Designing for early and late time symmetry control of delayed 2-shock pulse shapes relevant to double shell implosions

Current double shell experiments at the National Ignition Facility use a single shock pulse shape is used to drive the implosion. Peak laser power is reached while the shock is only midway through the ablator shell and under these conditions shock symmetry cannot be maintained. To improve shock symmetry significant changes must be made to the pulse shape, such as utilizing a multi-shock pulse. Our recent simulations have also shown that significant reductions in growth of the outer shell assembly joint are predicted with a particular type of 2-shock pulse where the main pulse is delayed until after first shock breakout by over 1 ns. However, to achieve these stability benefits we must use a long pulse duration that creates challenges for late time symmetry control and possibly high backscatter. In this presentation we will present results from integrated hohlraum simulations on various methods to reach adequate symmetry. Our current findings suggest a combination of outer cone repointing, 2-color and 4-color wavelength separation, and hohlraum gas fills above 0.45 mg/cc He4 are needed for main pulse delays of 0.7 ns after first shock breakout. We will also present symmetry sensitivity studies for double shell designs that use these 2-shock pulse shapes.