Femtosecond electron dynamics in hot solid-density plasmas

Thesis

X-ray Free Electron Lasers (XFELs) have shown to be an extremely versatile tool for the experimental study of warm and hot dense matter. Their femtosecond pulse length, high peak brightness and tunable wavelength open up an immense window of opportunity to study solid-density plasmas that are created in a well-controlled manner. At these high densities, the light-matter interaction can be modelled with a collisional-radiative code that describes the ion charge state populations, electron distribution function (EDF) and radiative properties.

In this thesis I will outline a numerical scheme based on the inhomogeneous, isotropic Fokker-Planck equation that describes the non-equilibrium evolution of the EDF in a warm dense system. The scheme encompasses a variety of collisional and radiative interactions between electrons, ions and photons in the system and implicitly conserves density and energy. The Fokker-Planck scheme has been integrated into a non-Local Thermodynamic Equilibrium (LTE) collisional-radiative code that computes the ion charge state populations of a warm dense system under irradiation of an XFEL. The two schemes together constitute a self-consistent method to compute the population dynamics, energy- and density balance of the combined electron and ion system.

The experimental section of this thesis describes a measurement of the collisional ionisation frequencies in a solid-density magnesium plasma. These rates are of great interest as they relate to the optical and electrical properties of high energy-density materials. The experiment relies on the use of an XFEL both to produce and probe the sample, and obtain a spectroscopic measurement of the rate and plasma conditions. This is the first direct measurement of the ionisation rate in the strongly-coupled regime where the rates are significantly enhanced by density effects such as ionisation potential depression.

Authors: van den Berg, Q

• DOI: 10.5287/ora-wxnjjaade
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: collisional ionisation; high energy density; x-ray spectroscopy; Fokker-Planck equation; x-ray free electron lasers; atomic kinetics
• Subjects: X-ray spectroscopy; Laser plasmas