Siddharth Karkare 1

OUTLINE Motivation and requirements Photocathode experimental facilities at Cornell Alkali-antimonide cathodes GaAs based photocathodes and photoemission theory 2

Why photocathodes? What we need from them? 3 4 th generation light sources powered by photoinjectors Photoinjector beam brightness – limited by photcathode Better photcathodes→brighter x-rays Other applications – Ultrafast Electron Diffraction Night vision Photon detection Process of photoemission not very well understood ERL photoinjector photocathode High QE (>1%) in visible Low MTE (<150meV) Short (<2ps) Response time Long lifetime High QE photocathodes – Alkali-antimonide NEA GaAs cathodes

Photocathode Facilities at Cornell 4 dedicated MBE system over in Wilson Lab actual injector over in Newman Lab Photocathode growth & analysis chamber over in Phillips Hall Cornell University campus Vacuum Suitcase Arsenic Cap

Photocathode diagnostics lab 2-D energy distribution from GaAs at 780nm Yo-Yo activation of GaAs QE surface scan of NaKSb cathode LEED pattern from GaAs Auger surface scan of K on a NaKSb cathode All connected in vacuum of less than torr 5

2-D energy analyzer 6 First Marking Electrode Varying Magnetic Field Second Marking Electrode Beam Current Detector

Transverse energy analyzer (TE-meter) 7 Cathode Grid (2-5kV) Screen Electron trajectories Focused laser Electron spot from TE-meter

Alkali-antimonides Exploring new materials elevated temperature, lifetime 90hrs high current operation lifetime 66hrs 8 Na2KSb cathode ~15% QE reduction Current (mA) QE Temperature (C)

Experimental Alkali-antimonide test chamber Use of MBE like effusion cells and pneumatically controlled shutters New alkali-antimonide growth test chamber for testing various alkali metal sources 9

Alkali-antimonides – Exploring new sources SAES dispensersALVATEC sources Alkali Azide (AN 3 ) Pure metal alkali sources 10

Growth using azide sources 11 Azides sputter big chunks all over the chamber Designed a cap for MBE furnaces to remove line of sight from chamber Successful growth using azides

S-20 photocathode NaKSb with CsSb layer 12 Wavelength (nm) QE First Results Ideal S-20 shows has cut- off in the infrared and QE upto 50% in the green

GaAs cathodes – Monte-Carlo simulations 13 3-Step photoemission model Excite electrons. Transport to surface – includes Monte-Carlo scattering with phonons, holes etc. Emission from surface. e-e- e-e- Higher photon energy -> Higher MTE, Higher QE, Shorter response time Can we manipulate electron transport to suit our needs?

14 Low MTE layered cathodes using MBE

Surface effects in GaAs photocathodes Small effective mass of electrons in GaAs and conservation of transverse momentum implies theoretical MTE <5 meV of transverse energy spread. Some experiments reproduce this Most measured values >120 meV. 15 Possible causes Surface roughness and cleanliness Eliminated by use of MBE grown / arsenic capped atomically flat samples Scattering at surface/ in Cs layer Needs to be explored

Surface scattering Cs islands – This can cause non- uniform work function leading to loss of momentum conservation 16 Scattering in amorphous Cs layer – LEED/RHEED measurements show that the Cs layer is amorphous. This could cause scattering in this layer.

Acknowledgements S. Karkare, I. V. Bazarov, L. E. Boulet, M. Brown, L. Cultrera, B. Dunham, N. Erickson, G. Denham, A. Kim, B. Lillard, T. P. Moore, C. Nguyen, W. Schaff, K. W. Smolenski, H. Wang. Dimitre. A. Dimitrov from Tech-X Corp, Boulder, CO Others in ERL team. NSF and DOE for funding. 17