OPTIMIZATION OF THE FREE-ELECTRON LASER INTERACTION VIA ELECTRON BUNCH TRAPPING

Electromagnetic wave amplification in free electron lasers with a reve rsed guide field and a right-hand polarized wiggler field is considere d both analytically and numerically. Electron bunch trapping by the hi gh frequency electromagnetic field is used for efficiency optimization . Based on motion stability criteria, the possibility of bunch trappin g by tapering the FEL parameters is shown. For this purpose any of the FEL parameters, such as the spatial periodicity of the wiggler or the electromagnetic wave, or the strength of the wiggler or guide field m ay be tapered. The stability analysis of the electron motion is based on Lyapunov theory, permitting the determination of the optimum taperi ng rate. A particle simulation was carried out for an FEL with a rever sed guide field with only the wiggler field tapered. It was found that under the theoretically predicted tapering all electrons of the beam were trapped and it was possible to achieve as high an efficiency as a llowed by the FEL mechanism. The interaction region lengthens in this case but is still experimentally realizable, In particular, for FEL pa rameters close to experimental ones (relativistic factor gamma = 4.75, wiggler field strength B-w = 2.8 kG, guide field strength B-0 = - 1.4 kG, operation wavelength lambda = 6.2 mm), it was found that the effi ciency grew to over 50%, compared with about 20% efficiency in the hom ogeneous wiggler field case.