Effect of tapering on optical guiding and sideband growth in a finite-pulse free-electron laser

The results of a numerical study of a high-current approximately kiloampere high-power, approximately gigawatt, short wavelength, approximately micrometer, tapered-wiggler, free-electron laser are presented. The development of an optical pulse of finite axial extent is studied by means of a numerical code. It is found that increasing the tapering rate reduces refractive guiding, causing the optical wavefronts to become more convex, thus spreading the optical field into a larger cross section. This increases the output power and efficiency. Concomitant with this, there is a significant reduction in the sideband modulation of the optical field. It is also found that as the tapering rate is increased there is a gradual transition from a refractive-guiding regime to one where gain focusing dominates, with optical power diffracting laterally along the convex wavefronts. The increased transverse extent of the optical field, rather than an increase in the field amplitude, is the major reason for efficiency enhancement as the tapering rate is increased