Measurements of the K-Shell Opacity of a Solid-Density Magnesium Plasma Heated by an X-Ray Free-Electron Laser

We present measurements of the spectrally resolved x rays emitted from solid-density magnesium targets of varying sub-mu m thicknesses isochorically heated by an x-ray laser. The data exhibit a largely thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell x rays within well-defined regimes of electron density and temperature, extremely close to local thermodynamic equilibrium conditions. The deduced opacities at the peak of the K alpha transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations.