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Siddharth Karkare
Photocathode theory and measurements
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OUTLINE
• Motivation and requirements• Photocathode experimental facilities at
Cornell• Alkali-antimonide cathodes• GaAs based photocathodes and
photoemission theory
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Why photocathodes?What we need from them?
4th 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
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Photocathode Facilities at Cornell
dedicated MBE system
over in Wilson Lab
actual injector
Phillips
Newman
Wilson
over in Newman Lab
Photocathode growth & analysis chamber
over in Phillips Hall
Cornell University campus
Vacuum Suitcase
Arsenic Cap Arsenic C
ap
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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
cathodeAll connected in vacuum of less than 10-10 torr
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2-D energy analyzer
First Marking
Electrode
Varying Magnetic
Field
Second Marking
Electrode
Beam Current
Detector
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Transverse energy analyzer (TE-meter)
Cathode
Grid (2-5kV)
Screen
Electron trajectories
Focused laser
Electron spot from TE-meter
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Alkali-antimonides Exploring new materials
elevated temperature, lifetime 90hrs
high current operation lifetime 66hrs
Na2KSb cathode
~15% QE reduction Cu
rren
t (m
A)
QEQE
Tem
pera
ture
(C)
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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
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Alkali-antimonides – Exploring new sources
SAES dispensers ALVATEC sources Alkali Azide (AN3)Pure metal
alkali sources
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Growth using azide sources
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
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S-20 photocathode• NaKSb with CsSb layer
Wavelength (nm)
QE
First Results
Ideal S-20 shows has cut-off in the infrared and QE upto 50% in the green
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GaAs cathodes – Monte-Carlo simulations
3-Step photoemission model
Excite electrons.
Transport to surface – includes Monte-Carlo
scattering with phonons, holes etc.
Emission from surface.
e-
e-
Higher photon energy -> Higher MTE, Higher QE, Shorter response timeCan we manipulate electron transport to suit our needs?
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Low MTE layered cathodes using MBE
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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.
Possible causesSurface roughness and cleanlinessEliminated by use of MBE grown /
arsenic capped atomically flat samples
Scattering at surface/ in Cs layer
Needs to be explored
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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.
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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.