Investigation of Electron-Ion Relaxation in Fast Electron-Driven Solid-Density Copper Using XFEL imaging*

Intense short-pulse laser-driven relativistic electron beams enable the creation of matter under extreme conditions for fundamental research, warm dense matter (WDM), and inertial fusion energy. In laser-solid interactions, relativistic electrons induce rapid heating via multiple mechanisms without changing the target density, known as isochoric heating. While the heating and ionization processes are broadly understood, the subsequent electron-ion energy relaxation timescale in WDM, particularly created by fast electrons, remains unclear. We investigated this relaxation using an X-ray Free Electron Laser (XFEL) at SACLA by measuring the expansion of isochorically heated copper foils. The hard X-ray beam, with micron-scale spatial resolution, allowed observation of motion only in dense plasma regions. Thin copper targets were irradiated with a relativistic intensity femtosecond laser. XFEL pulses probed the expansion in edge-on geometry by varying the XFEL-laser delay. Time-resolved X-ray images show no significant expansion before a 100 ps delay, much slower than expected for classical plasmas. The estimated shock velocity at 2×10¹⁸ W/cm² was 12-20 µm/ns. Experimental details, results, and hydrodynamic simulations will be presented.