51�������

Abstract:

Calculations of the patterns of x-ray diffraction from shocked crystals derived from the results of non-equilibrium molecular dynamics (NEMD) simulations are presented. The atomic coordinates predicted from the NEMD simulations combined with atomic form factors are used to generate a discrete distribution of electron density. A fast Fourier transform (FFT) of this distribution provides an image of the crystal in reciprocal space, which can be further processed to produce quantitative simulated data for direct comparison with experiments that employ picosecond x-ray diffraction from laser-irradiated crystalline targets. © 2008 IOP Publishing Ltd.

Abstract:

The electron temperature (Te) and average ionization (Z) of nearly Fermi-degenerate, direct-drive, shock-heated, and compressed plastic planar foils were investigated using noncollective spectrally resolved x-ray scattering on the OMEGA Laser System [T. R. Boehly, Opt. Commun. 133, 495 (1997)]. Plastic (CH) and Br-doped CH foils were driven with six beams, having an overlapped intensity of ∼1× 1014 W cm2 and generating ∼15 Mbar pressure in the foil. The plasma conditions of the foil predicted with a one-dimensional (1-D) hydrodynamics code are Te ∼10 eV, Z∼1, mass density ρ ∼4 g cm3, and electron density ne ∼3× 1023 cm-3. The uniformly compressed portion of the target was probed with 9.0-keV x rays from a Zn Heα backlighter created with 18 additional tightly focused beams. The x rays scattered at either 90° or 120° were dispersed with a Bragg crystal spectrometer and recorded with an x-ray framing camera. An examination of the scattered x-ray spectra reveals that an upper limit of Z∼2 and Te =20 eV are inferred from the spectral line shapes of the elastic Rayleigh and inelastic Compton components. Low average ionizations (i.e., Z<2) cannot be accurately diagnosed in this experiment due to the difficulties in distinguishing delocalized valence and free electrons. Trace amounts of Br in the CH foil (i.e., 2% atomic concentration) are shown to increase the sensitivity of the noncollective, spectrally resolved x-ray scattering to changes in the average ionization. The experimentally inferred electron temperatures are comparable to the 1-D predictions. © 2007 American Institute of Physics.