Exotic hollow atom states pumped by relativistic laser plasma in a radiation dominant regime

In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n $=$ 1 electrons) from aluminium targets was observed at high laser contrast, for intensities of 3x10$^{20}$ Wcm$^{-2}$ and micron thick targets. These spectral observations are interpreted using detailed atomic kinetics calculations suggesting these exotic hollow atom states occur at near solid density and are driven by an intense polychromatic x-ray field. We estimate that this x-ray radiation field has energy in the kilovolt range and has an intensity exceeding 10$^{18}$ Wcm$^{-2}$. The field may arise through relativistic electron Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface.