Cs–K–Te photo cathodes: a promising electron source for free-electron lasers

The characteristics of Cs–K–Te photo-cathodes when used in a photo-cathode linear accelerator will be presented together with a short review of their photo-emissive properties. The cathodes have been illuminated by light pulses obtained by frequency quadrupling the light of a Nd:YLF laser. The emitted electrons have been accelerated in a 1.3 GHz RF linear accelerator operating at an energy of 6 MeV. The quantum efficiency of the Cs–K–Te cathodes at 259 nm decreases exponentially with operation time, but recovers partially during storage in UHV between successive operating sessions. These two effects tend to compensate for long times, so that saturation of the quantum efficiency value at about 2% is reached after 20 h of operation. A Cs–K–Te cathode, degraded by use in the accelerator, can be rejuvenated to about 60% of its initial quantum efficiency by heating it to 120°C for 1 hour.