Fig. 2: Amplification of λX ≈ 2.2 nm X-rays with flying-focus and conventional laser undulators.
From: X-ray free-electron lasing in a flying-focus undulator
a A flying-focus pulse with UFF = 8 J and a conventional pulse with UC = 176 J drive exponential growth of the X-ray power to saturation, reaching P = 1 MW. For the conventional pulse with UC = 8 J, the power grows rapidly as the electron beam approaches the focal point (z = 0.625 cm) and moves into a progressively larger undulator strength, but no microbunching is observed. b, c The power spectral density of the X-rays produced by the conventional laser undulator with UC = 176 J and flying-focus undulator with UFF = 8 J, respectively. The left axes are normalized to the ideal gain bandwidth at saturation, 2ρ. The lineouts to the right show the spectra at saturation. Near saturation, the standard deviation of both spectral densities is ~2ρ. The dashed line in (b) traces the shift in the resonant wavelength due to amplitude variation in the conventional laser undulator [Eq. (2)]. The dashed line in (c) is constant: the resonant wavelength does not shift in the flying-focus undulator. The simulated parameters are provided in Table 1.
