Abstract: An electromagnetic-wave-wiggler free electron laser with a tapered axial guide magnetic field is proposed. A three- dimension self-consistent nonlinear simulation program about electromagnetic-wave-wiggler free electron laser is also developed. Both the scattered wave and the pump wave are considered to be disperse in the program. A set of coupled nonlinear differential equations is derived which governs the self-consistent evolution of either TE or TM modes in a loss- free cylindrical waveguide. The energy exchange of the electron beam with the pump wave and the scattered wave are fully taken into account. The simulation result of the output power for an untapered guide magnetic field is in agreement with the reported result in experiment. By using the simulation program, the electromagnetic-wave-wiggler free- electron laser with a tapered axial guide magnetic field is discussed. The results show that, compared to an untapered guide magnetic field, a tapered guide magnetic field can enhance the output power. However, the enhancement is related to the slope and the starting position of the taper of the guide magnetic field. !11
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