1. DEPOSITION OF Pb/Nb PHOTOCATHODES Jacek Sekutowicz, Robert Nietubyć,
(10.4.2)
Jacek Sekutowicz, Robert Nietubyć,

2. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Motivation
CW electron accelerator
High average power FEL
Lowly probable phenomena
Diluted samples
Special applications
Industrial applications
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3. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Eucard obligations
Deliverables D
QE data for Pb/Nb deposited photo cathode samples
(Report, M12) D
Cold test results for the test cavities with and without the deposited lead photo cathode
(Report, M36)
Milestones
M10.4.1
Lead deposition on samples for photocathode development
(M12)
M10.4.2
Lead deposition on half cells and 1.6 cell cavities
(M18)
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4. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Cathodic arc
Arc discharge
Ions emission from a small explosive centre
Rising electron emission: thermionic enhanced by the electric field and by ion impact.
I2R >
EXPLOSION – sudden transition to a dense plasma
Ion acceleration by pressure gradient, electron collisions, coulomb field
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5. Supersonic multiply ionised ions from the cathode
Cathodic arc introduction
Supersonic multiply ionised ions from the cathode
Electron driven transport in the plasma channel
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6. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Arc highlights
Hard landing
sub-plantation
diffusion with kinetic energy higher than displacement energy
cooling and condensation
Consequences for the film
regular,
dense,
adherent
Difference to magnetron sputtering
Small ion energy limited diffusion strong interaction with already deposited atoms – columns voids, defects
Absence of working gas residuals
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7. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Performance
Optimization of deposition system
transmission and deposition rate
micro-droplets filtering
temperature control
cleanliness and vacuum
Photocathodes preparation
deposition processes
after deposition treatment
chemistry
laser flashing
Measurements
surface diagnostics QE
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8. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
IPJ implementation –general description of the system
polarisation = -110 V
base pressure <10-7 mbar
rest gas QMS
arc current = 25 A
arc voltage = V
ion current = mA
deposition rate ≈ 0.5 nm/s
deposition time < 80 min
wall temperature <32 °C
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9. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Pb growth – x-ray diffraction studies
Dep. time
;lattice constant 3 15
4,96449 60
4,96423
120
4,96714
5×120
4,96144
10×60
4,95896
Lattice constant does not depends on thickness
Orientation distribution does not change neither
Observable amount of PbO appears in roughly 10 hours of the exposition to air
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SRF-EuCARD Annual Meeting, Daresbury, April 7,

10. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
8 niobium plugs coated with lead
M Lead deposition on samples for photocathode development
Plug
BNL-like
Location of the heater with lead stub for plasma formation No
Time [s]
Nb type
Distance
Setup
Pump 1
1800
poly
1.6 cell
Straight
oil 2 3
mono 4
2700
Bent
dry 5 6
0.5 cell 7 8
6000
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11. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Laser treatment and QE measurements
D QE data for Pb/Nb deposited photo cathode samples
QE =
number of emitted photoelectrons
number of incident photons
Laser: 213 nm 1 min 25 Hz 0.2 mJ/mm2 per pulse
keV 7 C O Pb Pb Nb
× 20 as compared to Nb
Three times less than the best one (coated at the minimal distance from Pb source)
Melting and re-crystallization of lead
Electron induced fluorescence spectrum showed a carbon contamination of the surface.
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12. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
We gained from the first series
Conclusions and question after the first series
QE = i.e better than Nb wall, and worse than it could be
We can at least double QE if the coverage is higher, film is to thin,
is it possible to avoid melting when laser treatment is gentle?
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SRF-EuCARD Annual Meeting, Daresbury, April 7,

13. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
30º-bent tube
30
Increased transmission to enable thicker films
To improve the coating efficiency, the system has been modified by replacing the rectangular knee by the 30º‑ bent tube. That solution enabled to increase 3 times the ion current saving the lead flux free of macro-particles. Chosen angle provides the minimal bend angle for which the lead droplets, which all move rectilinearly, cannot reach the target
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14. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Laser treatment
After cleaning
Before cleaning
Gentle laser treatment : 190 nm, 30 min, 300 Hz 0.01 mJ/mm2 per pulse
as compared to violent one:213 nm, 1 min, 25 Hz, 0.2 mJ/mm2 per pulse
QE higher than in the first series
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15. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Remaining faults
EDS (with 5 keV electrons) of cathode before (red) and after (blue) laser cleaning. The carbon peak is reduced by ~30% by laser cleaning.
In spite of sufficiently high ion current of 8 mA and extended deposition time, the layers thickness does not exceed 200 nm and does not rise
Further cleaning at this energy density failed to further increase the QE, suggesting that the optimum case would be a thicker lead coating and somewhat more aggressive cleaning.
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SRF-EuCARD Annual Meeting, Daresbury, April 7,
Robert Nietubyć, Jacek Sekutowicz, EuCARD 1st Annual Meeting, Daresbury, April 7,8 2010

16. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Aim and means
Conclusions question, and answer after the second series
QE = i.e 30 time greater than for niobium wall.
We can double QE if the coverage is complete, film is to thin,
is it possible to avoid melting when laser treatment is gentle?
very luckily if contamination is eliminated
how to icrease the thickness ?
yes
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17. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
Aim and means
Conclusions
In 2009 we gained:
Reproducible processes
Micro-droplets removal
Optimised Laser cleaning procedure
Reasonable QE in spite of significant film imperfection
In 2010 we have to continue with:
Contaminations removal
Fatting the film
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18. SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010
The crew
Andrzej Trembicki, Robert Nietubyć, Jan Witkowski, Bernard Kołakowski, Robert Mirowski, Mirosław Kuk, Katarzyna Nowakowska-Langier
Jacek Sekutowicz
TJNAL, BNL
Peter Kneisel, John Smedley …
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19. Thanks for the attention
The End
Thanks for the attention