Overview on development of metallic and superconducting photocathodes by the PLD technique for linear accelerator sources

In this paper, we report on the current status of the different configurations that have been suggested for the preparation of metallic and superconducting photocathodes (MPs and SCPs, respectively) by pulsed laser deposition. These strategic devices are used in the new generation of free-electron lasers and in research and development into plasma-based accelerators. To date, three different types of photocathodes have been used: conventional, hybrid and non-conventional configurations. In the case of the conventional configuration, the photocathode consists of a metallic bulk, typically Cu for normal conducting radio-frequency guns and Nb for superconducting radio-frequency guns. In the hybrid configuration, a thin emitting film with high photoemission performance, like Y or Mg, is located in the centre of the Cu flange of the photoinjector. For hybrid SCPs, the emitting material is a thin film of Pb, which has a quantum efficiency higher than that of Nb. Non-conventional configurations of MPs and SCPs have recently been engineered that consist respectively of a Y disc partially covered by a film of Cu and a Pb disc partially covered by a film of Nb. The central surfaces of the Y and Pb discs are not coated, thus providing an emitting area of the materials with higher quantum efficiency. These last devices inserted into a RF gun offer both the photoemission properties and work function of Y and the advantages of Cu in the case of MPs. In the case of SCPs, this new approach offers the relatively high photo-emissive properties of Pb and at the same time preserves all the advantages of Nb as a superconducting material.