Effect of electrostatic field on energy conversion efficiency in high current Raman regime free electron laser

The effect of the longitudinal electrostatic field on the energy conversion efficiency is investigated by use of a 1-dimensional FEL amplification code. It is found that the repulsive electrostatic interaction prevents the periodic bounce motion of trapped electrons in the ponderomotive potential especially in the high current Raman operation regime. The irregular rotation of the trapped electron increases the untrapped one, which removes the kinetic energy from the radiation field. Resultantly, the energy conversion efficiency averaged over a long wiggler distance stays in a lower level than that predicted by the trapping argument. It is also found that the finite beam energy spread reduces the electrostatic force in the nonlinear trapping stage. The reduction of the electrostatic force recovers the periodic electron bounce motion and the associated amplitude oscillation of the radiation field. (author)