2. Review of impedance aspects of NEG coatings (a surface scientist perspective)
Sergio Calatroni for the CERN NEG team

3. Cross section of the LHC warm LSS vacuum chamber3
NEG technology at CERN
Low activation temperature NEG coatings (TiZrV) were developed at CERN for the LHC circa 1998
Cross section of the LHC warm LSS vacuum chamber
after coating
Before coating
NEG
coating
Pedro Costa Pinto, WAMAS, CERN, November 2013

4. 3
NEG technology at CERN
Vacuum properties : pumping speed versus coverage.
Smooth (coated at 100oC)
Rough (coated at 300oC)
Pedro Costa Pinto, WAMAS, CERN, November 2013

5. 3
NEG technology at CERN
Anti electron multipacting properties: low Secondary Electron Yield (SEY).
Secondary electron yield
1.8
1.2
1.6
1.4
1.0
0.9
0.8
Primary electron energy (eV)
as received
after 120oC
after 160oC
after 200oC
after 250oC
SEYLHC=1.3
Pedro Costa Pinto, WAMAS, CERN, November 2013

6. Potential of NEG coatings4
Potential of NEG coatings
At CERN, large scale production for the LHC and experiments by DC Cylinder Magnetron sputtering (DCCM) from a target of Ti, Zr and V wires (more than 1300 chambers).
AND WORLD WIDE:
users
, in design/study
MaxLab
ISA
SOLEIL
DIAMOND
DESY
ANL
ESRF
ELECTRA
CERN
BNL
ALBA
ILSF
KEK
SESAME
NSRRC
LNLS AS
Pedro Costa Pinto, WAMAS, CERN, November 2013

7. Outline
NEG are metals ?
Evidence from experiments in various labs
Expected deviations from “simple” metallic behaviour
Grain size and grain boundaries
High frequency conductivity
Roughness
19 September 2014
Sergio Calatroni
19 September 2014
Sergio Calatroni 7

8. NEGs are metals?
Review of impedance measurements in various machines already reported at several workshops, conferences and papers
I borrow a couple of slides from the reviews of T. Perron (ESRF) E. Karantzoulis and F. Mazzolini (Elettra)
Dutifully acknowledge all the people involved in all the experiments at various machines and who wrote all the different papers and reports written on the topic
19 September 2014
Sergio Calatroni

9. A quote of a quote (of a quote…)
19 September 2014
Sergio Calatroni

10. Elettra experience
19 September 2014
Sergio Calatroni

11. Outline
NEG are metals !
Evidence from experiments in various labs
Expected deviations from “simple” metallic behaviour
Grain size and grain boundaries
High frequency conductivity
Thickness effects
Roughness
(Note: I will speak of surface impedance of the NEG, not of beam impedance)
19 September 2014
Sergio Calatroni
19 September 2014
Sergio Calatroni 11

12. 3
NEG technology at CERN
The Ti-Zr-V NEG coating can be activated at 180oC for 24 hours.
Crystallographic structure (XRD) 30 40 50 60
TiZrV
Substrate
 (degrees)
Well crystallised
grain size ≥ 100 nm 30 40 50 60
TiZrV
Nanocrystalline
grain size 3~5 nm
Pedro Costa Pinto, WAMAS, CERN, November 2013

13. Electrical resistivity: mean free path
For any metal : 𝜎= 1 𝜌 = 𝑛 𝑒 2 𝜏 𝑚 = 𝑛 𝑒 2 ℓ 𝑚 𝑣 𝐹
“Size effects” measurements (Fuchs model) of thin films allow extrapolating the quantity 𝜌∗ℓ= 𝑚 𝑣 𝐹 𝑛 𝑒 2
𝜌∗ℓ is usually in the order of m2
For NEG films  is typically 100÷150 µcm, confirmed in RF tests
(up to 350 µcm measured in different labs depending on chamber and coating geometry)
The calculated electron mean free path is ℓ ÷ 10-9 m, consistent with scattering dominated by lattice defects (typical for alloys)
19 September 2014
Sergio Calatroni
19 September 2014
Sergio Calatroni 13

14. Electrical resistivity: grain boundaries
Conduction electrons are scattered at grain boundaries
Grains effects simplified: 𝜌 𝑡𝑜𝑡𝑎𝑙 = 𝜌 𝑏𝑢𝑙𝑘 + 𝜌 𝑏𝑢𝑙𝑘 ℓ 𝑏𝑢𝑙𝑘 𝑑 𝑔𝑟𝑎𝑖𝑛𝑠
(Fuchs, Sondheimer, Mayadas & Shatzkes + Mathiessen’s rule)
For the smallest measured grain size we can expect up to a 30% increase of 
𝜌 𝑏𝑢𝑙𝑘 =100 𝜇Ω𝑐𝑚
ℓ 𝑏𝑢𝑙𝑘 =1 𝑛𝑚
19 September 2014
Sergio Calatroni
19 September 2014
Sergio Calatroni 14

15. High-frequency conductivity
For any metal at high frequency : 𝜎 ω = 𝑛 𝑒 2 𝜏 𝑚(1−iω𝜏)
DC conductivity valid only for ω𝜏≪1
For ω𝜏≫1 we can approximate: 𝜎 ω ≅ 𝑛 𝑒 2 𝑚 𝜔 2 𝜏 +𝑖 𝑛 𝑒 2 𝑚ω such that for large  the imaginary term dominates
Recalling: 𝜏= ℓ 𝑣 𝐹
Assuming a “standard” 𝑣 𝐹 =106 m/s  𝜏= s
DC conductivity value can be used for ω≪ Hz
19 September 2014
Sergio Calatroni

16. High-frequency conductivity
𝑍𝑠=(1+𝑖) 𝜔 𝜇 0 2𝜎(𝜔)
DC-like behaviour up to Hz
19 September 2014
Sergio Calatroni

17. Thickness effects
Surface impedance is well-behaved only when the skin depth  is ≪ or ≫ than the film thickness, depending on frequency
In the intermediate regime non-uniform thickness distribution of the NEG film over the vacuum chamber has to be taken into account
See presentations by Eirini and Sergio
10 m NEG
On Cu
1 m NEG
On Cu
19 September 2014
Sergio Calatroni

18. Effect of roughness
Key result: effective Rs depends on total path of surface currents
Key result: Rs may increase from flat-surface value when  ≲ rms roughness or smaller
Note: this is not the “roughness” effect identified for insertion devices (e.g. Karl Bane) where the relevant scale is much larger
Current flow  grooves (larger effect than )
Morgan J. Appl. Phys. 20 (1949) 352
Holloway IEEE trans waves 48 (2000) 10
19 September 2014
Sergio Calatroni

19. Intrinsic roughness
Smooth (coated at 100oC)
Depends mostly on substrate, then on coating parameters.
µm-scale rms roughness is usually obtained, or better
Change in Rs can be expected for f ≳ 10 GHz
Effect of roughness is of course frequency-dependent
See presentation by Eirini
Rough (coated at 300oC)
19 September 2014
Sergio Calatroni

20. Conclusion Textbook behaviour can be expected from NEG films
Some special cases (thickness changes, high frequency, roughness effects) need to be considered carefully in the design
19 September 2014
Sergio Calatroni

21. 19 September 2014
Sergio Calatroni

22. Potential of NEG coatings3
NEG technology at CERN 4
Potential of NEG coatings
At CERN, large scale production for the LHC and experiments by DC Cylinder Magnetron sputtering (DCCM) from a target of Ti, Zr and V wires (more than 1300 chambers).
AND WORLD WIDE:
producers under CERN license
ESRF (France)
LNLS (Brazil)
SAES getters
(Italy)
GSI (Germany)
DESY (Germany)
FMB Berlin (Germany)
Pedro Costa Pinto, WAMAS, CERN, November 2013
19 September 2014
Sergio Calatroni

23. ESRF experience 1
ESRF Al chambers (5m long and 8mm gap) Zeff =46 kΩ/m : The NEG should increase the result by 10 kΩ/m. (Coating thickness estimated 1 m)
19 September 2014
Sergio Calatroni