ELECTRON TRAJECTORIES IN A FREE-ELECTRON LASER WITH A REVERSED AXIAL-GUIDE FIELD

In order to interpret an experiment of Conde and Bekefi [Phys. Rev. Lett. 67, 3082 (199 1); 68, 418 (E) (1992)], electron trajectories are studied for a configuration with a reversed axial guide field. First, ignoring the radial dependence of the wiggler, an analytic model is constructed. The results are discussed and compared with those of numerical simulations. Close to ''antiresonance,'' two types of electron trajectories are considered and discussed. One type corresponds to a linearly polarized motion and leads to a moderate coupling with the radiation field. This first part shows how electrons, which remain close to the axis of the beam, can contribute to the observed dip in the power output. Then the radial dependence of the wiggler is considered. In the vicinity of ''antiresonance,'' electrons are shown to have ''chaotic'' trajectories when emitted far enough from the axis. As a consequence, the interaction efficiency is degraded for most particles. This explains the large dip in radiative power observed and predicted by our simulation computer code. The radial dependence also gives a possible explanation for the very good efficiency observed in the experiment.