Experimental Evidence of Harnessed Expansion of a High-$Z$ Plasma Using the Hollow Wall Design for Indirect Drive Inertial Confinement Fusion

The effectiveness of a dome-shaped wall covered by a thin gold foil (hollow wall) [M. Vandenboomgaerde et al., Phys. Plasmas 25, 012713 (2018)] in holding back the high-$Z$ plasma expansion in a gas-filled hohlraum is demonstrated for the first time in experiments reproducing the irradiation conditions of indirect drive at the ignition scale. The setup exploits a 1D geometry enabling record of the complete history of the gold expansion for 8 ns by imaging its emission in multiple x-ray energy ranges featuring either the absorption zones or the thermal emission regions. The measured expansion dynamics is well reproduced by numerical simulations. This novel wall design could now be tailored for the megajoule scale to enable the propagation of the inner beams up to the equator in low gas-filled hohlraum thus allowing the fine-tuning of the irradiation symmetry on the timescale required for ignition.

Figure 1

(a) Sketch of the hollow wall hohlraum geometry: the thin foil is located at $z=0$; (b) domed wall hohlraum structure as seen from the 80° diagnostic line of sight; (c) time evolution of the laser power.

Figure 2

2D x-ray images in the range [1, 2, 3, 4, 5, 6] keV recorded with the 80° diagnostic in the case of the plain wall (a)–(d) and of the domed wall (e)–(h). The images are integrated over 120 ps taken at $t=2\mathrm{ns}$ (a) and (e), $t=4\mathrm{ns}$ (b) and (f), $t=6\mathrm{ns}$ (c) and (g), and $t=8\mathrm{ns}$ (d) and (h). The signals are shown in a logarithmic scale normalized, at each of the acquisition times, to the emission measured in the plain wall target. In (h), the dotted line represents the edges of the observation window.

Figure 3

Same as Fig. 2 for the 2D x-ray images recorded with the 100° diagnostic in the range [1, 2, 3, 4, 5, 6] keV at $t=5\mathrm{ns}$ (a) and (e), $t=6\mathrm{ns}$ (b) and (f), $t=7\mathrm{ns}$ (c) and (g), and $t=8\mathrm{ns}$ (d) and (h).

Figure 4

Spatial repartition along the $z$ axis (taken at $r=0\mu \mathrm{m}\pm 18\mu \mathrm{m}$) of the x-ray signals measured with the 80° diagnostic in the energy ranges (a)–(d) [1, 2, 3, 4, 5, 6] keV and (e)–(h) [300–500] eV at $t=2$, 4, 6 ns, and 8 ns. At each instant of time and energy range, the signals are normalized to the maximum measured in the plain wall target.

Figure 5

Time evolution of the x-ray emission zones in the ranges (a) [1, 2, 3, 4, 5, 6] keV and (b) [300–500] eV for the plain and hollow walls. Vertical lines: locations where the emission is above 85% of the maximum in the experiment. Gray lines: locations of the maximum emission from the numerical simulations for the hollow (dashed line) and the plane (solid line) walls.