Overview of high-brightness, high-average-current photoinjectors for FELs

We are now nearing the end of the second decade since the inaugural photoinjector experiments were carried out at Los Alamos National Laboratory in 1985. The interim period has seen a dramatic increase in the number of photoinjectors in operation around the world, in our physical understanding of their operation, and, consequently, the brightness of their beams. For some time now, photoinjectors have been the solution of choice for electron beam applications requiring very high-quality electron beams, such as free-electron lasers (FELs). In recent years, there has been a renewed interest in high-average-power FELs and, as a result, high-average-current photoinjectors. Here, we present an overview of photoinjector development with particular emphasis on high-average-current devices in FEL applications.