Characterization of phase transitions in warm dense matter with X-ray scattering.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2012)
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|Item Type:||Ph.D. Thesis|
|Title:||Characterization of phase transitions in warm dense matter with X-ray scattering|
The topic of this thesis is the experimental generation and characterization of warm dense matter, i. e. the intermediate Regime between solid state materials and dense plasmas. These states are challenging for both theoretical models and experimental measurements in the laboratory. The physics of warm dense matter is of essential importance for inertial confinement fusion and the understanding of giant planets. For testing different theoretical models of this field, experimental data is urgently needed. In the framework of this thesis, the properties of carbon at a pressure around 100 GPa and a temperature of roughly 8000 K were investigated. Theoretical models predict a solid-liquid phase transition in this regime. This transition could be characterized for the first time in the laboratory by the experiments presented in this thesis. The desired matter states were produced by laser-driven shock compression of graphite samples with an initial density of 1.84 g/cm³. For this purpose, the laser system nhelix at the GSI Helmholtzzentrum für Schwerionenforschung was applied. Density and pressure inside the shock wave were determined by measurements of shock velocity and particle velocity. The carbon samples were compressed to a density of 3.9+/-0.2 g/cm³, i.e. about two times the initial density, and pressures from 80 GPa to 170 GPa were achieved. This is in very good agreement with one- and two-dimensional hydrodynamic simulations using the code packages HELIOS and MULTI2D, respectively. For the investigation of the microscopic structure inside the compressed carbon samples, intensive pulsed X-ray radiation was produced by irradiation of titanium foils with the PHELIX laser system. The applied titanium helium-alpha transition, which emits photons of the energy 4.75 keV, could be triggered very efficiently. In fact, a conversion efficiency from laser energy to X-rays of ~0.5% was achieved. The spectrally resolved measurement of the scattered radiation allows for determining the correlations of the carbon atoms via the structure factor. For the specially Chosen scattering angles of 105° and 126°, a strong change of the structure factor is expected to happen for the solid-liquid phase transition resulting in a strong change of the scattering cross section. This change could be verified experimentally in the framework of this thesis. Absolute values of the structure factor were obtained by determining the intensity ratios of elastically and inelastically scattered X-ray radiation. The existence of a liquid phase was experimentally observed for a density of 3.9+/-0.2 g/cm³ and a pressure of 145+/-17 GPa inside the shock wave. For a lower pressure of 86+/-11 GPa and a similar density of 3.9+/-0.2 g/cm³, a state of matter was produced which is very close to the melting line and just barely liquid. This is in very good Agreement with ab-initio simulations which were used to calculate structure factors for the investigated parameters. Thus, in the framework of this thesis an experimental method for the characterization of phase transitions in warm dense matter was demonstrated and the formation of liquid carbon under shock compression was reliably observed using graphite as initial state for the first time.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik|
|Divisions:||05 Department of Physics|
|Date Deposited:||20 Dec 2012 10:21|
|Last Modified:||20 Dec 2012 10:21|
|Referees:||Roth, Prof. Dr. Markus and Hoffmann, Prof. Dr. Dieter H. H.|
|Refereed:||26 November 2012|