Spectral content of buried Ag foils at 10(16) W/cm(2) laser illumination

Sources of 5-12 keV thermal Heα x-rays are readily generated by laser irradiation of mid-Z foils at intensities >10(14) W/cm(2), and are widely used as probes for inertial confinement fusion and high-energy-density experiments. Higher energy 17-50 keV x-ray sources are efficiently produced from "cold" Kα emission using short pulse, petawatt lasers at intensities >10(18) W/cm(2) [H.-S. Park, B. R. Maddox et al., "High-resolution 17-75 keV backlighters for high energy density experiments," Phys. Plasmas 15(7), 072705 (2008); B. R. Maddox, H. S. Park, B. A. Remington et al., "Absolute measurements of x-ray backlighter sources at energies above 10 keV," Phys. Plasmas 18(5), 056709 (2011)]. However, when long pulse (>1 ns) lasers are used with Z > 30 elements, the spectrum contains contributions from both K shell transitions and from ionized atomic states. Here we show that by sandwiching a silver foil between layers of high-density carbon, the ratio of Kα:Heα in the x-ray spectrum is significant increased over directly illuminated Ag foils, with narrower lines from K-shell transitions. Additionally, the emission volume is more localized for the sandwiched target, producing a more planar x-ray sheet. This technique may be useful for generating probes requiring spectral purity and a limited spatial extent, for example, in incoherent x-ray Thomson scattering experiments.