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.
Influences of parameters on the nonlinear power are simulated by using a Hamiltonian model. Results show that the saturation power is mainly determined by the mismatched parameter, and optimum of the mismatched parameter increases the saturation power and shortens the saturation-interaction length. Also, higher beam current leads to a higher saturation power and shorter saturation-interaction length.I.
Saturation-power enhancement of a free-electron laser (FEL) amplifier by using tapered wiggler amplitude is based on the postponement of the saturation length of the uniform wiggler. In this paper, we qualitatively and quantitatively demonstrate that the saturation-power enhancement can be approached by means of the parameters adjustment, which is comparable to that by using a tapered wiggler. Compared to the method by tapering the wiggler amplitude, the method of parameters adjustment substantially shortens the saturation length, which is favorable to cutting down the manufacture and operation costs of the device.
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