ELECTRON DYNAMICS 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. In the vicinity of ''antiresonance,'' electrons are shown to ha ve chaotic trajectories when emitted far enough from the axis. Thus, t he interaction efficiency is degraded for most particles. This explain s the large dip in radiative power observed and predicted by different simulation codes. The very good efficiency observed in the experiment is explained by a compromise between a high amplitude perpendicular v elocity and a regular enough axial momentum for the electrons. It is a lso explained by the fact that when photons are emitted, the axial kin etic energy is not too much affected. A new situation for which a smal l peak in efficiency should be observed is predicted. (C) 1996 America n Institute of Physics.