Collective Theory of the Free-Electron Laser

Abstract

The free-electron laser has the great advantages of high output power, continuous tunability over wide frequency range, and possible high efficiency of energy transfer from the kinetic energy of a relativistic electron beam to the electromagnetic wave energy. The amplification mechanism for the free-electron laser can be explained on the basis of the models of stimulated scattering of electromagnetic waves by a relativistic electron beam, namely, the stimulated Compton scattering in the shorter wavelength region and the stimulated Raman scattering in the longer wavelength region. The former scattering process treats the scattering by individual electrons composing a relativistic electron beam, while the latter scattering process considers the scattering by collective oscillation of electrons excited in the electron beam. In this chapter, we discuss the amplification mechanism for the free-electron laser in the longer wavelength region of millimeter to submillimeter waves, with the aid of the stimulated Raman scattering model. From a general point of view, the process of the stimulated Raman scattering in a relativistic electron beam can be regarded as a parametric interaction of the three waves, i.e., the pump wave, the scattered wave (positive-energy wave), and the electron plasma wave (negative-energy wave). In other words, under the influence of the pump wave, energy is exchanged between the scattered wave and the electron plasma wave, of which the former is extracted as the laser output. For the pump wave, an intense electromagnetic wave is used. However, in place of an electromagnetic wave pump, we can also use a transverse static magnetic field varied periodically in the direction of beam flow to get a laser action. This is because a transverse periodic static magnetic field behaves in the rest frame of the electron beam as an electromagnetic wave varying temporally with constant frequency, which plays the same role as the pump wave.KeywordsStimulate Raman ScatteringScattered WavePump WaveRelativistic Electron BeamCollective TheoryThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.