Time-Resolved Measurements of Shock-Compressed Matter using X-rays.

Abstract

Thermonuclear fusion occurs at extremely high pressures and densities. Producing thermonuclear fusion in the laboratory requires a detailed understanding of material properties beyond the scope of condensed matter or classical plasma physics, requiring experimental data to improve models describing matter in these extreme states. This thesis reports the development of two improved methods to probe highly compressed matter using x-ray diagnostics.

The first method uses time-resolved x-ray diffraction to infer the stresses in compressed polycrystalline materials. X-ray diffraction is capable of measuring strain states and densities in shock-compressed materials with significantly higher accurately than existing shock timing and velocimetry diagnostics. The analysis discussed in this thesis calculates Debye-Scherrer diffraction patterns from highly stressed polycrystalline samples in the Reuss (iso-stress) limit. In this limit, elastic anisotropy and sample texture effects are directly modeled using elastic constants to calculate lattice strains for all initial crystallite orientations. Example diffraction patterns showing the effects of probing geometry, deviatoric stresses, and sample texture are presented to highlight the versatility of the technique. Finally, I present the design of a recent experiment conducted at the Linac Coherent Light Source to measure the strength of polycrystalline diamond whose data can be analyzed using this technique.

The second method uses x-ray fluorescence (XRF) to measure density, ionization state populations, and electron temperature in shocked materials. Spatially resolved K-alpha intensity measurements enable measurements of ion density profiles. Ionization state distributions and electron temperatures are constrained by comparing K-alpha spectra to spectra from atomic-physics simulations using the computer code CRETIN. Analysis of experimental data from the Trident laser facility measuring Ti K-alpha emission spectra from shock-compressed foams demonstrates the use of the technique. This work shows that XRF spectroscopy is a useful technique to complement prior diagnostics to make equation of state measurements of shocked materials containing a suitable tracer element.