Demonstration of geometric effects and resonant scattering in the X-Ray spectra of high-energy-density plasmas

In a plasma of sufficient size and density, photons can be absorbed and reemitted multiple times—what is known in astrophysics as resonant scattering. This process alters the ratio of optically thick to optically thin lines, depending on the plasma geometry and viewing angle. This has significant implications for the spectra observed in a number of astrophysical environments and in laser-produced plasmas, but has not previously been studied in a controlled laboratory setting.

We report the results of a series of experiments at the OMEGA 60 laser on which we isolate the effect of the plasma geometry on x-ray line spectra. We generated cylindrical plasmas of the same thickness, temperature and density, but varied the radii, and measured their spectra from two perpendicular directions. We tracked the evolution of the ratio of optically thick to optically thin lines, and observed changes consistent with the predictions from 2-dimensional radiative transfer models.

This work highlights the importance of considering the plasma geometry in order to obtain reliable interpretations of spectroscopic measurements in high energy density plasmas.