FEL Efficiency Enhancement via Concurrent RF Acceleration

"The goal of this program is to analyze, design, and develop an efficiency enhancement scheme for free-electron lasers (FELs). In the proposed technique, a radio-frequency (RF) electric field is superimposed on the FEL wiggler field to reaccelerate theelectron beam as it loses energy. This reacceleration allows the electrons to remain in resonance with the wiggler and radiation fields over a longer distance, much as tapering the wiggler does in typical FEL enhancement schemes. The advantage to be gainedby the RF acceleration technique is improved control of the longitudinal electron beam dynamics (minimization of beam energy spread). During Phase I, the primary focus will be on use of the lower frequency FEL resonance to accelerate the electrons in anapplied RF field (inverse FEL mechanism). This approach will be contrasted with an alternative design in which an RF accelerating structure is added to the FEL interaction region. In Phase II, a preferred approach will be selected for detailed design,fabrication, and testing. Efficiency enhancement is crucial in high-average-power FELs, such as are being suggested for antiship cruise missile defense and other defense applications, as well as for industrial processing and other commercial uses. Animportant drawback of conventional tapered-wiggler efficiency enhancement is the large energy spread it imposes on the electrons, making subsequent acceleration or energy recovery very difficult.