Fig. 4: FEL quadratic charge dependence and longitudinal coherence.
From: Seeded free-electron laser driven by a compact laser plasma accelerator
a, ξFEL versus charge measured at the beamline exit (Extended Data Fig. 3) using two sets of data, both recorded at the optimum gap (4.3 mm) and optimum delay (τ = +0.1 ps). The first set recorded is shown as blue dots and the second set, recorded 1 h later, as red dots. Simulated ξFEL versus charge is shown using σe = 5% (r.m.s.) with a total charge of 100 pC, that is, a spectral charge density of 3 pC MeV−1 (black diamonds) and σe = 7.5% (r.m.s.) with a total charge of 100 pC, that is, spectral charge density of 2 pC MeV−1 (black stars), with other parameters as in Fig. 2. The quadratic fit of the simulated ξFEL is shown as a black dashed line using the fit formula ξFEL = a × charge, with a = 1.79 × 10−5 (resp. 7.25 × 10−6) for the first set (resp. the second set). b, FEL on-axis spectra corresponding to the ξFEL points in a, using the red and blue colour scale according to charge. c, Difference between the FEL on-axis spectra (b) and the associated seed on-axis spectra. All experimental data were sorted according to the seed peak intensity, keeping only data for seed intensity in the ±10% range around the optimum FEL generation.
