Assessing plasma filling in NIF cluster geometry

Abstract
The National Ignition Facility (NIF), currently under construction at Lawrence Livermore National Laboratory (LLNL) comprises four clusters of 48 laser beams in a symmetric geometry. Each cluster is itself capable of producing potentially useful conditions for hohlraum interaction experiments. The three dimensional nature of the laser illumination geometry in each cluster presents a challenge when modelled with a 2D hydrocode. Recent experiments have been fielded on the OMEGA laser facility to investigate the validity of using such codes in modelling the plasma filling of targets in similar geometries to a NIF cluster. The extent to which plasma filling within hohlraum type targets is a significant issue can potentially be addressed by the observation of laser plasma instabilities, such as Raman backscatter. This can be a significant energy loss mechanism for targets that have electron densities up to quarter critical. Linear Raman gain theory has been applied to the output from AWE's 2D lagrangian radiation hydrodynamics code, NYM, to produce qualitative and quantitative comparisons with these experiments. This provides a basis for judging the fidelity of the approach.