top of sufficiently high current portions of the electron beam and the second peak (at shorter wavelength) also appears.

As we remarked in the previous section, a linear energy chirp of the electron energy induces a spectral shift of the FEL emission. Hence, when they appear near the tail portion of the electron beam, both peaks are detuned towards shorter wavelengths because of the strong local negative slope of the electron beam chirp (see Fig. \[Figure\_ebeam\_measurement\]). For smaller relative timing delays (i.e., moving the seed from overlapping the tail toward the head of the bunch), the central wavelengths of the two peaks drift redward due to the increase of the value of the local linear chirp of the electron energy distribution. However, the separation between the two spectral peaks ($\Delta\lambda_6 \approx 0.17\,$nm), their relative heights and their respective widths remain practically constant. The wavelength drift flattens out about $100\,$fs before the first spectral peak disappears; this behaviour is mirrored $200\,$fs later in the second peak. This early disappearance might be caused by the small current ramp from the center of the bunch to its tail (see Fig. \[Figure\_ebeam\_measurement\]) and/or by degraded transverse properties in this region.

In this experiment, the 