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abstract:
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Electron beams are strongly microbunched near the high-gain free-electron laser
(FEL) saturation with a rich harmonic content in the beam current. While
the coherent harmonic emission is possible in a planar undulator, the third-
harmonic radiation typically dominates with about 1% of the fundamental power
at saturation. In this paper, we discuss the second-harmonic radiation in the
Linac Coherent Light Source. We show that by a suitable design of an second-
stage undulator with its fundamental frequency tuned to the second harmonic of
the first undulator, coherent second-harmonic radiation much more intense than
the third-harmonic is emitted. Numerical simulations predict that GW-level, sub-
Angstrom x-ray pulses can be generated in a relatively short second-harmonic
radiator.
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