Electron Thermal Transport Modeling of Laser-Plasmas

Abstract

A longstanding challenge in the field of high-energy-density physics is the development of a predictive simulation framework for inertial confinement fusion (ICF) experiments, which are complex multi-physics systems that span a large range of length scales and timescales. Computational approaches for modeling these integrated systems often employ simplified or reduced-order models for many of these physical processes, coupled with numerical multipliers to account for deficiencies in the physics modeling. For electron heat transport, flux-limiters are often employed in conjunction with the classical Spitzer-Harm electron conduction model. To address such deficiencies in electron heat transport modeling, this dissertation evaluates the effect of nonlocal deviations in the electron heat flux that emerges due to the presence of steep temperature gradients, as represented by the reduced-order nonlocal multigroup diffusion model proposed by Schurtz, Nicolai, and Busquet (SNB).

In current direct- and indirect-drive ICF approaches, lasers produce 353 nm light, which delivers the energy required to compress the fusion fuel. In the interaction of laser light with initially solid matter, steep temperature gradients--where nonlocal transport is expected--are produced. Historically, laser-irradiated sphere experiments have been used to study laser-plasmas at ICF-relevant conditions, specifically to study the radiative properties of high-Z elements. To focus on the effect of electron transport, our computational study focuses on low- to mid-Z spheres where modeling uncertainties from radiation transport and non local thermodynamic equilibrium (NLTE) atomic kinetics are smaller. In the laser-irradiated sphere, we benchmark a corrected form of the SNB model against Vlasov-Fokker-Planck kinetic modeling and find it matches kinetic heat flux predictions within 10%. This is an improvement to errors up to 40% from SNB heat flux predictions without these corrections. This work represents the first suite of integrated-modeling comparisons using this form of the SNB model. Compared to classical transport, we find that nonlocal electron transport produces a cooler expanded plasma corona due to anomalous heat flux reduction, and enhanced self X-ray emissions due to anomalous preheat. These nonlocal behaviors cannot be reproduced from classical electron heat flux predicted by the Spitzer-Harm model, with or without flux limiters.

Furthermore, when the electron heat transport becomes nonlocal the electron distribution becomes non-Maxwellian. And at ICF relevant laser intensities non-Maxwellian electrons are produced by the collisional absorption of laser light and by parametric laser-plasma-instabilities (LPI). Our approach employs atomic-kinetics simulations to assess the effect of such non-Maxwellian distributions on the radiative properties of the plasma from laser-irradiated spheres, using information from Vlasov-Fokker-Planck kinetic simulations or analytical theory. This approach is applied in zero-dimensional plasmas, as well as one-dimensional plasma profiles from the laser-sphere. From one-dimensional comparisons, we find the impact of non-Maxwellian electrons from nonlocal transport and from collisional laser absorption on the radiation emissions (<1%) and the $K-$shell line intensities (<10%) is minimal. These differences may not be experimentally significant, suggesting that the SNB model is sufficient for modeling electron transport in laser-irradiated spheres. Furthermore, our approach can be used to assess the effect of non-Maxwellian electrons from other physical processes, such as a number of LPI effects.