1. Photocathode R&D for bERLinPro
Julius Kühn
High Brightness Electron Beams (HBEB) Institute for Accelerator Physics
European Workshop on Photocathodes for Particle Accelerator Applications
The Cockcroft Institute, Daresbury, UK

2. Outline bERLinPro GunLab Transfer systems and vacuum suitcase
Photocathode lab
Cs-K-Sb photocathode preparation & characterization
Summary and Outlook

3. model of the building

4. construction site

5. 3d model of

6. Scientific highlight in 2016 and major milestone for bERLinPro
3d model of Gunlab (gun 1.0)
Scientific highlight in 2016 and major milestone for bERLinPro

7. cross-section of the gun modul
Support posts
80 K shield
Cathode transfer system #2
Helium gas return pipe
Cold steerer
SC Solenoid
HOM
absorber
Warm
Cold transition
2 layer magnetic
shielding
Cathode
Insert
w/ Plug
Coupler
with courtesy of A. Neumann (HZB)

8. Cold string assembly Cavity with helium vessel Gate valve Valve
Cathode beam tube
Coupler assembly
Cathode cooler
Cold part
of TTF-III
coupler
Next steps:
horizontal acceptance test
Mount complete string
Installation into module
Module RF tests
First beam this fall:
with courtesy of A. Neumann (HZB)

9. Transfer systems and vacuum suitcase overview
Cathodes must be stored and handled in < mbar UHV
Vacuum suitcase is available. Vacuum ~ 1*10-11 mbar
Stability with NEG + getter pump is good for >3 months
Cleaning and assembly in ISO 5 clean room conditions done!
commissioning of transfer system #1 at Preparation and Analysis System (PAS) is completed!
First parts for transfer system #2 have been delivered
Gun Module
Transfer System #2
PAS
Transfer System #1 9

10. cross-section transfer system #2
Plug transport
cart
Manipulator to take the plugs off the sampe holder
Gun Modul
Cathode insert w/ plug
Vacuum suitcase

11. PLUG Design For gun operation, consider:
Cathode insert w/ plug in GUN 1.0
For gun operation, consider:
Cooling of the thermal load from RF and drive laser:
Cu has low surface resisitivity and good thermal conductivity and contact
Properties as a cathode substrate:
Mo is favorable for cathode QE
Cu migrates into Antimony layer, results in 50% lower QE
PLUG - Options :
Mo-Plug: sputter cleaned and heat treatment (Mo)
Mo-plug: sputter-cleaned only at the center where the cathode will be deposited  lower field emission from rim (MoOx)
Cu-plug with Mo film deposited as a diffusion layer at the plug center  Mo layer deposited by e-beam evaporation
Scenario Gun 1:
6 W RF losses
1.2 W laser power
Scenario Gun 2:
24 W laser power

12. Our goals High QE CsK2Sb photocathodes Smooth substrates and films
Allow high bunch charge and current 100 mA means 25 W laser power at 1% QE
Smooth substrates and films
Low emittance, low field emission
Reproducible growth procedure & robust lifetime
Necessary for operation

13. Photocathode Lab in operation
February 2016
Bild aktualisieren!

14. Photocathode Lab: transfer system #1
PAS
Load lock
Transfer chamber
Bild aktualisieren!
Vacuum suitcase

15. We have prepared a number of photocathodes with varying results
Status of the photocathode lab
Photocathode Preparation and Analysis System (PAS)
Several Cs-K-Sb photocathodes have been prepared and characterized
Preparation chamber
effusion cell for Sb, SAES Dispenser for K and Cs
sequential growth and co-deposition of K and Cs
e-beam evaporator for Mo
bake-out for 72h at 120°C, p = 3x10-10 mbar
carefully degassing of all filaments and dispensers
monitor the growth process by mass spectrometer and photocurrent
Surface Analysis chamber
post- analysis by X-ray photoelectron spectroscopy (XPS)
Momentatron for emittance measurement (M. Schmeißer, Master Thesis, 2014)
Spectral response setup
Spectral response measurement from 250 – 600 nm
H. Kirschner, Master Thesis, ongoing
Poster
H. Kirschner
Transfer system #1 w/ vacuum suitcase
all parts were cleaned in the clean room
transfer system #1 is commissioned
We have prepared a number of photocathodes with varying results

16. Substrate characterization
For cathode research:
Polished polycrystalline Mo sheets, 10x10 mm
roughness : ~10 nm rms, can be 1-3 nm rms
Cleaning: heat to 450°C for 30 min, Ar+ sputter cleaning for 30 min or heat to 600°C for 60 min
Results in very low surface contamination, confirmed by XPS
AFM; Rq = 10,66 nm rms White Light Interferometer 20x; Rq = 8,30 nm
work done by Grzegorz Gwalt, Frank Siewert
Be aware: change in surface roughness due to heat treatment, sputter cleaning

17. Photocathode preparation on molybdenum sheet
P004 & P006 (Aug 2015)
Polished Mo substrate
10 nm Sb film evaporated at 100°C
P004 und P006 ~5% QE (532nm)
P005: oxygen contamination, 0.18% QE
P004 : Cs0.8K1.8Sb
P006 : Cs0.8K1.8Sb 17

18. Polished molybdenum plugs

19. PLUG characterization
The roughness in numbers looks promising, but :
Mo plug after turning:
Mo plug after polishing:
AFM
Roughness (rms):
11,9 nm
Problem: Loose grains break from sintered Mo and create new scratches
Forged Mo material preferable but grain boundaries limit the ultimate roughness
Single crystals can be obtained
with courtesy of Sven Lederer (DESY) 19

20. Phtocathode preparation on a plug
P007 & P008 (May 2016)
Polished Mo plug
10 nm Sb film evaporated at 100°C
P007 sequential growth, 1.64 % QE
P008 co-deposition of Cs und K, 1.2 % QE
P007 : Cs0.8K2.7Sb
P008 : Cs0.7K2.2Sb 20

21. Why was the QE on the plug so low?
Plug surface temperature 80°C when heater is set to 100°C
Temperature was calibrated in preparation of the next run
Thermal contact of the Plug on the sample holder has to be improved 21

22. Look into the preparation chamber
Aperture (7mm)/ Anode K Cs Mo
plug Sb
QCM

23. Summary & Outlook Transfer systems for Gun operation are on the way
Photocathodes with QE of 5% at 532 nm grown on Mo sheets by sequential growth
Photocathodes have been grown by sequential growth and co-deposition on Mo-Plugs
Spectral response measurement setup was commissioned
roughness of Mo plug increases due heating and sputtering
Plugs made from sintered Mo not ideal for polishing
JUN 16
Evaluation of the co-deposition growth procedure
Photocathode life time studies: Simulation of transfer to Gun 1.0
JUL 16
Setup and commissioning of transfer system #2 in the clean room
AUG 16
Setup and commissioning GunLAB

24. Thank you for your attention!
Acknowledgements
Martin Schmeißer, Hans Kirschner, Thorsten Kamps, Andreas Jankowiak
Axel Neumann, Michael Schuster, André Frahm (SRF Group)
Markus Bürger, Kerstin Martin, Daniel Böhlick (Engineering)
Grzegorz Gwalt, Frank Siewert (Nanometer Optics group)
Sven Lederer (DESY)
and bERLinpro project team
Thank you for your attention!