Improving performance of OMEGA direct-drive cryogenic implosions using embedded Si-doped plastic layers

Using Si-doped ablators in recent OMEGA DT-layered implosions has led to record neutron yields up to 3.1e14. Silicon increases collisional absorption and augments the final implosion velocity. However, radiation-hydrodynamic simulations (using LILAC) indicate that the presence of silicon enhances radiation emission from the conduction zone and coronal plasma, thereby preheating the DT ice and reducing the areal density. To reduce radiation preheat, a “sandwich” ablator has been designed where the Si-doped layer is embedded between an inner and outer layer of CD. The thicknesses of the sandwich target are chosen to optimize the laser energy absorption while minimizing radiation preheat. Only a slight decrease in the LILAC-predicted yields is observed as the amount of silicon present in the system is reduced. This controls radiative preheat and increases LILAC ρR. Simulations predicts the existence of an optimum thickness for the outer plastic layer and the embedded Si-doped layer. Increasing the thickness beyond its optimum value results in a significant reduction in neutron yield. Results from the first experiments using the sandwich targets will be presented and compared with rad-hydro simulations.