COPPER PHOTOCATHODES DEVELOPMENTS AT ASTEC
description
Transcript of COPPER PHOTOCATHODES DEVELOPMENTS AT ASTEC
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
2
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
13
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.