Experimental Reduction of Laser Imprinting and Rayleigh--Taylor Growth in Spherically Compressed, Medium-\textit{Z}--Doped Plastic Targets

The effect of medium-$Z$ doping of plastic ablators on laser imprinting and Rayleigh-- Taylor (RT) instability growth was studied using spherical direct-drive implosions on the OMEGA Laser System. The targets were spherical plastic (CH) shells, with an outer diameter of 860~\textit{$\mu $}m and a thickness of 22 \textit{$\mu $}m, doped with a varied concentration of Si (4.3{\%} and 7.4{\%}) and Ge (3.9{\%}). The targets were imploded with 48 beams with a low-adiabat, triple-picket laser shape pulse with a peak intensity of 4 $\times $ 10$^{14}$ W/cm$^{2}$ and a pulse duration of 2.5 ns. The shells were x-ray radiographed through a 400-\textit{$\mu $}m opening in the side of the target. The results show that impurity doping strongly reduces the shell-density modulation and the instability growth rate. The amplitude of the initial imprint is reduced by a factor of 2.5$\pm $0.5 for CH[4.3{\%} Si] targets and a factor of 3$\pm $0.5 for CH[7.4{\%} Si] and CH[3.9{\%} Ge] targets. At the end of the acceleration phase, the reduction factor becomes 3$\pm $0.5 and 5$\pm $0.5, correspondingly. The RT instability growth rate in doped targets is reduced by a factor of 1.5 compared to undoped ones. Simulations using the 2-D radiation--hydrodynamics code \textit{DRACO} show good agreement with the measurements. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.