Since 1992, the CLIC Test Facility (CTF) has used photocathodes (illuminated by laser) as intense sources of electrons. These cathodes are mounted in an RF gun at 3 GHz and subjected to high electric fields, greater than 100 MV/m, in order to produce a beam charge of more than 10 nC in less than 10 ps. Traditional photocathodes developed for photo-multipliers have been shown to be unsuitable for our application. After having tested several types of photocathode, metallic, semi-conductor or impregnated, we have selected cesium telluride material because of its compatibility with the specifications of the photoinjectors and, at the same time, with the technical performance of our laser installations, but it must be transferred under vacuum from the production laboratory to the RF gun.
A photocathode is characterized by:
The photoemission threshold - defines the wavelength range of the laser (IR, visible, UV) ; | |
The quantum efficiency ( the ratio of the emitted electrons over the incident photons) - defines the power of the laser ; | |
The lifetime - defines the working time and the photocathode production frequency ; | |
Working conditions - defines the robustness of the photocathode. |
Different kind of photocathodes are used :
Metallic photocathodes | |
Alkaline photocathodes | |
Photocathodes to produce Polarized electrons |
Photocathodes are developed and produced at the Photoemission laboratory, transported under UHV to the CTF, then used in the drive beam RF gun.
Publications with contribution of the Lasers & Photocathodes section members