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 on the cathodes, including high quantum efficiency (QE) >1% within a lifetime of 100 hours, the heat load to the superconducting Nb cavity, high robustness against high laser intensity and high rf field, low concentrations of carbon, oxygen and hydrogen on the photocathode surface, and also low field emission.
Since 2016 the SRF Gun II operated with magnesium (Mg) 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 the production of the 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 other semiconductor materials is gaining more and more interest, especially for the operation of high bunch charges . For example Cs2Te with QE between 1 and 10 % has been developed for the SRF gun-I. Besides Cs2Te photocathodes, the potential usage of GaAs (Cs, O) and GaN (Cs) photocathodes are also studied in our laboratories to achieve better photocathode performances for high current electron sources.
Different Photocathodes in HZDR
 Xiang, R. et al. Review of Recent Progress on Advanced Photocathodes for Superconducting RF Guns. Micromachines 13, 1241 (2022). https://doi.org/10.3390/mi13081241
 Teichert, J. et al. Successful user operation of a superconducting radio-frequency photoelectron gun with Mg cathodes. Phys. Rev. Accel. Beams 24, 033401 (2021). https://doi.org/10.1103/PhysRevAccelBeams.24.033401
 Schaber, J. et al. Review of photocathodes for electron sources in particle accelerators. J. Mater. Chem. C (2023) https://doi.org/10.1039/D2TC03729G