Classical molecular dynamics simulations of hydrogen plasmas and development of an analytical statistical model for computational validity assessment

Abstract

Classical molecular dynamics simulations of hydrogen plasmas have been performed with an emphasis on the analysis of the equilibration process. The theoretical basis of the simulation model as well as numerically relevant aspects, such as the proper choice and definition of simulation units, are discussed in detail, thus proving a thorough implementation of the computer simulation technique. Because of the lack of experimental data, molecular dynamics simulations are often considered as idealized computational experiments for benchmarking of theoretical models. However, these simulations are certainly challenging and consequently a validation procedure is also demanded. In this work we develop an analytical statistical equilibrium model for computational validity assessment of plasma particle dynamics simulations. Remarkable agreement between model and molecular dynamics results including a classical treatment of the ionization-recombination mechanism is obtained for a wide range of plasma coupling parameter values. Furthermore, the analytical model provides guidance to securely terminate simulation runs once the equilibrium stage has been reached, which in turn gives confidence in the statistics that potentially may be extracted from time histories of simulated physical quantities.

Introduction to spectral line shape theory

“…Generally, simulations will generate classical particles that move around in a finite-sized box. It is rare that a simulation will take into account all N-body interactions; Stambulchik et al [28] and Gigosos et al [29] are exceptions. Rather, the simulation will generate particles that move on straight-path (neutral radiator) or hyperbolic (charged radiator) trajectories inside a finite-sized box.…”

“…One advantage of the non-interacting simulation is that the distribution of velocities remains Maxwellian. Therefore, for fully interacting simulations, care must be taken to avoid numerical heating or cooling [29].…”

Hydrogen Line Shape Uncertainties in White Dwarf Model Atmospheres

“…expensive computationally (Gigosos et al, 2018), and only a small number of plasma conditions can be calculated in this way. In addition, simulation codes such as Xenomorph need thousands of such simulations for identical plasma conditions in order to approximate an ensemble average over the surrounding plasma, and this is usually prohibitively expensive.…”

Exploring extreme magnetization phenomena in directly-driven imploding cylindrical targets

“…It is also worth noting that despite the high B-field values achieved in the compressed core (> 10 kT around stagnation), Stark-Zeeman spectroscopy cannot be used to obtain an estimation of compressed B-field in an experimental scenario. Given the range of electron densities achieved, we performed systematic calculations of Stark-Zeeman line profiles based on molecular dynamics simulations [70] and checked that the broadening of Ar K-shell lines produced by the Stark effect blurs the characteristic Zeeman pattern even in the case of the expected achievable B-field. Zeeman features will be further washed out by radiation transport effects through the imploded core and instrumental broadening.…”