Enhanced durability NEA surface layer for GaAs photocathodes using Cs, O$_2$, and Li*

Photocathodes based on GaAs are used in photo-electron sources to supply spin-polarized, high-current electron beams for accelerator applications. A thin surface layer, typically comprised of Cs and an oxidant, needs to be applied to such photocathodes during an activation process in order to achieve negative electron affinity (NEA). The NEA layer and the associated quantum efficiency (QE) deteriorates over time. This decay is characterized by the so-called lifetime, a crucial parameter for photo-electron source operation. An increase of surface-layer durability and, hence, lifetime by adding Li during activation has been successfully shown in previous studies. \\ Results of recent lifetime studies of bulk GaAs photocathodes activated with Cs, O$_2$, and Li using the activation chamber of the Photo-CATCH setup at Technische Universität Darmstadt will be presented, as well as additional measurements planned in the future.