Ab-Initio Calculation of the Flux-Limiter Determining Thermal Diffusion in High Energy Density Plasmas

Full-scale simulations of high energy density plasmas (HEDP) use approximate models models for some important plasma processes. Indeed, the inaccuracy of such models could play a role in the discrepancy between simulated and measured drive temperatures in gas filled hohlraums on the National Ignition Facility (NIF). A specific example of such a model is the application of a flux limiter to thermal transport, limiting the flux to some fraction $f $of the free-streaming limit which is then tuned ``post hoc'' to fit data from a particular experiment. The recent modification of $f$ in NIF simulations from the more usual 0.05 to 0.15 to obtain such a fit demonstrates the limited predictive capability of a flux-limited heat flow model [1]. The value of the flux limiter is also important in direct drive inertial fusion experiments and in shock ignition and so ``first-principles'' calculations of the flux limiter are potentially important in many HEDP scenarios. We will show how an existing fluid code can be modified to include an ab-initio calculation of the flux limiter by replacing the energy equation with a direct solve of the kinetic Vlasov-Fokker-Planck (VFP) equation. Sample simulations of laser-solid interactions with the resulting hybrid VFP-fluid code will be presented, demonstrating for the first time that a VFP simulation framework can be used for realistic simulation of HEDP experiments.\\[4pt] [1] M.D. Rosen, HEDP, 180 (2011)