Abstract
The longitudinal coherence of free-electron laser (FEL) radiation can be enhanced by seeding the FEL with high harmonics of an optical laser pulse. The radiation produced by high-harmonic generation (HHG), however, has a fast-varying temporal profile that can violate the slowly varying envelope approximation and limited frequency window that is employed in conventional free-electron laser simulation codes. Here we investigate the implications of violating this approximation on the accuracy of simulations. On the basis of both analytical considerations and 1D numerical studies, it is concluded that, for most realistic scenarios, conventional FEL codes are capable of accurately simulating the FEL process even when the seed radiation violates the slowly varying envelope approximation. We additionally discuss the significance of filtering the harmonic content of broadband HHG seeds.
Highlights
The ability of free-electron lasers to produce coherent, high-power pulses of tunable radiation in spectral regions not accessible with conventional sources makes them highly attractive to many areas of research [1,2]
The radiation produced by high-harmonic generation (HHG), has a fast-varying temporal profile that can violate the slowly varying envelope approximation and limited frequency window that is employed in conventional free-electron laser simulation codes
We have considered seeding of free-electron lasers with radiation pulses containing frequency components that strongly violate the approximations that lead to the slowly varying envelope approximation (SVEA)
Summary
The ability of free-electron lasers to produce coherent, high-power pulses of tunable radiation in spectral regions not accessible with conventional sources makes them highly attractive to many areas of research [1,2]. At the other extreme— wavelengths much longer than that corresponding to FEL resonance—it is not clear that the strong radiation components in the HHG spectrum at the low harmonics of the drive laser can be safely neglected in terms of their effects upon the particle dynamics and, if not, whether the combination of spectral filtering and undulator-period averaging accurately capture these effects. We summarize a linearized Vlasov-Maxwell model whose details are given in the Appendix that is used for later comparison of results
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