Structure and ionisation in dense plasmas: a molecular dynamics approach

Thesis

This thesis investigates the structural and thermodynamic behaviour of partially-ionised dense plasma using advanced molecular dynamics frameworks formulated within a chemical picture. Emphasis is placed on the explicit treatment of bound electronic states and the self-consistent determination of ionisation equilibria in strongly interacting systems relevant to warm dense matter. First, a general methodology is developed to compute model-dependent ionisation states through free energy minimisation and thermodynamic integration. To demonstrate the approach, two representative models for hydrogen plasma are constructed in which electrons are either bound in the hydrogen ground state or represented as a uniform charge-neutralising background. These models are used to study the transition from atomic gas to ionised plasma, highlighting the role of neutral interactions beyond commonly employed chemical models. Concepts such as ionisation potential depression and pressure ionisation are also examined. Subsequently, a wavepacket molecular dynamics formalism is developed to model the structural properties of dense plasmas with a modified wavefunction that directly incorporates bound states. Using hydrogen as a prototypical system, self-consistent charge state distributions and radial distribution functions are calculated, and shown to depend on the confining potential, an empirical parameter in the model. These results are compared directly with path integral Monte Carlo data, enabling a critical assessment of the underlying approximations. Finally, a statistical theory for the dynamical degrees of freedom in wavepacket molecular dynamics is derived. Statistical distributions for the variables that parameterise the Gaussian wavepacket widths are obtained for both isotropic and anisotropic formulations. These are shown to agree with simulation data under warm dense matter conditions. The results provide a practical means of assessing the effect of the confining potential. Together, the developments presented in this thesis provide, and critically assess, a set of computational tools for investigating partially-ionised dense plasma.

Authors: Plummer, D

• DOI: 10.5287/ora-no9y5aob4
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: warm dense matter; dense plasmas; ionisation; wave packet molecular dynamics; molecular dynamics
• Subjects: Computational physics; Plasma (Ionized gases)