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

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.

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

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, 085114(2013)

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) 015103 measurement on K2CsSb 0.39 microns / mm rms (accel/drift) - 0.40 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

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., 84107, 134101(2015)

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

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.

Tradition method heavily depends on personal skill

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

New method: tri-element co-deposition

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

Photocathode installed to the photoinjector

photocathode tested in photoinjector F.Sanniable

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

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

Cathode roughness

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 ? .

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