The influence of gas fill on hohlraum coupling and symmetry in high efficiency hohlraums*

Indirectly driven inertial confinement fusion experiments on the National Ignition Facility (NIF) have demonstrated improved coupling of laser energy to x-ray flux and finally a roughly 14% increase in capsule absorbed energy by using a high efficiency cylindrical hohlraum (x-ray cavity).  To achieve this improvement, we decreased the hohlraum losses compared the highest performing inertial confinement fusion experiments by using both a smaller, 2.7 mm diameter, laser entrance hole and by reducing the hohlraum diameter to 6.2 mm from 6.4 mm.  In past experiments, the same hohlraum fill density (0.3 mg/cc helium) was used in these high efficiency hohlraums as is typically used in high performing experiments to reduce expansion of the hohlraum wall and reduce filling from capsule material blowing in.  Unfortunately, the results were asymmetric implosions with high implosion symmetry swings.  Following this result, we conducted experiments at twice the fill density, 0.6 mg/cc, to improve inner beam propagation by additionally tamping the wall blow-in.  Simulations and simple scaling predicted a significant improvement to symmetry.  Experimental results at the higher fill density resulted in even poorer symmetry, but with improved wall tamping as expected.  Experimental data, analytic scaling and experiments will be presented.