Chaotic electron motion caused by sidebands in free electron lasers
Abstract
The electron dynamics in a Free Electron Laser (FEL) is studied in the case when the radiation field contains many modes. This situation arises when unstable modes (sidebands) are excited during operation. It is observed that when the strength of these sidebands exceeds certain levels, the electron motion becomes chaotic. This may lead to extensive particle detrapping and loss of amplification for the FEL signal. The threshold for the onset of stochastic electron motion is computed. The evolution of the trapped electron distribution exhibits a diffusive behavior. The rate of particle detrapping is parameterized by the diffusion coefficient D in action space. It is found that the diffusion rates are connected to the type of the sideband spectrum. The diffusion coefficient is always proportional to the ratio of the sideband power in all frequencies to the power of the carrier signal. The coefficient of the proportionality however scales differentially to the FEL parameters for each of the three spectral categories: a narrow, a broad discrete and a broad continuous spectrum. The diffusion coefficient is computed analytically for the last two cases and is in good agreement with numerical results. The narrow spectrum yields the highest and the broad continuous the measured in wiggler periods, is independent of the electron energy.
 Publication:

Naval Research Lab. Report
 Pub Date:
 October 1988
 Bibcode:
 1988nrl..reptS....R
 Keywords:

 Carrier Frequencies;
 Charged Particles;
 Continuous Spectra;
 Electron Distribution;
 Electron Energy;
 Free Electron Lasers;
 Particle Diffusion;
 Particle Motion;
 Sidebands;
 Trapped Particles;
 Diffusion Coefficient;
 Electrodynamics;
 Electron Mobility;
 Numerical Analysis;
 Radiation Spectra;
 Radio Signals;
 Stochastic Processes;
 Lasers and Masers