Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

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.

Abstract:

Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas ($\mathrm{Pm} < 1$). However, the same framework proposes that the fluctuation dynamo should operate differently when $\mathrm{Pm} \gtrsim 1$, the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports a new experiment that creates a laboratory $\mathrm{Pm} \gtrsim 1$ plasma dynamo for the first time. We provide a time-resolved characterization of the plasma's evolution, measuring temperatures, densities, flow velocities and magnetic fields, which allows us to explore various stages of the fluctuation dynamo's operation. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude from its initial value and saturate dynamically. It is shown that the growth of these fields occurs exponentially at a rate that is much greater than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized MHD simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems.