1. Jun Feng Advanced Light Source Lawrence Berkeley National Laboratory
Simultaneous element evaporation and deposition of multialkali cathodes
Jun Feng
Advanced Light Source
Lawrence Berkeley National Laboratory
P3 workshop Oct 17,2016

2. Motivation We needs multi-alkali cathode to be used in APEX Gun.
Only if we can successfully make good multi-alkali cathode (QE, emittance, Lifetime, survivability, robustness, operational properties, etc.), we can study and improve it.

3. Deposition System for photoinjector
Plasma source and
UV monochromator
Plug deposition system
APEX plug
Plug heater/ cooler 2 1
Momentum
Measurement
system 3 4
Suitcase transfer 5
Momentum system transfer
Deposition system transfer
UHV suitcase

4. A tunable light source for photocathode fabrication and emittance measurement
In-house designed and built in-vacuum monochromator
Aberration free
Plasma source:
135 x 230 mm FWHM
free space
fiber
High brightness plasma source with stigmatic
optical system gives high flux in a small focus
ARPES (Scienta R3000): Cu(111), 1 eV excess energy
J.Feng, et al, Rev. Sci. Instrum., 84, (2013)

5. Emittance measurement in the lab: Accelerating and drift method
solenoid added to momentum measurement system
J. Feng et al, ‘A novel system for measurement of the transverse electron momentum distribution from photocathodes’, Rev, Sci. Instrum., 86(1) (2015)
measurement on K2CsSb
0.39 microns / mm rms (accel/drift)
microns / mm rms (solenoid)
check on accelerate and drift emittance measurement
allows imaging of the focused photon source
allows imaging of extended defocus source for uniformity measurements

6. Measured and Theoretical Transverse Momentum: Sb
transverse position (mm)
transverse position (mm)
transverse position (mm)
transverse position (mm)
J.Feng,J.Nasiatka, W.Wan, S.Karkare, J.Smedley, H.Padmore, APL., , (2015)

7. Photocathode transferring
Laser
Photocurrent
monitor
UHV
suitcase
UHV valve
to APEX gun
UHV cold
gauge

8. Tradition method to make multi-alkali cathode
a) Deposited Sb on a substrate at ~90C with ~20nm
b) Deposited K on a substrate at ~130C, until maximum photocurrent, forming K3Sb, has two phase
c) Deposited Cs on a substrate at ~100C, until maximum photocurrent, forming K2CsSb.
We could make multi-alkali cathode with good QE from time to time, but we don’t know when we can make it.

9. Tradition method heavily depends on personal skill

10. New method: tri-element co-deposition
Deposition curve
~5h
~1.5h
532nm laser
Sb:320 0
Cs:4.7A
K:5.5A
Mo:75 0
Triple element deposition doesn’t need to baby-sitting and Yo-Yo optimization, can grow different thick cathodes

11. New method: tri-element co-deposition

12. New method: tri-element co-deposition
Spectra
Life time

13. Photocathode installed to the photoinjector

14. photocathode tested in photoinjector
F.Sanniable

15. Our cathode eclolor 1. Different K-CS-SB cathode color
Sequential method
2. Just by eye, new method is smoother than old method

16. In-situ AFM to measure roughness
Sample transfer
Deposition
chamber
AFM
controller

17. Cathode roughness

18. Conclusion
- We have developed a deposition system with tunable light source, emittance measurement, and UHV suitcase for multi-cathode.
We have developed a method for 3 element simultaneous deposition
No baby sitter need,
Repeatable, doesn’t depend on personal’s skill and luck.
Need much less deposition materials
Routinely fabricated and tested in photoinjector
We can provide cathode to people who needed it.
Our new method open the door to study the chemical dynamics of multi-alkali materials.
Is the reaction one step or two steps?
is the stoichiometry same as sequential method?
How much can the roughness be improved by this new method?
What is the robustness and life time difference between sequential deposition and simultaneous deposition ? .

19. Thanks for you attention!
Acknowledgement
H.Padmore, Jamie Nasiatka, S.Karkare, Zihao Ding, J.Smedley, S.Schubert, F.Sannibale
Thanks for you attention!