Photocathodes for SRF Gun
Photocathodes play a key role in all photo-injector systems. With the strong connection to the laser system, the choice of the optimum cathode type and its further improvement is a main issue for the progress in photoinjectors. Especially the SRF guns have rigorous requirements for the cathodes, including quantum efficiency (QE) >1% in a lifetime over 100 hours, the least contamination and heat load to superconducting Nb cavity, being robust against high laser intensity and high rf field, no C or H element on the surface of the plug, and also low field emisstion.
Since 2016 the SRF Gun II has been operated with magnesium photocathodes. Mg has a low work function of 3.6 eV and is the metal with the highest QE ever used as a photocathode in accelerator electron sources. With ultraviolet laser light at 260 nm, the expected QE of Mg is over 0.1 %. The combination of the present UV laser and the Mg cathode allows for production of beam with up to about 300 pC, which is limited by the space charge effect in the gun with an acceleration gradient of 8 MV/m (20.5 MV/m peak field on axis).
On the other hand, the high QE of semiconductor material is attractive for high bunch charge operation. For example Cs2Te with QE between 1 and 10 % which has been developed for the SRF gun-I. In our laboratory GaAs (Cs, O) and GaN (Cs) photocathodes are also exploited to search new materials with high QE for high current electron sources.