Abstract
The optical klystron enhancement to self-amplified spontaneous emission (SASE) free electron lasers (FELs) is studied in theory and in simulations. In contrast to a seeded FEL, the optical klystron gain in a SASE FEL is not sensitive to any phase mismatch between the radiation and the microbunched electron beam. The FEL performance with the addition of four optical klystrons located at the undulator long breaks in the Linac Coherent Light Source (LCLS) shows significant improvement if the uncorrelated energy spread at the undulator entrance can be controlled to a very small level. In addition, FEL saturation at shorter x-ray wavelengths (around $1.0\text{ }\AA{}$) within the LCLS undulator length becomes possible. We also discuss the application of the optical klystron in a compact x-ray FEL design that employs relatively low electron beam energy together with a shorter-period undulator.
Highlights
An x-ray free electron laser (FEL) operated in the selfamplified spontaneous emission (SASE) mode is the primary candidate for the next-generation light source and is under active development around the world [1,2,3]
Motivated by the very small uncorrelated energy spread of the electron beam that has been measured in a photocathode rf gun [10], we study the possible optical klystron enhancement to SASE x-ray FELs
This feature distinguishes an optical klystron in a SASE FEL from that in a seeded FEL, which is always subject to phase matching unless the dispersively enhanced microbunching dominates over the radiation by more than an order of magnitude
Summary
An x-ray free electron laser (FEL) operated in the selfamplified spontaneous emission (SASE) mode is the primary candidate for the next-generation light source and is under active development around the world [1,2,3] In such a device, a high-brightness electron beam passing a long undulator develops energy and density modulations at the radiation wavelength and amplifies the spontaneous emission into intense, coherent radiation. Motivated by the very small uncorrelated energy spread of the electron beam that has been measured in a photocathode rf gun [10], we study the possible optical klystron enhancement to SASE x-ray FELs. In Sec. II, we generalize the previous high-gain optical klystron theory to a SASE FEL having a wide bandwidth. We summarize our studies and conclude that the optical klystron is a promising approach to enhance the x-ray FEL performance
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