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Siddharth Karkare

Photocathode theory and measurements. Siddharth Karkare. OUTLINE. Motivation and requirements Photocathode experimental facilities at Cornell Alkali-antimonide cathodes GaAs based photocathodes and photoemission theory. Why photocathodes?. What we need from them?.

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Siddharth Karkare

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  1. Photocathode theory and measurements SiddharthKarkare

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

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

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

  5. Photocathode diagnostics lab QE surface scan of NaKSb cathode 2-D energy distribution from GaAs at 780nm Auger surface scan of K on a NaKSb cathode All connected in vacuum of less than 10-10torr Yo-Yo activation of GaAs LEED pattern from GaAs

  6. 2-D energy analyzer

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

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

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

  10. Alkali-antimonides – Exploring new sources Pure metal alkali sources Alkali Azide (AN3) SAES dispensers ALVATEC sources

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

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

  13. GaAs cathodes – Monte-Carlo simulations 3-Step photoemission model e- e- Excite electrons. Transport to surface – includes Monte-Carlo scattering with phonons, holes etc. Emission from surface. 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

  15. 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.

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

  17. 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.

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