Transport coefficient challenges in high energy density plasmas

Coefficients for thermal and particle transport in high energy density (HED) plasmas play a critical role in computational modeling of HED experiments. Radiation hydrodynamics simulations are our primary tool for such modeling due to the complexity and computational expense of more detailed methodologies. Nevertheless, such simulations typically rely on single material tables or asymptotic formulae to obtain coefficients for transport phenomena. These coefficients are frequently used beyond the domains where they are constrained by data and their uncertainties are often masked by adjusting simulation parameters. Indeed, the uncertainties are large enough that they could play a key role in explaining many remaining discrepancies between simulations and experiments. Several examples of recent work that explore these sensitivities and experimental evidence for the importance of expanding and improving available coefficients will be presented. In some cases, the impact of material configurations at scales below the computational grid size is more important than single material coefficient values. Detailed comparisons between simulation and experiment exhibit deficiencies in shock and viscosity modeling[1,2], a need to better constrain thermalization[3], strong sensitivities to conductivity coefficients[4,5], a need for the development of strategies to modify coefficients in the presence of magnetic fields[6], etc. [1]Gatu Johnson et al, HEDP 36:100825, 2020 [2]Haines et al, Phys Plasmas 21:092306 2014 [3]Haines et al, Nature Comm 11:544 2020 [4]Haines et al, Phys Plasmas in review 2022 [5]Dhakal et al, Phys Plasmas 26:092702 2019 [6]Sadler et al, Phys Plasmas 27:072707 2020