Simulation study on efficiency enhancement of 10–140 GHz FEL with wiggler tapering and a prebunched e-beam

Efficiency enhancement with wiggler tapering and wiggler reductions with prebunching of e-beam in an induction linac-based FEL are assessed numerically in detail. The design of the wiggler with a prebunched beam is related to the upgrading of JAERI microwave FEL because it is crucial for the generation of extremely high-power RF pulse anticipated therein. Simulation studies to optimize the device parameters confirm that (1) an independent use of the prebunching is effective to reduce the rise distance of RF pulse along the wiggler, without significant enhancement of the saturation level, (2) wiggler tapering would lead to the extraction efficiency in energy as high as 50% when well optimized, and (3) the use of the optimized wiggler with the prebunched beam will realize a compact and high-power microwave FEL system. The tapering pattern depends on all of the FEL parameters and specifically on the initial distribution of the beam in phase space. The mechanisms involved such as the particle trapping in the ponderamotive potential well are discussed.