of the seed power upon double spectro-temporal pulse formation. However, other parameters can influence the production and separation of the peaks. One is the strength of the dispersive section, as already discussed in [@PRLDP] and experimentally demonstrated here in Fig. \[Figure\_R56\_experiment\]. Indeed, the dispersive section has an effect very similar to that of the seed power: the higher its strength, the lower the required seed power to produce a double pulse. Some small differences in the trend may be observed essentially because changing the seed intensity may affect the non-linear optical material dispersion and thus the chirp, while this does not happen when the $R_{56}$ is varied. Similarly, when employing a strong dispersive section, a moderate seed power can be sufficient to increase the peaks separation. Fig. \[Figure\_chirp\_experiment\] shows a different way to modify the spectral separation of the two pulses, i.e., by changing the seed chirp. However, increasing the seed chirp above a given limit does not produce any significant change in the FEL spectrum. This can be expected from the limitation of $\Delta\lambda$ shown in Fig. \[Figure\_seed\_data\_without\_alphabeta\].

![Double peak for different tuning conditions of the radiators. The seed was in its standard configuration so that $\sigma_{\lambda} \approx 0.45\,$nm 