Long wavelength free-electron lasers in 1995

A summary of the current status and most important future directions for long wavelength (≥0.5 mm) free-electron lasers is presented. The distinction between long and short wavelengths is a natural one since different physical processes may be important. Space-charge effects may be important for the high currents typically employed at long wavelengths, and the dominant interaction mechanism is often coherent Raman scattering. In addition, dispersion due to the dielectric effects and finite transverse dimensions in the drift tubes and cavities are important at longer wavelengths. The principal goals at long wavelengths are to achieve much higher average powers with good overall efficiency in a compact design, and the highest average power produced in an FEL to date (36 W) has been recorded in the Ku-band.