Simulations of Dense Plasmas
A dense plasma forms a peculiar media, exhibiting features of a solid, liquid, and ionized
gas. The density approaches or exceeds that of solid matter (1022-1026 atoms/cm3) yet the
media behaves as a freely-flowing fluid. Due to the proximity of the atoms or to the
temperature, the system also ionizes, yielding its plasma nature. Dense plasmas play a crucial
role in such diverse regimes as white dwarf stars, planetary interiors, interial confinement
fusion, pulse-power generators, laser-matter interfaces, sonoluminescence, and certain liquid
metals. Plasmas are generally considered classical entities. However, for this case, the very
close packing of the atoms so that their electronic wavefunctions strongly overlap forces a
quantum mechanical treatment. The media resembles a quantal "fluid", where all species must
be treated democratically.
To model such a complex media requires very sophisticated techniques so that all possible
mechanisms such as molecular bonding and dissociation, cluster formation,
ionization-recombination, and attachment-detachment receive treatment on an equal footing. We have
employed quantum molecular dynamics (QMD) techniques to effect this simulation. The
electronic component of a composite system of atoms is treated within a
periodically-replicated reference cell by a finite-temperature density functional method in the local
density approximation, and the nuclear motion by classical mechanics. We have also
investigated more approximate electronic structure methods such as tight-binding and various
zero-differential-overlap forms. An example of a typical QMD simulation for a system heated
above the melting point appears in Figure 1. The system begins in a solid state of high
symmetry (blue), passes through a molecular stage (yellow), and ends in a disordered
collection of atoms (red).
Figure 1
We have focused on hydrogen (H) compressed to over ten times its solid density at
temperatures up to hundreds of thousands of degrees Kelvin. At very low temperatures, we
find a complex fluid, permeated with long chains or filaments of atoms. Increasing the
temperature yields an insulating diatomic molecular fluid and eventually a purely atomic,
highly conducting media. Even at the elevated temperatures, transient associations of atoms
form that profoundly affect both radiative and transport properties.
We also have examined liquid alkali metals, impurities such as argon in H plasmas, and the
insulator to metal transition in rare gases. The former exhibits even more complex behavior
than H at low temperatures. For example, an expanded sodium (Na) fluid fills with bound
clusters of between two and eight atoms. Figure 2 illustrates this effect for a single snapshot
taken during a long trajectory simulation, displaying the atoms (red) and their associated
bonds (white).
Figure 2
Quantum molecular dynamics provides a powerful tool for exploring intricate properties of
matter at conditions of high temperatures or pressures.
Collaborators include I. Kwon (Georgia); N. Troullier (IBM); and O.
Pfaffenzeller and D. Hohl (Juelich, Germany).
Address scientific comments and questions to
Lee Collins
<lac@lanl.gov> or
Joel Kress
<jdk@lanl.gov>
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