Saturable Absorption of an X-Ray Free-Electron-Laser Heated Solid-Density Plasma

High-intensity $\approx$10$^{17}$ Wcm$^{-2}$, short duration (100 fsec) x-ray pulses from the LCLS x-ray free-electron laser, with photon energies ranging from below to above the K-edge of cold Al (1560 eV), are used to generate and probe a solid-density aluminum plasma. The photon-energy-dependent transmission of the heating beam is studied through the use of a photodiode. Saturable absorption is observed, with the resulting transmission differing significantly from the cold case, with the increased transmission being due to the K-edge energy of the dominant ion species shifting in time as the solid-density target is heated, in good agreement with atomic-kinetics simulations [1]. \\[4pt] [1] D.S. Rackstraw {\it et al.}, Phys. Rev. Lett., {\bf} 114, 015003 (2015)