High-power free-electron laser amplifier using a scalloped electron beam and a two-stage wiggler

High-power free-electron laser (FEL) amplifiers present many practical design and construction problems. One such problem is possible damage to any optical beam control elements beyond the wiggler. The ability to increase the optical beam’s divergence angle after the wiggler, thereby reducing the intensity on the first optical element, is important to minimize such damage. One proposal to accomplish this optical beam spreading is to pinch the electron beam thereby focusing the radiation as well. In this paper, we analyze an approach that relies on the natural betatron motion to pinch the electron beam near the end of the wiggler. We also consider a step-tapered, two-stage wiggler to enhance the efficiency. The combination of a pinched electron beam and step-taper wiggler leads to additional optical guiding of the optical beam. This novel configuration is studied in simulation using the medusa code. For a representative set of beam and wiggler parameters, we discuss (i) the effect of the scalloped beam on the interaction in the FEL and on the focusing and propagation of the radiation, and (ii) the efficiency enhancement in the two-stage wiggler.