Ray-based scattering approach to modeling the filamentation instability in speckled laser beams*

In laser fusion experiments such as those conducted at the National Ignition Facility (NIF), small-scale intensity fluctuations (speckles) in the laser beams result from the use of random phase plates to shape the beams' far-field focal spot. These speckles are often above the critical power threshold for self-focusing and so trigger the filamentation instability, which can cause increased beam spreading and result in less power delivered to the target. Traditional ray-tracing codes do not model the speckle structure in the laser beam and therefore neglect any effects due to filamentation. We present a novel model of filamentation based on ray scattering that can be incorporated into traditional ray-tracing codes to account for filamentation-induced spreading. We describe how to incorporate additional effects, such as polarization smoothing and smoothing by spectral dispersion (SSD), into the model. We validate the model by comparing its predictions to those of paraxial wave solvers and show how the model can be used to inform hohlraum simulations.