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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.