypton calculated for Hext = 0.5 G
~32 n at 35 MV/m
BCS + Fluxon losses
~52 n at 35 MV/m
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16. The residual resistance
The BCS surface resistance and the fluxon-induced losses have a well determined dependence on the electron mean free path .
The residual resistance has no correlation with the mean free path , or any other superconducting parameter.
The residual resistance has been seen to be influenced by:
The quality of the copper surface: roughness and defects
The oxide at the Nb/Cu interface
The hydrogen content of the niobium film
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17. The residual resistance: definitions
200
400
600
800
1000 20 40 60 80
100
Rs [nOhm]
RF field [mT]
The residual resistance is calculated by subtracting the BCS contribution to the measured 1.7 K surface resistance
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18. The residual resistance: effect of roughness5 10 15 20 25 30 40 60 80
100 R
res 1 (n W
m MV -1 )
Standard coatings using argon as sputter gas
Circles: spun cavities
Triangles: hydroformed cavities
Crosses: electroplated cavities
Spun cavities: 0.2 µm
Hydroformed cavities: 0.8 µm
Electroplated cavities: 0.2 µm
Spun cavities Hydroformed cavities
Rres0 17±3 n 28±6 n
Rres1 5±1 n m MV ±2 n m MV-1
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19. The residual resistance: effect of electropolishing
Standard coatings using
Xe, Kr, Ar, Ne as sputter gas
Average roughness of
chemically polished spun
cavities: 0.2µm
chemically polished hydroformed cavities: 0.8µm
electropolished spun
cavities: 0.04µm
Absence of defects (etching pits)
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20. High gradients: High Pressure Water Rinsing
HPWR influences Rres0
Improved HPWR extends the operating range
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21. The new HPWR station 100 bar HPWR Closed cycle operation
Before and after coating
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22. High gradient cavities
Coatings performed using krypton
Rinsed with upgraded HPWR installation
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23. Subtracting the BCS contribution, we see that this is not yet a limit
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24. High gradient cavities
Subtracting the BCS contribution 10 9 11 5 15 20 25 Q E
acc
[MV/m]
(1.7K)
residual
BCS
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25. Conclusions We built on the successful LEP technology.
RBCS has an intrinsic slope, similar for bulk and for films, which may be a future limitation at high fields.
Fluxon-induced losses can be described in terms of . Coatings produced using krypton provide the lowest losses.
Rres is mostly influenced by the quality of the substrate, and very small values of Rres0 and Rres1 can be obtained by electro-polishing the copper substrate.
High accelerating fields can be reached with a more effective HPWR.
No evidence for any fundamental limitation has been found.
Sergio Calatroni - CERN