Theoretical study of free-electron lasers with reversed guide field

Abstract

Free-electron laser (FEL) amplifier performance with a reversed axial guide field is studied theoretically and numerically. The theoretical analysis focuses on an analytic nonlinear calculation of the particle motion at negative resonance which predicts the amplitude of oscillation as a function of guiding center location. Numerical studies based on a new simulation code, which follows the three-dimensional particle motion and the self-consistent amplitude and phase of the rf field, reproduce the high efficiency results recently reported by Conde and Bekefi [1] for a reversed field FEL. These studies indicate that the saturated power is rather sensitive to the model used for the space-charge forces. The reported results for Group I orbits can be reproduced only by assuming an anomalously large emittance, while the Group II results are not consistent with the single-frequency model.