The objective of this research program is to investigate the performance of a free-electron laser for which a gaseous medium has been introduced into the region of the wiggler, or undulating magnetic field. Placing a gas in an FEL wiggler alters the phase matching condition. This is a consequence of the fact that the gas introduces an index of refraction which retards the phase velocity of the electromagnetic wave. By changing the pressure of the gas, and so changing the index of refraction, the oscillation wavelength can be tuned, potentially from the infrared to the ultraviolet, without altering either the energy of the electron beam or the wiggler parameters. In addition to the tunability, the gas allows for a significant reduction in beam energy, resulting in a lower cost and size for the accelerator, and a higher gain per unit length for the FEL. As part of the program on FEL performance with a gas, there will be an investigation of the effects of operating near an electronic resonance in the gas. In this case a high refractive index can be obtained at relatively low gas densities. There will also be a study of the possibility of using interferometer mirrors with on-axis holes to facilitate development of a compact in-line, high-gain system. In addition, the experiments will provide information concerning the propagation of picosecond electron beams through gases at up to one atmosphere pressure; investigate methods for increasing FEL gain with high emittance beams; and study the optical properties of interferometers with on-axis holes.
