Gaussian CHG pulse at wavelength $\lambda_n$ with slightly shorter duration[@Stupakov] . However, this statement (which, in the temporal domain, is mainly a prediction of numerical simulations, due to the lack of temporal diagnostics in the vacuum-ultraviolet spectral range), is true only under certain limits and depends on the regime of operation of the FEL. Indeed, when increasing the seed power, the temporal edges of the seed pulse are also able to produce strong bunching at entrance to the radiator so that a larger part of the electron beam emits intense FEL radiation, giving a longer FEL pulse. For very high seed powers, the electron-beam energy modulation at the peak of the seed (i.e., the central position for a Gaussian) becomes too strong for the particular $R_{56}$ value characterizing the strength of the dispersive section. Then there is an overbunching effect and the resultant bunching in the radiator is strongly depressed. Thus, the FEL emission from this central position falls. On the other hand, there are two temporal positions with lower seed power, symmetric with respect to the peak of the seed, that match the optimum bunching condition for a given $R_{56}$. Accordingly, the FEL signal grows and reaches the maximum 