Efficiency enhancement in free-electron lasers driven by electromagnetic-wave wigglers

A scheme for efficiency enhancement in free-electron lasers which are driven by electromagnetic-wave wigglers is described which employs a tapered axial guide magnetic field. While tapered magnetostatic wiggler designs have been proven to be an effective means of enhancing the efficiency of free-electron lasers, practical difficulties occur in the design of tapered electromagnetic-wave wigglers. For this reason, it is shown that a tapered axial guide field can be used in concert with an electromagnetic-wave wiggler for the purpose of efficiency enhancement.