Opacity Measurements of a Hot Iron Plasma Using an X-Ray Laser

The temporal evolution of the opacity of an iron plasma at high temperature (30–350 eV) and high density ($0.001–0.2\mathrm{g}{\mathrm{cm}}^{-3}$) has been measured using a nickel-like silver x-ray laser at 13.9 nm. The hot dense iron plasma was created in a thin (50 nm) iron layer buried 80 nm below the surface in a plastic target that was heated using a separate 80 ps pulse of 6–9 J, focused to a $100\mu \mathrm{m}$ diameter spot. The experimental opacities are compared with opacities evaluated from plasma conditions predicted using a fluid and atomic physics code.

Figure 1

The footprint of the x-ray laser as it passes through the opacity target at a time of 100 ps after the peak of the sample target irradiation. The brighter zone corresponds to the region where the target has been heated and a plasma produced, while the most intense component of the x-ray laser beam can be seen towards the left of the image passing through the unheated material. The central region of the heated region (as indicated by the intersection of the arrows) has been used to deduce the transmission of the heated target by comparison to transmission through unheated regions on the same shot.

Figure 2

The variation of transmission of a 13.9 nm x-ray laser with time from the peak of the heating pulse through the iron component of the sample target at 45° incidence is shown for both experimental (data points) and simulated data (continuous curves). Experimental data with pulse energies in the range 7–9 J are shown as open circles, while a data point with pulse energy 6 J is shown as a solid circle. Simulations are shown for heating laser energy of 6.5 J using the TOPS data (dashed curve) and 9.2, 8.4, and 5.1 J using our developed code (as labeled). A measurement of transmission through an unheated target is shown as a data point at time 0 ps.

Figure 3

The calculated transmission of 13.9 nm soft x-ray radiation through the iron layer at time 100 ps at 45° incidence as simulated using the ehybrid fluid code (assuming a pulse energy of 8.4 J) and our model of opacity with (dotted line) and without (dot-dashed line) the opacity effect of a large number of bound-bound transitions. The ehybrid code evaluation of the average degree of plasma ionization is also shown (solid line).