Drive Asymmetry in Directly-Driven Cylindrical Implosion Experiments at the National Ignition Facility*

We present results from recent directly-driven cylindrical implosion experiments fielded at the National Ignition Facility (NIF) that demonstrate a clear m=8 drive asymmetry. Cylinders are used to measure hydrodynamic instability growth in regimes relevant to inertial confinement fusion implosions, as they include the effects of convergence while retaining direct diagnostic access. The drive asymmetry is observed for both 3 mm and 4 mm outer diameter cylinders, and it is most readily apparent in nominally smooth targets that lack any machined perturbation. The m=8 asymmetry appears with the same phase for three different laser drive configurations, despite vastly different initial intensity patterns, and we conjecture that the asymmetry arises from differential absorption of the 44 degree beams (relative to the cylinder axis) and the 50 degree beams that are used to drive the cylinder. Radiation-hydrodynamics simulations in 2D can reproduce the observed pattern using an ad hoc variation of the laser power in the different cones, but 3D laser-driven simulations are needed to accurately model the drive asymmetry. Several alternative laser drive configurations are proposed for future experiments, and preliminary simulation predictions under these conditions are presented.