High-Areal-Density, Fuel-Assembly Experiments for the Fast-Ignitor Concept

Fast-ignition targets must be imploded to high-areal densities, $\sim $0.5 g/cm$^{2}$, to stop either $\sim $1-MeV electrons or $\sim $18-MeV protons, generated by an intense ultrashort laser pulse. Simulations have shown that high-density and high-areal-density fuel assembly can be achieved by imploding thick cryogenic shells with low velocity on a low adiabat.\footnote{ R. Betti and C. Zhou, Phys. Plasmas \textbf{12}, 110702 (2005).} A scaled noncryogenic version of the proposed design$^{1}$ was tested experimentally. Fuel-assembly experiments using 40-\textit{$\mu $}m-thick, 0.9-mm-diam plastic shells filled with various gas pressures were performed on the OMEGA Laser Facility, using an optimized low-speed spherical implosion. High-areal densities with temporally and spatially averaged values of $\sim $130 mg/cm$^{2}$ were measured with proton wedged range filters\footnote{ F. H. S\'{e}guin \textit{et al}., Rev. Sci. Instrum. \textbf{74}, 975 (2003).} for D$_{2}$ and D$^{3}$He fills of various pressures in the range from 10 to 33 bar. The areal densities compare favorably to one-dimensional, hydrodynamic-simulation predictions if the measured temporal-neutron-production history is taken into account. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.