Photocathodes for SRF gun
Advanced electron injectors are based on photocathode electron guns. These guns can produces electron beams with short pulses and high brightness. For all of the different photo-injector systems the photocathodes play a key role. In strong connection with the laser system the choice of the optimum cathode type and its further improvement is a main issue for the progress in photo-injectors. In this website an overview of the existing knowledge of photo cathodes for rf photoelectron injectors is presented.
The SRF-gun works in the ultra high vacuum, which does benefit to the operation lifetime of the photocathode. But it also has rigorous requirements for the cathodes.
1. Enough quantum efficiency (Q.E.>1%) in needed lifetime (~100 hours).
2. Robust against high laser intensity
3. The least contamination to Nb cavity
4. Plug without C or H element on surface.
5. The least heat load to superconducting cavity
6. Tolerance to high RF field
7. Low reflection for the drive laser
In Rossendorf SRF gun, Semiconductor Cs2Te photocathodes are the standard electron sources. Cs2Te is a p-type semiconductor with the work function of 3.5 eV. It keeps a good Q.E. in hundred of hours, and has the possibility to stand the high RF field. Besides the present Cs2Te photocathodes, GaAs(Cs, O) attracts a lot of attention because of its high Q.E. in the visible light range and the probability to produce high polarization beams.