Long wavelength free electron lasers in 1994

A short summary of the current status and most important future directions for long wavelength free-electron lasers is presented. For the purposes of the discussion, long wavelength refers to wavelengths longer than 0.5 mm. The distinction between long and short wavelengths is not entirely arbitrary since different physical processes may be important. For example, higher current beams are typically employed at long wavelengths and space-charge effects may be important. Indeed, the dominant interaction mechanism is often coherent Raman rather than coherent Compton scattering. In addition, dispersion due to the beam dielectric effects and finite transverse dimensions in the drift tubes and cavities are important effects at longer wavelengths. The ultimate goals of long wavelength FEL research 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. At the present time, electrostatic accelerators and long pulse modulators appear to be the prime candidates for drivers for these systems; however, advances in induction linac technology which lead to higher repetition rates and improved beam quality could alter this conclusion.