and $\sigma_t \approx 90\,$fs with an energy per pulse of $\approx 70\,\mu$J. The configuration of the rest of the machine was different than the one described in Table \[Table\_FEL1\_parameters\]: 6 radiators were used, tuned to the $8^{th}$ harmonic of the seed ($32.6\,$nm), $R_{56} = 60\,\mu$m and the electron beam had the following parameters: mean energy $1175\,$MeV, $\chi_1  \approx 0\,$MeV/ps, $\chi_2 \approx 10\,$MeV/ps$^2$, FWHM duration $0.8\,$ps, peak current $\approx 600\,$A.[]{data-label="Figure_detuning_experiment"}](Figure_detuning_experiment){width="30.00000%"}

Also the relative heights of the peaks can be easily controlled, favouring the growth of one over the other, by tuning the wavelength corresponding to FEL resonance of the undulators. The spectrum represented by the dotted line in Fig. \[Figure\_detuning\_experiment\] corresponds to a central tuning of the undulators. The data represented by the solid line were taken at slightly negative detuning; while both peaks can be seen, the one at shorter wavelength is significantly stronger. The dashed curve shows a situation where the detuning is positive, for which the peak of the gain curve has moved to longer wavelengths. Here the shorter wavelength pulse has been almost completely suppressed.

Simulations
===========

The formation of the double peak is well reproduced by simulations [@PRLDP] carried out using both 3D [@genesis] and 1D 