Free Electron Laser

Recent observations of amplification of submillimeter 1 and infrared 2 electromagnetic waves using a relativistic electron beam (REB) have created interest in applying the mechanism to produce a high-power, tunable laser in the infrared to visible range as well as in speculating the possibility of constructing an X-ray laser. This paper introduces the basic mechanism of the amplification processes and discusses the limitations in the power and frequency referring to the presently available REBs. A nonspecialist should be able to follow the contents without referring to special references. Section II introduces Lorentz transformation of various variables between the beam and the laboratory frames, which are used in succeeding sections. One of the important discussions presented here is the distinction between the stimulated Compton and stimulated Raman scattering. When the scattering occurs by an excitation of a single particle state, uncorrelated free-streaming motion of electrons, it is called the stimulated Compton scattering; if it occurs by an excitation of plasmon, the collective plasma oscillation of the electrons, it is called the stimulated Raman scattering. In most cases, the stimulated Compton scattering has a gain which is too small to be useful for practical purposes. Hence, the limitation in the output frequency is decided by whether or not the relativistic electron beam can be operated in the stimulated Raman regime. The beam current density and the energy spread is the decisive factor for this, as shown in Section III.