Ionization dynamics in ultra-relativistic Cu plasmas

I will present time- and space-resolved resonant X-ray opacity and emission spectroscopy measurements from solid-density copper foils irradiated by relativistic femtosecond laser pulses at intensities exceeding 10^20 W/cm², performed at the HED instrument of the European XFEL. By tuning the XFEL probe to K–L transitions of Cu 21+ and Cu 22+ ions, we access transient electron temperatures during the first picoseconds of plasma evolution. The data show rapid electron cooling and spatially confined ionization, in stark contrast to 2D PIC simulations which predict significantly stronger and deeper heating. The discrepancy highlights critical limitations in current models, particularly in accounting for energy transport, recombination, and dimensional effects. Our results benchmark the need for full 3D treatments and improved atomic physics in simulations of relativistic laser-solid interactions.