Fig. 2: Spatio-spectral distributions of the radiation at the undulator exit.

a,b, Spatio-spectral distributions for an undulator gap of 4.3 mm (K_u = 2.35) and an optimum delay of +0.1 ps: experimental measurements (a) and simulation (b) of SR only (i), seed only (ii), SR with seed (iii) and the difference between the (iii) and (ii) images (iv). c, On-axis spectral intensity I extracted along the red line in a and blue line in b with integration over Δy = 0.3 mm and median filtering of the simulated profile. In a,b,c(i–iii), distributions are normalized to their maximum intensity and displayed in logarithmic (dB) scale. In a,b,c(iv), the distributions are displayed in a linear scale. Simulation parameters (electron-beam parameters given at the source point): E_e = 188.8 MeV, charge = 150 pC, σ_z = 2 μm (r.m.s.), normalized emittance ϵ_x, y = (1.5; 1.0) mm mrad, divergence \({\sigma }_{x^{\prime},\,y^{\prime}}\) = (1.5; 1.0) mrad (r.m.s.), σ_e = 5% (r.m.s.), R₅₆ = −1.8 mm, QUAPEVA 2 strength detuned by −2%, E_seed = 0.5 μJ, λ_seed = 269 nm, Δλ_seed = 3.9 nm (FWHM) and ΔT_seed = 1.0 ps (FWHM).