Effect of plastic coating on the density of plasma formed in Si foil targets irradiated by ultra-high-contrast relativistic laser pulses

The formation of high energy density matter occurs in inertial confinement fusion, astrophysical, and geophysical systems. In this context, it is important to couple as much energy as possible into a target while maintaining high density. A recent experimental campaign, using buried layer (or “sandwich” type) targets and the ultrahigh laser contrast Vulcan petawatt laser facility, resulted in 500 Mbar pressures in solid density plasmas (which corresponds to about $4.6\times {10}^7\mathrm{J}/\mathrm{c}{\mathrm{m}}^3$ energy density). The densities and temperatures of the generated plasma were measured based on the analysis of x-ray spectral line profiles and relative intensities.

Figure 1

Schematic comparison of an intense laser pulse interacting with a solid density target and the formation of preplasma on the target surface in case of a (a) plain foil target and (b) sandwich type target (a preplasma forms at the inner interface between the outer and inner layer in a sandwich target).

Figure 2

Experimental schematic showing the relative positions of the target foil, laser pulse, plasma mirror, and the three FSSR spectrometers.

Figure 3

Experimental x-ray spectra obtained by laser pulse irradiation of a (i) 0.5 µm $\mathrm{Si}{\mathrm{N}}_3$ uncoated foil (orange curve) without plasma mirror, (ii) 0.5 µm $\mathrm{Si}{\mathrm{N}}_3$ uncoated foil (green curve) with plasma mirror, (iii) 2 µm Si foil coated on both sides with 1.4 µm CH plastic layers (red curve), and (iv) 2 µm Si uncoated foil (black curve). For ease of comparison, the intensity of the curve (i) was divided by factor 2 and the intensity of the curve (iii) was increased by factor 2.

Figure 4

Comparison of numerically calculated and experimental spectra (an orange area) for $\mathrm{L}{\mathrm{y}}_{\beta }$ and $\mathrm{H}{\mathrm{e}}_{\beta }$ lines emitted by the 0.5 μm $\mathrm{Si}{\mathrm{N}}_3$ foil during irradiation by a high contrast laser beam (plasma mirror was not used). The spectra are modeled for different initial plasma parameters (color curves) in the approximation of plasma adiabatic expansion approach (initial plasma parameters are presented in the legend); solid line represents the best fit.

Figure 5

Simulation data calculated in the stationary approximation (colored lines) in comparison with experimental spectra (orange regions) for (a) an uncoated 2 μm Si foil, case (iii), and (b) 1.4 µm CH + 2 µm Si + 1.4 µm CH, case (iv). Solid green lines are best fits; red and blue curves are over- and underestimations, respectively. The ion density, electron temperature, and plasma thickness denoted by $n_i, T_e$, and $l$, respectively, are given in the legend.

Figure 6

The dependence of Lya, Heb linewidths, and $\mathrm{L}{\mathrm{y}}_{\alpha }$ to $\mathrm{L}{\mathrm{y}}_{\beta }$ intensity ratio on plasma density as calculated for (a) 2 um and (b) 3 um optically thick Si plasma and (c) Heb to Lyb intensity ratio for a set of electron temperatures depending on plasma density. The range of values obtained in experiment with a 2 um Si CH-coated target and corresponding plasma densities are shown by filled bars.