Stochastic electron detrapping in FELs (Free Electron Lasers) caused by sidebands
Abstract
The growth of sidebands in a free electron laser (FEL) above a certain threshold will result in stochastic electron motion. This may lead to significant electron detrapping and loss of amplification for the radiation field. The threshold in the sideband amplitude for the stochastic transition is computed. The rate trapped electrons leak outside the sepratrix is measured by the diffusion coefficient in action space. This approach is general and covers both tapered and untapered wigglers. Three general types of spectra are examined: a narrow, a broad discrete and a broad continuous. Each type is associated with a particular scaling of the diffusion coefficient on the FEL parameters. Numerical results, obtained for constant radiation amplitudes, show good agreement with the theoretical predictions. This diffusion rate is always proportional to the total sidebandtocarrier power ratio, with different coefficients of proportionality for each spectral type. The narrow types of spectrum causes the highest diffusion rates and the broad continuous spectrum causes the lowest diffusion rates under constant total sideband power. The diffusion length, measured in wiggler periods, is independent of the beam energy gamma. Serious deterioration in the FEL efficiency will result when the diffusion length becomes shorter than the wiggler length.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 July 1988
 Bibcode:
 1988STIN...8914439R
 Keywords:

 Amplification;
 Amplitudes;
 Deterioration;
 Diffuse Radiation;
 Electrons;
 Energy Dissipation;
 Free Electron Lasers;
 Sidebands;
 Stochastic Processes;
 Trapped Particles;
 Coefficients;
 Continuous Spectra;
 Diffusion Coefficient;
 Numerical Analysis;
 Radiation Spectra;
 Rates (Per Time);
 Spectral Theory;
 Lasers and Masers