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

In order to interpret the experiment of Conde and Bekefi, electron tra jectories are studied for a configuration with a reversed axial guide field. First, ignoring the radial dependence of the wiggler, an analyt ic model is constructed. The results are discussed and compared with t hose of numerical simulations. This first part shows how electrons, wh ich remain close to the axis of the beam, can contribute to the dip in the power output observed. Then, the radial dependence of the wiggler is considered. In the vicinity of ''anti-resonance'', electrons are s hown to have ''chaotic'' trajectories when emitted far enough from the axis. Thus, the interaction efficiency is degraded for most particles . This explains the large dip in radiative power observed and predicte d by our simulation code. The radial dependence also gives a possible explanation for the very good efficiency observed in the experiment.