Preheat Studies Using Low-Adiabat Plastic-Shell Implosions with Triple-Picket Pulses on OMEGA

The effect of laser--plasma interactions in the underdense coronal plasma on direct-drive target performance has been systematically studied on OMEGA. Room-temperature D$_{2}$-filled, 27-\textit{$\mu $}m-thick plastic shells were irradiated using triple-picket laser pulse shapes. The intensity on the main pulse is varied between 3.5 $\times $ 10$^{14}$ W/cm$^{2}$ and 1.1 $\times $ 10$^{15}$~W/cm$^{2}$, while picket energies are kept nominally the same to maintain similar shell adiabat in all designs. Time-resolved reflected light and its spectrum, neutron-rate histories, areal densities, ion temperatures, neutron yield, and time-resolved hard x-ray signals have been simultaneously measured on these implosions. At lower intensities below the two-plasmon-decay (TPD) threshold, only cross-beam transfer induced by laser--plasma interactions influences target performance, whereas both affect target performance at higher intensities. In particular, fast-electrons generated by TPD can potentially preheat the shell reducing compression at high intensities ($\sim $1 $\times $ 10$^{15}$ W/cm$^{2})$. This work was supported by the U.S. Department of Energy under Cooperative Agreement Nos. DE-FC02-04ER54789 and DE-FC52-08NA28302.