Photocathode RF guns

Free-electron oscillators and amplifiers require electron accelerators capable of delivering pulse trains of electron bunches of high charge density in a wiggler or undulator. A high electron density implies a high peak current (100 A to 2000 A) and a low transverse beam emittance (< 80 π x mm x mrad, determined by matching the transverse size of the electron beam to the optical beam in the wiggler). Electron beam collider machines also require high peak currents (> 8 nC in picoseconds) with extremely small emittances (< 10 π x mm x mrad). Several approaches have been proposed to attain such performance. This article discusses the use of photocathodes in attaining the aforementioned performance requirements. Photocathodes have been used as electron sources in lasertrons and for the production of spin-polarized electrons. A photocathode is a light-activated electron source that gives unprecedented control over all aspects of the electron distribution: peak current, spatial profile, and temporal profile. This control is possible because the electron distribution is not determined by grids or a cathode, but rather by an incident laser pulse on the photocathode. 44 references, 21 figures