Electron Dynamics and Harmonic Generation in the FreeElectron Laser.
Abstract
The freeelectron laser (FEL) is a device for converting kinetic energy in a relativistic electron beam directly into laser light. A classical theory of the FEL is discussed. This theory uses the Boltzmann distribution to describe the electrons and Maxwell equation to describe the evolution of the laser field. The harmonic expansion of the Boltzmann equation leads to a set of "quasiBloch" equations describing the electron distribution. The behavior of the momentum distribution is discussed theoretically in the smallsignal regime of the FEL. The distribution function changes in nontrivial ways. The electron distribution does not only experience a recoil, but is also subject to spread through the amplification process. The recoil plays a relatively minor role compared to the spread. This behavior of the electron distribution may play an important role in the efficiency of the FEL. Freeelectron lasers, except for those using helical wigglers, are predicted in most cases to generate higher harmonics, of the fundamental optical frequency, in the forward direction. The basic equations describing this process are derived by using the multiplescaling perturbation theory, which leads to the slowlyvarying Maxwell and Boltzmann equations. Harmonic generation in the FEL offers a possible means to extend the wavelength range of the device towards high frequency. Numerical calculations are shown for CW operation using a linearly polarized wiggler. Higher harmonic emission becomes enhanced as the magnetic field is increased and as the energy spread in the electron beam is reduced. Coherent pulse propagation in the picosecond pulse regime of the FEL is treated. Coherent transient effects such as laser lethargy are discussed. The effect of laser lethargy is seen to play an important role in the pulsed FEL, as it does in conventional sweptgain amplifiers based on an atomic medium. Numerical calculations of the harmonics in the pulsed FEL are presented. The optical pulses show a ringing behavior which is sensitive to the reflectivity of the cavity mirrors and to the electron current.
 Publication:

Ph.D. Thesis
 Pub Date:
 1982
 Bibcode:
 1982PhDT........26A
 Keywords:

 Physics: Optics