Efficiency enhancement in a two-stream free electron laser with a helical wiggler

Efficiency enhancement in a two-stream Free Electron Laser (FEL) by a tapered wiggler prior to saturation regime is treated. Wave-particle interaction is described by a set of nonlinear coupled differential equations in 1D approximation. The scaled vector potential correspond to the out-put power is plotted versus scaled axial distance. It was found, that, the output power was enhanced to a level of almost 51.79%. Bunching parameter was plotted and compared with the case of untapered wiggler field. It is found, that, bunching parameter increased when tapering of the wiggler is executed. Bunching of the electrons in a phase space is shown, too.