Investigation of Energy Relaxation in Laser-Irradiated Copper Foils with Femtosecond XFEL Imaging*

Accurate measurements of rapidly transient, high-density target conditions heated by laser-driven fast electrons, such as electron temperatures and ionization states, are crucial for advancing knowledge of electron-driven warm dense matter properties and fast ignition in inertial fusion energy research. These measurements are challenging due to limited diagnostics with adequate spatial and temporal resolutions. Recently, we developed spatiotemporally resolved measurements of fast electron transport in a solid-density copper foil using an X-ray Free Electron Laser (XFEL) from SACLA. The femtosecond, collimated X-ray pulse captures an expanding electron heat front at near the speed of light.[1] By varying the X-ray beam’s photon energy near the Cu K-edge, we inferred the plasma conditions behind the heat front.[2] In this study, we focused on energy relaxation physics in a high-intensity laser-irradiated thin Cu foil by measuring target expansion in edge-on and surface deformation in face-on geometries. Both edge-on and face-on measurements show significant target modification at laser intensities ranging from 2×10¹⁸ to 5×10¹⁹ W/cm², occurring 30~50 ps after laser irradiation, significantly longer than the energy relaxation time for ideal plasmas. Details of the measurements and electron-ion energy relaxation in warm dense matter will be discussed.