Comparison of free-electron laser amplifiers based on a step-tapered optical klystron and a conventional tapered wiggler

• Main Author: H. P. Freund
• Format: Article
• Language: English
• Published: American Physical Society 2013-06-01
• Series: Physical Review Special Topics. Accelerators and Beams
• Online Access: http://doi.org/10.1103/PhysRevSTAB.16.060701

Free-electron laser amplifiers have been operated at high efficiency at wavelengths from the microwave through the visible. Typically, these amplifiers require long tapered sections and produce spent beams with large energy spreads that are 4–5 times the electronic efficiency. In addition, while optical guiding during exponential growth in the uniform wiggler section confines the optical mode, the guiding disappears in the tapered wiggler section resulting in a relatively large optical mode at the wiggler exit. Optical klystrons consist of a Modulator wiggler that induces a velocity modulation on the electron beam followed by a magnetic dispersive section that enhances the velocity modulation prior to injection into a second, radiator wiggler. Optical klystrons have been operated over a broad spectral range; however, no optical klystron has been built with a tapered radiator wiggler. A comparison between a optical klystron with a step-tapered Radiator wiggler and a conventional tapered wiggler amplifier is analyzed in this paper. The purpose of the step taper is to both enhance the efficiency and to extend the range of the exponential gain and so preserve the optical guiding over a longer interaction length. The step-tapered optical klystron and a tapered wiggler amplifier are compared for a nominal set of parameters to determine the differences in the efficiency, interaction length, spent beam energy spread, and the size of the optical mode at the wiggler exit.