Spectroscopic Investigation of High-Energy-Density Plasmas using Collisional-Radiative Model and X-ray Diagnostics Jang Hyeob Sohn Gwangju Institute of Science and Technology

Abstract

High-energy-density (HED) plasma refers to a state of plasma at extremely high pressures (≥ 1 Mbar) and energy densities (≥ 10¹¹ J/m³). Advances in energetic sources have enabled the efficient generation of well-defined HED plasmas in laboratories, providing valuable insights into matter under extreme conditions. X-ray spectroscopic techniques have been instrumental in revealing key information about these plasmas, but interpret- ing their spectra is complex. To address this, collisional-radiative models have been developed to describe atomic processes accurately. Consequently, combining experimental X-ray spectroscopy with theoretical collisional-radiative modeling is essential for precise diagnostics and comprehensive analysis of HED plasmas. This thesis investigates the spectroscopic characteristics of HED plasmas through both theoretical and experimental approaches, utilizing the generalized collisional-radiative model FLYCHK and advanced X-ray techniques. FLYCHK is improved to extend its applicability to strongly coupled plasmas and used to generate versatile opacity tables for various environments and elements spanning from atomic numbers Z=1 to 79, compared with those from other calculation codes. The thesis also presents an investigation of EUV-emitting tin plasmas produced by near-infrared-wavelength lasers using radiation-hydrodynamic simulations and FLYCHK calculations to explore optimal ionization strategies for the efficient generation of highly charged tin ion populations essential to EUV emissions. Additionally, the K-shell emission spectra from silicon nanowire arrays irradiated at relativistic intensities are measured and compared with FLYCHK calculations to investigate the physical properties and depths of optically thick HED plasmas. Finally, the high-energy-resolution off-resonant spectroscopy technique with self-seeded X-ray free-electron laser pulses is demonstrated to more efficiently, accurately, and sensitively probe the valence electronic structure of warm dense copper foil compared to conventional X-ray absorption spectroscopy. ©2025 Jang Hyeob Sohn ALL RIGHTS RESERVED