Requirements for high-surface electric fields for high-brightness FEL injectors

For FEL (free-electron-laser) operation, the advantages of photocathodes over thermionic cathodes for the emission of electrons with the required properties are presented. Simulation codes have been used to design a photoinjector prototype for testing the highest surface electric fields an RF gun cavity could sustain at 144 MHz. From the available information on different RF systems operating at high fields (continuous wave or pulsed beams), the authors infer that 15 MV/m should be a realistic value for the peak surface field. However, their photoinjector system is designed to study higher field limits as they are of crucial importance for FEL operation with the required characteristics