Electron beam brightness in linac drivers for free-electron-lasers

Linear accelerators delivering high brightness electron beams are essential for a number of current and proposed accelerator applications, such as free electron lasers (FELs). In this kind of facilities the charge density is high enough to drive collective effects (wakefields) that, notwithstanding the high beam rigidity at energies up to the GeV range, may increase the beam emittance relative to the injection level, eventually degrading the nominal beam brightness. New theoretical developments and experimental capabilities, driven by the recent construction of vacuum-ultra-violet and X-ray linac-driven FELs, have advanced the present knowledge. This article describes the progress in the field of ultra-relativistic electron beam manipulation to maximize the final beam brightness, with a focus on the most recent techniques including optics design, pulse shaping and brightness optimization strategies. The theoretical models are supported by a review of the experimental results in now-running FEL facilities.