Exotic Dense-Matter States Pumped by a Relativistic Laser Plasma in the Radiation-Dominated Regime J. Colgan, 1 J. Abdallah, Jr., 1 A. Ya. Faenov, 2,9 S. A. Pikuz, 2 E. Wagenaars, 3 N. Booth, 4 O. Culfa, 3 R. J. Dance, 3 R. G. Evans, 5 R. J. Gray, 6 T. Kaempfer, 7 K. L. Lancaster, 4 P. McKenna, 6 A. L. Rossall, 3 I. Yu. Skobelev, 2 K. S. Schulze, 7 I. Uschmann, 7,10 A. G. Zhidkov, 8 and N. C. Woolsey 3 1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow 125412, Russia 3 York Plasma Institute, Department of Physics, University of York, York YO10 5DD, United Kingdom 4 Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, United Kingdom 5 Department of Physics, Imperial College, London SW7 2AZ, United Kingdom 6 SUPA, Department of Physics, University of Strathclyde, Glasgow G4 ONG, United Kingdom 7 Helmholtzinstitut Jena, Fro ¨belstieg 1, D-07743 Jena, Germany 8 PPC Osaka University and JST, CREST, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan 9 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Kizugawa, Kyoto 619-0215, Japan 10 Institut fu ¨r Optik und Quantenelektronic, Friedrich-Schiller-Universita ¨t Jena, Max-Wien Platz 1, D-07743 Jena, Germany (Received 31 October 2012; published 18 March 2013) In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n ¼ 1 electrons) from thin Al foils was observed at pulse intensities of 3  10 20 W=cm 2 . 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  10 18 W=cm 2 and is in the 3 keV range. DOI: 10.1103/PhysRevLett.110.125001 PACS numbers: 52.20.Hv, 34.80.Dp, 52.25.Jm The properties of high energy density plasma have been under increasing scrutiny in recent years due to their importance to our understanding of stellar interiors, the cores of giant planets [1], and hot plasma in inertial con- finement fusion devices [2]. Such studies are also relevant to photoionized plasmas found in active galactic nuclei and x-ray binaries [3]. Currently, powerful x-ray free-electron- laser sources (XFELs) are being used to create such high energy density matter [4,5] using x rays at intensities greater than 10 17 W=cm 2 . The radiation intensity domi- nates standard collisional atomic processes creating exotic states of matter, composed of hollow atoms, which are diagnosed through the observation of unique spectral lines [6]. Although the definition of a hollow atom is not unique [7–9], it is usually taken to mean atoms (or ions) in which K and/or L shell electrons have been removed in prefer- ence to the valence electrons. In this work, we report the observation of hollow atoms with double K-shell (principal quantum number n ¼ 1) vacancies (KK atoms) formed with long-wavelength laser fields. This might be regarded as unexpected since the small photon energies of the laser pulse cannot directly ionize the target K-shell electrons. Our detailed atomic kinetics simulations suggest that an intense short- wavelength radiation field is the only plausible mechanism for production of hollow atoms. We provide a detailed argument for the generation of this intense x-ray radiation field through the energy loss of the highly accelerated field- ionized electrons produced from the intense long- wavelength laser field. This work complements the recent observations of such exotic states using XFELs [4,5]. We note that the radiation field generated in our experiments is polychromatic with an intensity that is comparable to or exceeding that of current XFELs. Our work implies that matter dominated by exotic hollow atom states, as well as radiation-dominated atomic physics, can be accessed and probed with high-power optical lasers. Indeed, as laser intensities continue to increase above 10 22 W=cm 2 , radia- tion processes will start to govern plasma interaction phys- ics as well as the atomic physics [10–12]. The proposed concept and experimental layout is schematically shown in Fig. 1. The measurements were made at the Vulcan Petawatt (PW) Laser Facility at Rutherford Appleton Laboratory, which provides a beam using optical parametric chirped pulse amplification technology with a central wavelength of 1054 nm and a pulse FWHM duration of 0.7 ps. The optical parametric chirped pulse amplification approach enables an amplified spontaneous emission to peak- intensity contrast ratio exceeding 1:10 9 several nanosec- onds before the peak of the laser pulse. The laser pulse was focused with an f=3 off-axis parabola providing up to 160 J on the target. The maximum laser intensity of 3  10 20 W=cm 2 was achieved with a laser focus PRL 110, 125001 (2013) PHYSICAL REVIEW LETTERS week ending 22 MARCH 2013 0031-9007= 13=110(12)=125001(5) 125001-1 Ó 2013 American Physical Society