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

High-intensity x-ray pulses from an x-ray free-electron laser are used to heat and probe a solid-density aluminum sample. 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, in good agreement with atomic-kinetics simulations.

Figure 1

The transmission of a $1\mu \mathrm{m}$ thick aluminum sample oriented at 45° to the FEL beam. The red line shows the experimental trend, neglecting outlying data points significantly far from the majority. The green line is from scfly simulations of the transmission. The blue line indicates the CXRO database values for the cold transmission.

Figure 2

Simulated effective attenuation length as a function of fluence at a range of central photon energies. The actual attenuation length will vary over the duration of the pulse.

Figure 3

Time-dependent opacities, with contributions from different charge states, and charge state distributions from simulations at a photon energy of 1670 eV at 20% of peak intensity (top two panels) and the full intensity (bottom two panels). The FEL pulse is on for the first 100 fs and constant in intensity.

Figure 4

The rates of ionization of the $4+$ charge state through photoionization (P), Auger (A), and collisional (C) processes. Also shown are the populations of the two superconfigurations of the $4+$ charge state.