A Green’s function analysis for understanding the time-dependent x-ray drive asymmetries and error bars in indirectly driven implosions on the NIF*

Time-dependent, low-mode asymmetries, are believed to play a leading role in limiting the performance of current ICF implosions on NIF. These long wavelength modes are initiated and driven by asymmetries in the x-ray flux from the hohlraum; however, the underlying hydrodynamics of the implosion also acts to modify and amplify these asymmetries. We present here a simulation-based Green’s function model relating the time-dependent drive asymmetries seen by the capsule to the measured inflight and hot spot asymmetries. In this approach, we evaluate the response of the capsule to a time sequence of impulsive asymmetry inputs. This method is general and can be applied to other types of problems, such as direct drive implosions. Our model sheds new light on the sensitivity of the implosion to the drive asymmetry at different times during the pulse. By inverting the problem, finding the drive asymmetry needed to match the experimental data, it also allows us to tightly constrain the drive asymmetry experienced by the capsule providing an error estimate on the result. With this approach, we are able to identify when and how the complex hohlraum simulations deviate from the drive asymmetry needed to match the experimental data. We can also identify where in the pulse additional measurements would have the largest impact in reducing the error bars on the possible drive scenarios. We expect to use this model to identify approaches to reduce the time-dependent low mode asymmetry of indirectly driven implosions and to provide a better understanding of the connection between asymmetry evolution and the underlying implosion hydrodynamics with a view to improving target designs on the NIF.