High-resolution radial Kα spectra obtained from a multi-keV electron distribution in solid-density titanium foils generated by relativistic laser-matter interaction

We studied temperature and Kα yield radial profiles of thin titanium foils as a result of femtosecond high-power laser pulse irradiation at several 10¹⁹ W/cm² by high-resolution x-ray spectroscopy. Laser-accelerated electrons heat the cold solid to bulk temperatures of up to ∼50 eV. The plasma strongly affects the shape of the emitted Kα doublet, which is surveyed by x-ray spectroscopy with both high spectral (E/ΔE ≥ 15,000) and 1D spatial (Δx ≤ 13.5 μm) resolutions. Temperature-dependent spectra modeled by line-shape calculations are compared with Abel-inverted experimental spectra and provide a radial temperature distribution. The radially resolved Kα yield shows a depletion of the Kα₁-line at the position of the laser focus. The density gradients induced by prepulses are modeled by hydrodynamic simulations, and density-dependent line-shape models are applied. The x-ray yield as function of foil thickness is explained by partial refluxing of a multi-keV electron distribution inside the foil, supported by Monte-Carlo simulations. Finally, we derive parameters to optimize the peak brilliance of such a laser-driven thin foil x-ray source.