Colgan, J., Abdallah, J., Faenov, A. Ya, Pikuz, S.A., Wagenaars, E., Booth, N., Culfa, O., Dance, R.J., Evans, R.G., Gray, R.J., Kaempfer, T., Lancaster, K.L., McKenna, P., Rossall, A. K., Skobelev, I.Yu, Schulze, K.S., Uschmann, I., Zhidkov, A.G. and Woolsey, N.C. (2013) Exotic Dense-Matter States Pumped by a Relativistic Laser Plasma in the Radiation-Dominated Regime. Physical Review Letters, 110 (12). p. 125001. ISSN 0031-9007

In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n=1 electrons) from thin Al foils was observed at pulse intensities of 3×1020  W/cm2. The observations of spectra from these exotic states of matter are supported by detailed kinetics calculations, and are consistent with a picture in which an intense polychromatic x-ray field, formed from Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface, drives the KK hollow atom production. We estimate that this x-ray field has an intensity of >5×1018  W/cm2 and is in the 3 keV range.