Since the electromagnetic energy gained by the laser wave in a free-electron laser (FEL) is transferred from the kinetic energy loss of a relativistic electron beam, the stability of electron motion is one of the key factors that affect FEL performance. In this paper the stability of electron motion is compared for different focusing regimes. It is demonstrated that the natural focusing regime of a three-dimensional wiggler is easily broken by the self-field of the electron beam. The magnetic focusing regime of an axial guide magnetic field is based on the superposition of a strong Larmor rotation on the transverse quiver motion of the electrons, while the electric focusing regime of an ion-channel guiding field generates an electric force to counteract the divergent effect of the beam self-field. In comparison with the magnetic focusing regime of an external magnetic system, the electric focusing regime of an ion-channel guiding field may yield smaller instantaneous Larmor radius and slighter Larmor-centre deviation from the axis and provide better motion stability.
Free Electron Laser for a Mildly Relativistic Electron Beam
