Evolution of a finite pulse of radiation in a high-power free-electron laser

The development of an optical pulse of finite axial extent is studied by means of an axisymmetric time-dependent particle simulation code for different rates of tapering of the wiggler field. The results illustrate a number of physical phenomena underlying the free-electron laser mechanism. These include: suppression of the sideband instability: the role of gain focusing versus that of refractive guiding: efficiency enhancement; and pulse slippage. It is found that a significant reduction in the sideband modulation of the optical field can be achieved with a faster tapering of the wiggler parameters. Increasing the tapering rate also reduces refractive guiding, causing the optical wavefronts to become more convex, thus spreading the optical field into a larger cross section. The corresponding enhancement of the peak output power is associated with an increased lateral extent of the optical field rather than an increase in the field amplitude