COPPER PHOTOCATHODESDEVELOPMENTS AT ASTEC

R. ValizadehAccelerator Science and Technology

Centre Science & Technology Facility Council, UK

• Overview of work done at other accelerator centres

• Photocathode preparation program at Daresbury Lab

Outline

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Important Properties of Photocathodes• High Quantum efficiency • Fast response time• Uniform emission• Low as possible emittance (atomically flat)

• Have reasonably high operating life time

at vacuum level 10-9 to 10-10 Torr range.• Easy to fabricate, reliable, reproducible QE

and emittance after cathode cleaning and re-activation.

Why Metal Photocathodes• Metal cathodes have fast response time

10-15 to 10-14 seconds.• They are robust and can survive years at

high cathode fields to produce high brightness beam

• However due to the high work function UV photons are needed for reasonable QE.

Metal Photocathodes

QE and thermal emittance

Variation of Emittance and QE with laser wavelength and spot size

Effect of surface roughness on Emittance.

Survivability and lifetime

Dependence of lifetime on residual operating gas species

Effect of laser surface cleaning for restoring QE

H-ion bombardment and Ozone

Annealing and Ar plasma Cleaning

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Photocathode preparation and diagnostic system

QE Measurment setup

Vacuum suitcase for transportation of prepared photocathode to RF Gun.

Initial Programme• Establish reliable and reproducible sample preparation

procedure to minimise the final surface carbon contamination. (done at SLAC)

• Prepare atomic flat surfaces for single crystal and poly-crystalline copper.

• Produce engineered rough surface.• Determine QE for different method of surface regeneration

(Ar, H and Ozone cleaning)• Determination of surface roughness after each procedure.• Identify the effect each individual residual gas species(H2,

CO, CO2, H2O and CH3) on QE degradation.• Optimization of wide band gap coating (CsBr) on copper.• Study the effectiveness of the Vacuum suitcase.