Modifications of bound states in dense plasma environments

• Main Author: Baggott, Rory
• Published: University of Warwick 2017
• Subjects: 530.4; QC Physics
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720507

Electronic bound states in dense plasmas are modified, due to screening of ionic potentials by nearby free charge carriers. In particular, screening typically reduces the ionization energies of bound electrons. In this thesis, we explore the key physics underlying bound state modifications and ionization energies in dense plasmas. A theoretical framework is developed to study the free electrons and ion structure, and to calculate the resulting electrostatic fields in the plasma. The influence of such external fields on electronic bound states is then calculated. The framework is extended to include nonequilibrium plasmas, with populations of high-energy electrons. The screened potential around a central test ion is studied using the nonlinear Poisson equation. This approach allows different treatments for the free electrons, ions and the potential due to the central test ion to be evaluated. Quantum mechanical interactions between the bound and free electrons are found to be important in determining the screened potential, as are correlations between ions. The modified bound states are then calculated using a Hartree-Fock method, which takes into account the finite extent of bound state wavefunctions. The relationship between the modification of bound states and the modelling of x-ray scattering is also explored. We show how the modified bound state energies could be obtained from quantities that can be determined from x-ray Thomson scattering experiments. The nonlinear model is applied to an analysis of recent experiments on compressed plastics; on compressed and solid-density aluminium; and on shocked iron. These experiments had provided seemingly contradictory evidence regarding the applicability of existing screening models. The nonlinear screening model is found to partly reconcile this apparent contradiction. A screening model suitable for the treatment of nonequilibrium systems is applied to a model distribution function. The resulting ionization energies are found to be insensitive to the shape of the high-energy part of the distribution function. However, the role of hot electrons as an energy sink is found to be important in modelling screening in nonequilibrium systems. The presence of hot electrons reduces the bulk temperature relative to an equilibrium system, resulting in stronger screening overall.