The role of hot electrons in the generation of anomalous X-ray spectra from ultra-intense laser-plasma interactions*

An accurate description of excited atomic states in high-energy-density matter remains an experimental and theoretical challenge. Using the ALEPH laser at Colorado State University, we perform high-resolution X-ray spectroscopy of exotic matter produced via laser-solid interactions at ultra-relativistic intensities (I $\sim 10^{21}$ W/cm$^2$). We examine the origin of anomalous X-ray emission from copper foil, foam and buried layer targets through precise measurements of K-shell fluorescence and hot electron emission, as well as spectroscopy of XUV plasma emission. These measurements also elucidate the generation and propagation of hot electrons under self-generated electric and magnetic fields, providing crucial constitutive data for HED matter in an ultra-high intensity regime.