Direct Ion-Temperature Measurements Using High-Resolution Inelastic X-ray Scattering
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Authors
Griffin, Travis Dell
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
High-energy-density , Nonequilibrium , Short-pulse Lasers
Alternative Title
Abstract
Temperature in high-energy-density (HED) materials has been a long-standing problem. Direct temperature measurements are challenging because HED samples are often opaque and have extremely short lifetimes. Historically, ion temperature has been inferred from structural measurements that serve only as proxies. This approach becomes especially problematic when studying the evolution of highly nonequilibrium states generated by short-pulse laser–matter interactions. In such states, electrons can reach multi-eV temperatures while the lattice remains essentially unchanged. Understanding the dynamics of non-equilibrium HED systems is critical for laboratory astrophysics, materials science, and inertial fusion energy research. A reliance on inferred ion temperatures has led to disagreements across many thermodynamic processes. For example, within the field of electron–ion equilibration, indirect temperature determination has produced more than an order-of-magnitude variation in predicted equilibration rates. Likewise, experiments investigating interatomic bond strength have reached conflicting conclusions due to uncertainties in inferred ion temperatures. Direct, model-independent temperature measurements are needed to resolve these discrepancies. This dissertation presents the development of a high-resolution inelastic-scattering platform capable of directly measuring ion temperature in HED matter. Designed for free-electron laser facilities, the platform uses a backscattering geometry to measure the ion velocity distribution. When the distribution follows Maxwell–Boltzmann statistics, the temperature can be extracted through standard Doppler-broadening relationships. These truly model-independent lattice-temperature measurements enable unambiguous determination of thermodynamic behavior in metallic samples under extreme conditions. Using this platform, we performed a series of experiments measuring the temporal evolution of ion temperature in laser-irradiated metallic samples. Gold temperature measurements were obtained at the LINAC Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. Analysis of the resulting temperature profiles provides insight into the thermal properties of gold in a highly nonequilibrium regime. The measurements offer conclusive evidence of extreme superheating—ion temperatures far exceeding the melt point while the material maintains a crystalline structure. The full temporal profiles also allow us to study electron–ion equilibration dynamics directly, addressing the long-standing order-of-magnitude discrepancies in published equilibration rates. Furthermore, by combining the temperature measurements with structural information from X-ray diffraction, we observe changes in interatomic bond strength—a topic of significant debate over the past several decades. The final chapter discusses extending this platform to laser-shock experiments. A model-independent method for determining the temperature behind the shock front is essential for accurately constraining equation-of-state measurements.