The longitudinal phase space distribution of ultra-relativistic electron bunches for three different relative timings (−97 fs, −21 fs and 168 fs) between seed pulse and electron bunch are shown in panels (a–c). The panels (d–f) display the corresponding reconstructed power profiles. The two-dimensional histogram of panel g shows the correlation between seeded FEL peak power and the relative position of the seed laser pulse maximum in time. The histogram uses Gaussian smoothing with an rms size of 1 pixel. The white line indicates the seeded FEL performance predicted with the semi-analytical Ming-Xie formalism using electron beam parameters extracted with no FEL action at all. Panel h shows the measured energy spread from the unmodulated reference bunches and the reconstructed emittance used to calculate the prediction in panel g. The inset shows the longitudinal profile of the slice emittance in the core region of the bunch. It has its minimum at about −50 fs and increases towards the outer wings of the scan in agreement with the measured reduction of the emitted FEL power in these regions.