Abstract
In this paper, we systematically study the echo-enabled harmonic generation (EEHG) free electron laser (FEL). The EEHG FEL uses two modulators in combination with two dispersion sections that allow one to generate in the beam a high harmonic density modulation starting with a relatively small initial energy modulation of the beam. After presenting an analytical theory of the phenomenon, we address several practically important issues, such as the effect of incoherent synchrotron radiation in the dispersion sections, and the beam transverse size effect in the modulator. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows one to generate a fully (both longitudinally and transversely) coherent radiation. As an example, we demonstrate that 5 nm coherent soft x rays with GW peak power can be generated directly from the 240 nm seeding laser using the proposed EEHG scheme.
Highlights
There has been continually growing interest in generating coherent and powerful short wavelength radiation using the free electron laser (FEL) scheme, as reflected by the many proposals and funded projects worldwide [1]
We have demonstrated a new working scheme of harmonic generation in FELs
The enabled harmonic generation (EEHG) FEL significantly improves the performance of single-stage harmonic generation FEL
Summary
There has been continually growing interest in generating coherent and powerful short wavelength radiation using the free electron laser (FEL) scheme, as reflected by the many proposals and funded projects worldwide [1]. The up-frequency conversion efficiency for this classic HGHG scheme is relatively low: generation of the nth harmonic of the seed laser requires the energy modulation amplitude approximately equal to n times the slice energy spread of the beam. In order to get higher harmonics while keeping the energy spread growth within an acceptable level, a double-undulator HGHG scheme was recently suggested [7] where the modulator is subdivided into two pieces with a phase shifter between them. This allows for generation of substantial bunching at higher harmonics while simultaneously limiting the growth of the energy spread.
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