Free Electron Lasers: New Sources of Millimeter and Submillimeter Waves

Free electron lasers, combining the stimulation of radiation by an electron beam in an undulator and a Doppler upshifting of the stimulated radiation, presently provide efficient and tunable generators of millimeter and submillimeter waves. The interaction between the waves and the electrons can be either Compton scattering (magnetic bremstrahlung), the dominant effect at shorter wavelengths or, Raman (or Cerenkov) scattering with high gain at longer wavelengths. For both Compton and Raman regimes, the gain is a strong function of the beam current and beam emittance, i.e., of the beam brightness. Short wavelengths could be obtained, even with modest voltages, if short-period undulators can be developed. Recent new results are presented and briefly discussed.