Abstract
We analytically examine the bunching factor spectrum of a relativistic electron beam with sinusoidal energy structure that then undergoes an echo-enabled harmonic generation (EEHG) transformation to produce high harmonics. The performance is found to be described primarily by a simple scaling parameter. The dependence of the bunching amplitude on fluctuations of critical parameters is derived analytically, and compared with simulations. Where applicable, EEHG is also compared with high gain harmonic generation (HGHG) and we find that EEHG is generally less sensitive to several types of energy structure. In the presence of intermediate frequency modulations like those produced by the microbunching instability, EEHG has a substantially narrower intrinsic bunching pedestal.
Highlights
External seeding schemes like high gain harmonic generation (HGHG) [1] and echo-enabled harmonic generation (EEHG) [2,3] have been developed to improve the spectral brightness and spectral stability of high-gain free electron lasers (FELs)
Uncontrolled energy structures lead to reductions in the spectral density for FEL seeding systems because they are translated to the broader FEL output spectrum
A final monochromator is needed to further filter the spectrum, but this comes with a limited efficiency (≤10%) that significantly reduces the overall gains in spectral brightness. Understanding these effects is important for evaluating HGHG and EEHG as competitive external seeding schemes, as they differ in their sensitivity to phase space structures
Summary
External seeding schemes like high gain harmonic generation (HGHG) [1] and echo-enabled harmonic generation (EEHG) [2,3] have been developed to improve the spectral brightness and spectral stability of high-gain free electron lasers (FELs). Can amplify broadband perturbations that end up peaked around ≳1 μm wavelengths In either case, such energy structures can be difficult to remove completely, and they mix extra frequencies into the FEL gain bandwidth that can spoil the high-contrast, narrowband performance improvements offered by seeding in general. A final monochromator is needed to further filter the spectrum, but this comes with a limited efficiency (≤10%) that significantly reduces the overall gains in spectral brightness Understanding these effects is important for evaluating HGHG and EEHG as competitive external seeding schemes, as they differ in their sensitivity to phase space structures. EEHG is compared with simple HGHG to highlight the general distinctions and to provide a conceptual framework for more detailed future comparisons, for ultrahigh harmonics that require cascading of multiple seeding stages
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