A THERMIONIC-CATHODE RF GUN FOR COMPACT FEL USE

Conventionally designed thermionic-cathode RF guns, such as the one us ed in the Stanford Far-Infrared Free-Electron Laser (FIRFEL) experimen t, typically generate electron beams with large energy spreads. In ord er to use such a gun as the sole accelerator, most of the beam must be discarded to reduce the energy spread to acceptable levels. It is pos sible to design a thermionic-cathode RF gun, which produces an electro n beam essentially monochromatic in energy. One-and three-dimensional simulations indicate that an electron beam can be produced with more t han 90% of the beam current in a 1% energy spread, over a range of kin etic beam energies of 2-6 MeV. This may be accomplished while retainin g reasonable beam emittances. Such a gun is ideal for use with a compa ct FEL, such as the FIRFEL experiment. Potential benefits include a la rger operational wavelength range, higher power output, greater effici ency, and shorter beamlines.