Photocathodes for free electron lasers

Many different photocathodes have been used as electron sources for FELs and other electron accelerator systems. In choosing one, a compromise between lifetime and quantum efficiency has been unavoidable. High quantum efficiency photocathodes such as K2CsSb, Cs3Sb, and cesiated GaAs have short operational lifetimes and require an ultrahigh-vacuum environment. Long lifetime photocathodes such as LaB6, Cu, and Y have relatively low quantum efficiencies. However, recently, cesium telluride was found to be an exception. Initial results from CERN and now at Los Alamos have shown that Cs2Te is reasonably rugged with a high quantum efficiency below 270 nm. Further studies carried out at Los Alamos have determined that its performance as an electron source for the Los Alamos Advanced FEL is excellent.