Three-dimensional Modeling of the Impact of Beam Incidence Angle in Laser-Driven Cylindrical Implosions Using the FLASH Code & Ramifications for HED Experiments

The advent of high-power laser facilities such as the National Ignition Facility (NIF) has ushered in a new and exciting era in high-energy-density (HED) physics research, and the flexibility of the NIF allows many distinct targets to be fielded beyond the standard indirect-drive inertial confinement fusion (ICF) configuration. Often these physical systems are modeled in more tractable two-dimensional (2D) simulations with assumed symmetry, but this simplification risks inadvertently masking crucial features. Here, we show experimental evidence of a 3D asymmetry in directly driven cylindrical implosions which was not predicted with 2D modeling, and we accurately reproduce this feature in 3D radiation-hydrodynamics simulations. The asymmetry arises as a consequence of the NIF beam geometry and the dependence of laser absorption on beam incidence angle, and it clearly delineates the acceptable limits of 2D design work. This has significant implications for targets with off-normal beam pointing such as polar direct-drive ICF, and it may also be important for a more complete understanding of indirect-drive systems. In particular, differences between experimental data and synthetic data generated from 2D simulations can be misattributed to deficiencies in physics models rather than 3D effects. Our 3D simulations use the FLASH code, which is widely used in the design of laser driven HED systems, and they also show that there is a north/south skew to the drive asymmetry due to the beam configuration. This skew is obscured in radiographs that image down the cylinder axis, complicating inferences of instability growth. Recent experimental results control the drive asymmetry by increasing the power in the 45-degree beams to partially offset the reduced coupling efficiency, in agreement with FLASH simulations. More sophisticated asymmetry mitigation schemes are also discussed.