Time-Resolved XUV Opacity Measurements of Warm Dense Aluminum

Area of Science:

• Plasma Physics
• Astrophysical Plasmas
• Fusion Energy Research

Background:

• Free-free opacity is crucial for energy transport in stellar interiors and inertial confinement fusion (ICF).
• Existing theoretical models for dense plasmas are conflicting, and experimental data for validation is scarce.
• Accurate opacity data is needed to refine models used in astrophysics and ICF research.

Purpose of the Study:

• To experimentally measure the opacity of dense aluminum plasmas under extreme conditions.
• To validate theoretical models of plasma opacity in the dense plasma regime.
• To investigate the influence of temperature and plasma state on opacity.

Main Methods:

• Time-resolved transmission measurements of solid-density aluminum (Al) heated by a femtosecond XUV free-electron laser (FEL).
• A novel functional optimization approach was used to extract the temperature-dependent absorption coefficient from single-shot measurements.
• Measurements captured plasma evolution on ultrafast (femtosecond) and short (picosecond) timescales.

Main Results:

• A significant enhancement in plasma opacity was observed as aluminum was heated to temperatures near its Fermi energy.
• Plasma heating and opacity enhancement occurred on ultrafast timescales, consistent with the XUV FEL pulse duration.
• Further opacity increases on picosecond timescales were attributed to material melting and the formation of warm dense matter.

Conclusions:

• The experimental results provide crucial data for validating theoretical models of free-free opacity in dense plasmas.
• The findings demonstrate a pronounced opacity enhancement linked to Fermi energy effects in heated aluminum.
• The study highlights the formation of warm dense matter and its impact on opacity dynamics.