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 emitted within the system have a probability of being reabsorbed and reemitted multiple times—a phenomenon known in astrophysics as resonant scattering. This effect alters the ratio of optically thick to optically thin lines, depending on the plasma geometry and viewing angle, and has significant implications for the spectra observed in a number of astrophysical scenarios, but has not previously been studied in a controlled laboratory plasma. We demonstrate the effect in the x-ray spectra emitted by cylindrical plasmas generated by high power laser irradiation, and the results confirm the geometrical interpretation of resonant scattering.

Figure 1

Schematic drawing of the targets used in the experiment. The $\mathrm{Sc}/\mathrm{V}$ disk is shown in brown whereas the Be tamper is shown in green. The thickness of the $\mathrm{Sc}/\mathrm{V}$ disk has been exaggerated for clarity.

Figure 2

Thickness $H$ (2a), radius $R$ (2b) and aspect ratio $H/R$ (2c) of the R125 (blue) and R175 (red) targets as a function of time. The dot-dashed lines in both (b) indicate the initial radius of each target, while the green dot-dashed line in (c) marks an aspect ratio of $\pi /2$. The uncertainty bars correspond to shot to shot variations. Times are indicated with respect to the start of the laser pulse.

Figure 3

Te (left) and density of scandium ions (right) for both types of targets as a function of time. Both targets evolve following the same temperature and density curves within error bars, making them comparable.

Figure 4

Example of face-on (solid lines). The blue lines correspond to the R125 targets, while the red lines show the spectra from the R175 targets. The spectra are normalized to the peak of the $y$ line. Additional dotted lines have been introduced in the plot to indicate the peak of the $w$ line. These spectra were taken 1.6 ns into the laser pulse. Although both targets are at the same conditions, the $w$ line is more enhanced with respect to the $y$ line in the R175 targets, as their LOS is smaller with respect to the mean chord.

Figure 5

Line-integrated $w/y$ flux ratio for the face-on emission from both types of targets, and predictions for the optically thin case. Although the LOS and plasma conditions are the same in both cases, the $w$ line is more enhanced for the R175 targets than for the R125 targets, owing to their different mean chords.

Figure 6

Quotient between the face-on and side-on $w/y$ ratio for both types of targets. The horizontal line corresponds to a value of 1, and defines a boundary between disklike and pipelike cylinders.