Aphotoinjector[1] is a type of source for intenseelectron beams which relies on thephotoelectric effect. Alaser pulse incident onto the cathode of a photoinjector driveselectrons out of it, and into the accelerating field of theelectron gun.[2] In comparison with the widespreadthermionic electron gun, photoinjectors produce electron beams of higher brightness, which means more particles packed into smaller volume of phase space (beam emittance). Photoinjectors serve as the main electron source for single-passsynchrotron light sources, such asfree-electron lasers[3] and for ultrafastelectron diffraction setups.[4] The firstRF photoinjector was developed in 1985 atLos Alamos National Laboratory and used as the source for a free-electron-laser experiment.[5][6] High-brightness electron beams produced by photoinjectors are used directly or indirectly to probe the molecular, atomic and nuclear structure of matter for fundamental research, as well as material characterization.
A photoinjector comprises a photocathode, electron gun (AC or DC), power supplies, driving laser system, timing and synchronization system, emittance compensation magnets. It can include vacuum system and cathode fabrication or transport system. It is usually followed by beam diagnostics and higher-energy accelerators.
The key component of a photoinjector is aphotocathode, which is located inside the cavity of electron gun (usually, a 0.6-fractional cell for optimal distribution of accelerating field). Extracted electron beam suffers from its ownspace-charge fields that deteriorate the beam brightness. For that reason, photoelectron guns often have one or more full-size booster cells to increase the beam energy and reduce the space-charge effect. The gun'saccelerating field isRF (radio-frequency) wave provided by aklystron or other RF power source. For low-energy beams, such as ones used in electron diffraction and microscopy,electrostatic acceleration (DC) is a suitable.
Thephotoemission on the cathode is initiated by an incident pulse from the drivinglaser. Depending on the material of thephotocathode, the laser wavelength can vary from 1700 nm (infrared) down to 100-200 nm (ultraviolet). Emission from the cavity wall is possible with laser wavelength of about 250 nm for copper walls or cathodes.Semiconductor cathodes are often sensitive to ambient conditions and might require a clean preparation chamber located behind the photoelectron gun. The optical system of the driving laser is often designed to control the pulse structure, and consequently, the distribution of electrons in the extracted bunch. For example, afs-scale laser pulse with an elliptical transverse profile creates a thin "pancake" electron bunch, that evolves into a uniformly filled ellipsoid under its own space-charge fields.[7] A more sophisticated laser pulse with a comb-like longitudinal profile generates a similarly shaped, comb electron beam.[8][9]