Validation of hydrodynamic and kinetic simulations with a plasma interpenetration ICF hohlraum experiment

TL;DR

This study compares hydrodynamic and kinetic plasma simulations with experimental data from an ICF hohlraum experiment, finding kinetic models better replicate certain plasma states and highlighting the importance of non-Maxwellian effects in diagnostics.

Contribution

It demonstrates the effectiveness of 1D kinetic simulations in replicating experimental plasma diagnostics and explores the impact of non-Maxwellian distributions on Thomson-scattering spectra.

Findings

Kinetic simulations match experimental diagnostics well.

Gas-fill experiments are better captured by kinetic models.

Non-Maxwellian spectra differ significantly from Maxwellian assumptions.

Abstract

We report on simulations of counter-propagating laser-produced plasmas in an inertial confinement fusion (ICF) hohlraum surrogate, aiming to replicate observations reported by Le Pape et. al in recent work. The conditions of the colliding plasmas are relevant to ICF hohlraums used for indirect-drive ignition, and are obtained both with and without low-density He-gas fill. We compare experimental diagnostics to outputs from simulations using the 1D-2V Vlasov-Fokker-Planck kinetic code iFP and the xRAGE radiation-hydrodynamics code. These include the inferred radial lineouts of inferred ion number fraction and ion and electron temperatures, as well as the reported experimental Thomson-scattering (TS) spectra (compared via synthetic TS diagnostics). We observe that 1D kinetic simulations capture the plasma states reported in the experimental diagnostics quite well. Counter-intuitively, the kinetic simulations capture the gas-fill experiment (expected to be more `hydro-like') better than the vacuum experiment, while the reverse is observed for hydrodynamic simulations. This is attributed to the presence of non-trivial multi-dimensional hydrodynamic effects which are more dominant in the vacuum experiment. These effects are somewhat inhibited in the gas-fill experiment, permitting quasi-1D kinetic plasma transport to play more of a role in producing plasma interpenetration. Differences between the effects of Maxwellian vs. non-Maxwellian (`full f') synthetic TS diagnostics are investigated for the kinetic simulations. We find non-Maxwellian TS spectra differ non-trivially from Maxwellian spectra, which suggests caution may be warranted when applying Maxwellian TS models to infer plasma conditions via backward modeling when kinetic effects may be present.