Improving the accuracy of hohlraum simulations by calibrating the `SNB' multigroup diffusion model for nonlocal heat transport against a VFP code

Nonlocal heat transport, occurring when temperature gradients become steep on the scale of the electron mean free path (mfp), has proven critical in accurately predicting ignition-scale hohlraum energetics. A popular approach, and modern alternative to flux limiters, is the `SNBu0027 modelfootnote{Schurtz emph{et al.} textbf{Phys. Plasmas} 7, 4238 (2000)}. This is implemented in both the HYDRA code used for simulating National Ignition Facility experiments and the CHIC code developed at the CELIA laboratory. We have performed extensive comparisons of the SNB heat flow predictions with two VFP codes, IMPACTfootnote{Kingham u0026 Bell textbf{J. Comp. Phys.} 194 (2004)} and KIPPfootnote{Chankin emph{et al.} textbf{Contrib. Plasma Phys.} 52, 500 (2012)} and found that calibrating the mfp to achieve agreement for a linear problem also improves nonlinear accuracy. Furthermore, we identify that using distinct electron-ion and electron-electron mfpu0027s instead of a geometrically averaged one improves predictive capability when there are strong ionisation (Z) gradients.