Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of x-ray to ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, and find good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV.

Abstract:

Abstract:

Laser peak powers rise inexorably higher, enabling the study of increasingly exotic high-energy-density plasmas. This thesis explores one such phenomenon, that of the interaction between a relativistically intense laser pulse and a solid-density plasma. The laser pulse is reflected. Both the reflected radiation and the electron bunches that induce the interaction have fascinating properties. Through the application of theory, simulation and experiment, this thesis strives to extend our understanding of this mechanism and thus direct the community towards potential applications for these sources. Of primary interest is the development of novel diagnostic tools. Theories have been developed and tested to describe the production of low emittance nano-Coulomb charge electron bunches. Such properties are comparable to forefront synchrotron sources but on a considerably more compact scale. These results have wide-reaching implications for future particle accelerator science and associated technologies. Furthermore, these electron bunches will initiate QED processes on next-generation laser facilities. The radiation they produce is composed of high harmonics of the incident laser pulse. This radiation can be coherently focused to unprecedented intensities and is of ultra-short duration, possibly even entering the zeptosecond regime. The intensity of X-ray harmonics has been measured on the ORION laser facility producing results consistent with theory and enabling the benchmarking of peak intensity simulations with real data. The work of this thesis has amassed interest within the community and in June 2024 its ideas will be tested on the GEMINI PW laser facility.

Abstract:

The prototypical α → ω phase transition in zirconium is an ideal test-bed for our understanding of polymorphism under extreme loading conditions. After half a century of study, a consensus had emerged that the transition is realized via one of two distinct displacive mechanisms, depending on the nature of the compression path. However, recent dynamic-compression experiments equipped with in situ diffraction diagnostics performed in the past few years have revealed new transition mechanisms, demonstrating that our understanding of the underlying atomistic dynamics and transition kinetics is in fact far from complete. We present classical molecular dynamics simulations of the α → ω phase transition in single-crystal zirconium shock-compressed along the [0001] axis using a machine-learning-class potential. The transition is predicted to proceed primarily via a modified version of the two-stage Usikov-Zilberstein mechanism, whereby the high-pressure ω-phase heterogeneously nucleates at boundaries between grains of an intermediate β-phase. We further observe the fomentation of atomistic disorder at the junctions between β grains, leading to the formation of highly defective interstitial material between the ω grains. We directly compare synthetic x-ray diffraction patterns generated from our simulations with those obtained using femtosecond diffraction in recent dynamic-compression experiments, and show that the simulations produce the same unique, anisotropic diffuse scattering signal unlike any previously seen from an elemental metal. Our simulations suggest that the diffuse signal arises from a combination of thermal diffuse scattering, nanoparticle-like scattering from residual kinetically stabilized α and β grains, and scattering from interstitial defective structures.